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1 From the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University, Boston
2 Address reprint requests to SN Meydani, Nutritional Immunology Laboratory, JM USDA-HNRCA at Tufts University, 711 Washington Street, Boston, MA 02111. E-mail: simin.meydani{at}tufts.edu.
ABSTRACT
In recent years, numerous studies have been published on the health effects of yogurt and the bacterial cultures used in the production of yogurt. In the United States, these lactic acid-producing bacteria (LAB) include Lactobacillus and Streptococcus species. The benefits of yogurt and LAB on gastrointestinal health have been investigated in animal models and, occasionally, in human subjects. Some studies using yogurt, individual LAB species, or both showed promising health benefits for certain gastrointestinal conditions, including lactose intolerance, constipation, diarrheal diseases, colon cancer, inflammatory bowel disease, Helicobacter pylori infection, and allergies. Patients with any of these conditions could possibly benefit from the consumption of yogurt. The benefits of yogurt consumption to gastrointestinal function are most likely due to effects mediated through the gut microflora, bowel transit, and enhancement of gastrointestinal innate and adaptive immune responses. Although substantial evidence currently exists to support a beneficial effect of yogurt consumption on gastrointestinal health, there is inconsistency in reported results, which may be due to differences in the strains of LAB used, in routes of administration, or in investigational procedures or to the lack of objective definition of "gut health." Further well-designed, controlled human studies of adequate duration are needed to confirm or extend these findings.
Key Words: Yogurt • gut function • gut immunity • gastrointestinal diseases • gut microflora
INTRODUCTION
Components of the human intestinal microflora and of the food entering the intestine may have harmful or beneficial effects on human health. Abundant evidence implies that specific bacterial species used for the fermentation of dairy products such as yogurt and selected from the healthy gut microflora have powerful antipathogenic and antiinflammatory properties. These microorganisms are therefore involved with enhanced resistance to colonization of pathogenic bacteria in the intestine, which has led to the introduction of novel modes of therapeutic and prophylactic interventions based on the consumption of monocultures and mixed cultures of beneficial live microorganisms as "probiotics." Probiotics are defined as "living microorganisms, which on ingestion in sufficient numbers, exert health benefits beyond inherent basic nutrition" (1).
Yogurt is one of the best-known of the foods that contain probiotics. Yogurt is defined by the Codex Alimentarius of 1992 as a coagulated milk product that results from the fermentation of lactic acid in milk by Lactobacillus bulgaricus and Streptococcus thermophilus (2). Other lactic acid bacteria (LAB) species are now frequently used to give the final product unique characteristics. As starter cultures for yogurt production, LAB species display symbiotic relations during their growth in milk medium (3). Thus, a carefully selected mixture of LAB species is used to complement each other and to achieve a remarkable efficiency in acid production. Furthermore, to increase the number of LAB that survive the low pH and high acidity of the gastrointestinal environment, some LAB species that are indigenous to the human intestine have been used in yogurt production. To meet the National Yogurt Association's criteria for "live and active culture yogurt," the finished yogurt product must contain live LAB in amounts 108 organisms/g at the time of manufacture (3), and the cultures must remain active at the end of the stated shelf life, as ascertained with the use of a specific activity test.
In many modern societies, fermented dairy products make up a substantial proportion of the total daily food consumption. Furthermore, it has long been believed that consuming yogurt and other fermented milk products provides various health benefits (4). Studies from the 1990s on the possible health properties of yogurt added to this belief (1, 5).
Probiotic therapy is based on the notion that there is such a thing as a "normal" healthy microflora, but normal healthy microflora has not been defined except perhaps as microflora without a pathogenic bacterial overgrowth. The development of novel means of characterizing and modifying the gut microflora has opened up new perspectives on the role of the gut microflora in health and disease. Numerous studies suggested beneficial therapeutic effects of LAB on gut health. However, results have been inconsistent, which may be due to differences in the strains of LAB, routes of administration, and investigational procedures used in these studies.
Several LAB species are currently used in the production of yogurt. This review focuses on the current evidence suggesting that yogurt and specific LAB species that are used for the fermentation of milk may or may not have valuable health-promoting properties or therapeutic effects on various gastrointestinal functions and diseases.
NUTRITIONAL VALUE OF YOGURT
The nutrient composition of yogurt is based on the nutrient composition of the milk from which it is derived, which is affected by many factors, such as genetic and individual mammalian differences, feed, stage of lactation, age, and environmental factors such as the season of the year. Other variables that play a role during processing of milk, including temperature, duration of heat exposure, exposure to light, and storage conditions, also affect the nutritional value of the final product. In addition, the changes in milk constituents that occur during lactic acid fermentation influence the nutritional and physiologic value of the finished yogurt product. The final nutritional composition of yogurt is also affected by the species and strains of bacteria used in the fermentation, the source and type of milk solids that may be added before fermentation, and the temperature and duration of the fermentation process.
B vitamins
Dairy products have generally been considered an excellent source of high-quality protein, calcium, potassium, phosphorus, magnesium, zinc, and the B vitamins riboflavin, niacin, vitamin B-6, and vitamin B-12 (6). A much greater loss of vitamins than of minerals may occur during the processing of yogurt because vitamins are more sensitive to changes in environmental factors than are minerals. Some of the factors that are important during the processing of milk and that are known to have adverse effects on the vitamin content of dairy products in general include heat treatment and pasteurization, ultrafiltration, agitation, and oxidative conditions. In addition, bacterial cultures used during the fermentation process of yogurt can influence the vitamin content of the final product (6).
LAB species do require B vitamins for growth, but some cultures are capable of synthesizing B vitamins (6). An example of a B vitamin that is utilized by LAB is vitamin B-12 (7, 8). Vitamins required for the growth of LAB cultures vary from one strain to another. Significant losses of vitamin B-12 can be corrected by the careful use of supplementary LAB cultures that are capable of synthesizing vitamin B-12 (9).
Folate is the best example of a B vitamin that some LAB species synthesize (10, 11). Depending on the bacterial strains used, the folate content of yogurt can vary widely, ranging from 4 to 19 µg/100 g (8). The major form of folate present in milk is 5-methyl-tetrahydrofolate (12). In a recent study, bacterial isolates from various species used for milk fermentation and yogurt production were examined for their ability to synthesize or utilize folate (11). S. thermophilus and Bifidobacteria were folate producers, whereas Lactobacilli depleted folate from the milk media. A combination of folate-producing cultures resulted in even greater folate content of the final fermented product. Further studies on the effect of changes in the vitamin B content of milk on fermentation would be of great practical significance.
Lactose
Dairy products and foods prepared with the use of dairy ingredients are an exclusive source of the disaccharide lactose in human diets. Before absorption, lactose is hydrolyzed by the intestinal brush border ß-galactosidase (lactase) into glucose and galactose. These monosaccharides are absorbed and used as energy sources.
Before fermentation, the lactose content of the yogurt mix generally is 6% (3). One example of a significant bacteria-induced change that occurs during the fermentation process is the hydrolysis of 20-30% of the disaccharide lactose to its absorbable monosaccharide components, glucose and galactose (2). In addition, a portion of the glucose is converted to lactic acid. Depending on other ingredients added, this hydrolysis results in lower lactose concentrations in yogurt than in milk, which in part explains why yogurt is tolerated better than milk by persons with lactose maldigestion (13-15). However, other factors also seem to play a role. For example, lactose-intolerant subjects exhibited better tolerance for yogurt with a relatively high amount of lactose than for milk containing a similar amount of lactose (13, 15). In another example, bacteria present in yogurt, such as L. bulgaricus and S. thermophilus, expressed functional lactase, the enzyme that breaks down lactose (16). This expression may also contribute to better tolerance of lactose in yogurt than of lactose in milk by persons with lactose maldigestion (15).
Protein
The protein content of commercial yogurt is generally higher than that of milk because of the addition of nonfat dry milk during processing and concentration, which increases the protein content of the final product. It has been argued that protein from yogurt is more easily digested than is protein from milk, as bacterial predigestion of milk proteins in yogurt may occur (8, 17). This argument is supported by evidence of a higher content of free amino acids, especially proline and glycine, in yogurt than in milk. The activity of proteolytic enzymes and peptidases is preserved throughout the shelf life of the yogurt. Thus, the concentration of free amino groups increases up to twofold during the first 24 h and then doubles again during the next 21 d of storage at 7 °C (18). Some bacterial cultures have been shown to have more proteolytic activity than do others. For example, L. bulgaricus was shown to have a much higher proteolytic activity during milk fermentation and storage than does S. thermophilus, as indicated by elevated concentrations of peptides and free amino acids after milk fermentation (19).
During fermentation, both heat treatment and acid production result in finer coagulation of casein, which may also contribute to the greater protein digestibility of yogurt than of milk. Proteins in yogurt are of excellent biological quality, as are those in milk, because the nutritional value of milk proteins is well preserved during the fermentation process (20). Both the caseins and the whey proteins in yogurt are rich sources of all the essential amino acids, and the intestinal availability of nitrogen has been reported as being high (93%; 21, 22). Labeling of milk proteins with the stable isotope 15N has made it possible to discriminate between exogenous and endogenous nitrogen fractions in serum after ingestion of 15N-labeled milk or 15N-labeled yogurt proteins. In a study of human subjects, Gaudichon et al (23) found that proteins from both milk and yogurt were rapidly hydrolyzed after ingestion, but the gastroduodenal transfer of dietary nitrogen was slower when yogurt was fed than when milk was fed.
Lipids
Milk fat also goes through biochemical changes during the fermentation process. Minor amounts of free fatty acids are released as a result of lipase activity (3). Because most of the yogurt sold in the United States is of the low-fat and nonfat varieties, hydrolysis of lipids contributes little to the attributes of most yogurt products. However, yogurt has been shown to have a higher concentration of conjugated linoleic acid (CLA), a long-chain biohydrogenated derivative of linoleic acid, than does the milk from which the yogurt was processed (24). A fermented dairy product from India, referred to as dahi, has also been shown to have higher CLA content than does nonfermented dahi (25). The major sources of CLA in our diets are animal products from ruminants, in which CLA is synthesized by rumen bacteria. Increased consumption of dairy fat was shown to be associated with increased concentrations of CLA in both human adipose tissue (26) and human milk (27). It was hypothesized that biohydrogenation also occurs during fermentation of milk and results in higher concentrations of CLA in the final product (28).
CLA was reported to have immunostimulatory and anticarcinogenic properties (29). In a recent study of breast and colon cancer cells, Kemp et al (30) showed that the anticarcinogenic properties of CLA may be due to the ability of some CLA isomers to inhibit the expression of cyclins and thus halt the progression of the cell cycle from G1 to S phase. In addition, CLA induced the expression of the tumor suppressor p53.
Minerals
In addition to being a good source of protein, yogurt is an excellent source of calcium and phosphorus. In fact, dairy products such as milk, yogurt, and cheese provide most of the highly bioavailable calcium in the typical Western diet. Because of the lower pH of yogurt compared with that of milk, calcium and magnesium are present in yogurt mostly in their ionic forms.
One of the major functions of calcium is the role it plays in bone formation and mineralization. The calcium requirements during growth, pregnancy, and lactation are increased. However, the average calcium intake of women of childbearing age is consistently less than is recommended (31). In addition, calcium intake of women tends to fall even lower during the postmenopausal years (32). This is especially important for postmenopausal women, who are at increased risk of bone loss and osteoporosis. Dietary fiber has an adverse effect on calcium absorption, whereas lactose may enhance the absorption of calcium (33). In the rat model, calcium retention was greater with consumption of a diet in which lactose made up half the total carbohydrates ingested than with consumption of the control diet (34). Schaafsma et al (35), investigating the effect of dairy products on mineral absorption by using rat models, reported that lactose enhances the absorption of calcium, magnesium, and zinc. Because yogurt has a lactose content lower than that of milk, the bioavailability of these minerals may be negatively affected, although the effect is likely to be small.
The acidic pH of yogurt ionizes calcium and thus facilitates intestinal calcium uptake (36). The low pH of yogurt also may reduce the inhibitory effect of dietary phytic acid on calcium bioavailability. Vitamin D plays a major regulatory role in intestinal calcium absorption. The active, saturable, transcellular route of calcium absorption in the duodenum and proximal jejunum requires calbindin-D, a vitamin D-dependent calcium-binding protein (37). In the United States, milk and infant formula are fortified with vitamin D, and hence they serve as good dietary sources, with 2.5 µg (100 IU) vitamin D/237-mL serving. However, other dairy products, such as yogurt, typically are not fortified with vitamin D.
Few studies have investigated the effect of yogurt-derived calcium on bone mineralization in animals (34, 38). Kaup et al (34) reported that yogurt-fed rats showed greater bone mineralization than did rats fed a diet containing calcium carbonate. These studies may suggest that the bioavailability of calcium in yogurt is greater and yogurt may increase bone mineralization more than do nonfermented milk products. However, there are currently no published studies that show a superior effect of yogurt on bone mineralization in human subjects.
MECHANISTIC RATIONALE FOR POTENTIAL BENEFITS OF YOGURT ON GUT FUNCTION AND HEALTH
It has been suggested that yogurt and LAB contribute to several facets of gastrointestinal health: the makeup of the gastrointestinal flora, the immune response, and laxation.
Gut microflora
Lactobacilli are among the components of microbial flora in both the small and large intestines. The ability of nonpathogenic intestinal microflora, such as LAB, to associate with and bind to the intestinal brush border tissue is thought to be an important attribute that prevents harmful pathogens from accessing the gastrointestinal mucosa (39). For LAB to have an effect, they must adapt to the host intestinal environment and be capable of prolonged survival in the intestinal tract (40-43). LAB survival is influenced by gastric pH as well as by exposure to digestive enzymes and bile salts (42), and LAB species differ in their ability to survive in the gastrointestinal environment (43).
When 4 strains of Bifidobacterium (B. infantis, B. bifidum, B. adolescentis, and B. longum) were compared, B. longum was the most resistant to the effects of gastric acid (44). Bifidobacterium animalis was reported to have a high survival rate during intestinal transit in human subjects (45).
The effect of feeding yogurt fermented with S. thermophilus, L. bulgaricus, and Lactobacillus casei on the fecal microflora of healthy infants aged 10-18 mo was investigated by Guerin-Danan et al (46). Whereas the number of infants with fecal Lactobacillus increased after the feeding, the total numbers of anaerobes, Bifidobacteria, bacteroides, and enterobacteria were not affected by yogurt intake. In a group of elderly patients with atrophic gastritis and hypochlorhydria, Lactobacillus gasseri survived passage through the gastrointestinal tract, but S. thermophilus and L. bulgaricus were not recovered (43). Bifidobacterium sp has also been shown to survive passage through the gastrointestinal tract: fecal concentrations were detectable for 8 d after the cessation of intake (47).
Another important factor that limits the survival of lactobacilli within the upper gastrointestinal tract is the inherent ability of the organisms to adhere to intestinal epithelial cells (42). With the use of scanning electron microscopy, Plant and Conway (48) screened 16 strains of Lactobacillus for their capacity to associate with Peyer's patches and the lymphoid villous intestinal tissues in mice. Two of the 16 strains investigated, Lactobacillus acidophilus and L. bulgaricus, are of interest because they relate to yogurt. It was found, in both in vitro and in vivo models using BALB/c mice, that L. bulgaricus did not associate with Peyer's patches or with the lymphoid villous intestinal tissues. L. acidophilus had a low degree of association with Peyer's patches and no association to the lymphoid villous intestinal tissue. Nevertheless, the authors stated that the strains of Lactobacillus tested showed high rates of survival when Lactobacillus was administered orally.
The ability of LAB to decrease the gastrointestinal invasion of pathogenic bacteria has also been described (39, 49). Bernet et al (39) reported a dose-dependent L. acidophilus-mediated inhibition of the adherence of enteropathogenic Escherichia coli and Salmonella typhimurium to the enterocyte cell-line Caco-2. In addition, L. acidophilus inhibited the entry of E. coli, S. typhimurium, and Yersinia pseudotuberculosis into Caco-2 cells. In another report (49), the same authors described similar inhibitory effects when 2 different strains of Bifidobacteria (B. breve and B. infantis) were used. In addition, long-term feeding of yogurt does not result in a significant change in the results of breath-hydrogen tests, which indicates the absence of a significant change in the intestinal survival of the yogurt organisms (50). Furthermore, it is possible that the ability of LAB to compete with pathogens for adhesion to the intestinal wall is influenced by their membrane fluidity. This possibility was suggested by studies indicating that the type and quantities of polyunsaturated fatty acids in the extracellular milieu influence the adhesive properties of LAB to the epithelium (51, 52).
Gut-associated immune response
The mucosal lymphoid tissue of the gastrointestinal tract plays an important role as a first line of defense against ingested pathogens. The interactions of LAB with the mucosal epithelial lining of the gastrointestinal tract, as well as with the lymphoid cells residing in the gut, have been suggested as the most important mechanism by which LAB enhances gut immune function. Several factors have been identified as contributing to the immunomodulating and antimicrobial activities of LAB, including the production of low pH, organic acids, carbon dioxide, hydrogen peroxide, bacteriocins, ethanol, and diacetyl; the depletion of nutrients; and competition for available living space (1, 5, 53).
The gastrointestinal tract is a complex immune system tissue. The main site of the mucosal immune system in the gut is referred to as gut-associated lymphoid tissue (GALT), which can be divided into inductive and effector sites. In the small intestine, the inductive sites are in the Peyer's patches, which consist of large lymphoid follicles in the terminal small intestine. The best-defined effector component of the mucosal adaptive immune system is secretory immunoglobulin A (sIgA). sIgA is the main immunoglobulin of the humoral immune response, which together with the innate mucosal defenses provides protection against microbial antigens at the intestinal mucosal surface (54). In a healthy person, sIgA inhibits the colonization of pathogenic bacteria in the gut, as well as the mucosal penetration of pathogenic antigens. At least 80% of all the body's plasma cells, the source of sIgA, are located in the intestinal lamina propria throughout the length of the small intestine. IgA is the most abundantly produced immunoglobulin in the human body. The production of intestinal sIgA requires the presence of commensal microflora (55), which indicates that the production of intestinal sIgA is induced in response to antigenic stimulation. It is not yet clear, however, how lamina propria B cells are activated to become IgA-secreting plasma cells or how the intestinal microflora influence this process. Most studies on the effect of fermented milk or specific LAB on gut immune function have centered on their immune adjuvant effects in the gut.
The ability of LAB to modulate IgA concentrations in the gut has also been the subject of several studies. Orally administered L. acidophilus and L. casei and the feeding of yogurt increased both IgA production and the number of cells secreting IgA in the small intestine of mice in a dose-dependent manner (5). Similarly, a report by Puri et al (56) indicated that S. typhimurium-induced serum IgA concentrations were significantly higher in mice fed yogurt over a period of 4 wk than in milk-fed control mice. This report suggests that the IgA secreted by the challenged intestinal B cells enters the circulation and increases the concentrations of IgA in the serum. Thus the IgA-enhancing effect of yogurt intake may have both an effect on the gut and a systemic effect. The same study also showed that intestinal lymphocytes from mice fed yogurt had a higher mitogen-induced proliferative response after a challenge with S. typhimurium than did those from control-fed mice.
In a study using human subjects, Link-Amster et al (57) showed that the specific anti-IgA titer to S. typhimurium was 4 times greater in subjects fed fermented milk containing L. acidophilus than in control subjects fed diets without fermented milk. Total sIgA concentrations also increased in subjects fed fermented milk.
Macrophages play an important role as a part of the innate immune response in the gut, and they represent one of the first lines of nonspecific defense against bacterial invasion. The effects of feeding milk fermented with either L. casei or L. acidophilus or both on the specific and nonspecific host defense mechanisms in Swiss mice were investigated by Perdigon et al (58). They showed that feeding milk fermented with L. casei, L. acidophilus, or both for 8 d increased the in vitro and in vivo phagocytic activity of peritoneal macrophages and the production of antibodies to sheep red blood cells. The activation of the immune system began on day 3, peaked on day 5, and decreased somewhat on day 8 of feeding. Phagocytic activity was further boosted in mice given a single dose of fermented milk on day 11 of feeding.
Modulation of cytokine production by yogurt and LAB has also been the focus of several studies. In addition to interleukin (IL)-1ß and tumor necrosis factor (TNF) , which are mainly produced by macrophages, T lymphocytes are the source of most cytokines investigated in those reports. T cells are frequently classified into 2 categories—type 1 (Th1) and type 2 (Th2) helper T cells. On activation, these cells produce 2 diverse patterns of cytokines (59). Th1 cells are the main producers of interferon- (IFN-) and IL-2, and Th2 cells produce IL-4, IL-5, IL-6, and IL-10. The Th1 cytokines boost cell-mediated immunity, and the Th2 cytokines augment humoral immunity. IFN- plays a critical role in the induction of other cytokines and in mediation of macrophage and natural killer cell activation.
Several reports indicated that consumption of yogurt or intake of LAB by themselves modulates the production of several cytokines, such as IL-1ß, IL-6, IL-10, IL-12, IFN-, and TNF- (60-63). Moreover, the production of IFN- in an in vitro culture system using human lymphocytes was reported to be greater with cultures in the presence of LAB (L. bulgaricus and S. thermophilus) than with those without LAB (64). Yogurt containing live L. bulgaricus and S. thermophilus was also reported to augment IFN- production by purified T cells from young adults after 4 mo feeding (62).
Effects of yogurt consumption on the modulation of cytokine production in the human gastrointestinal tract, whether by cells of the GALT or by others, have not been investigated. These types of studies, although feasible with the use of biopsy samples from the intestines of healthy subjects (65), are difficult to carry out, and good animal models currently do not exist.
Even though cytokines play diverse roles in regulating immune functions, some cytokines, eg, IL-1ß, IL-6, and TNF-, have been given more attention than others because they have traditionally been classified as proinflammatory and as such are known to be associated with inflammatory conditions such as Crohn disease and ulcerative colitis (66). Another diverse family of immune modulators that play important roles in the health of the gastrointestinal tract consists of chemokines and their receptors (67). Currently, only limited data have been published on the effect of yogurt or its components on chemokine modulation in the gastrointestinal tract. The effects of different strains of Lactobacillus on chemokine production by the intestinal epithelial cell-line, HT-29, were investigated by Wallace et al (68). All 3 LAB species investigated—L. acidophilus, Lactobacillus rhamnosus, and Lactobacillus delbrueckii—had suppressive effects on the production of 2 chemokines, RANTES (a member of the IL-8 superfamily of cytokines) and IL-8, by activated HT-29 cells. As is the case with proinflammatory cytokines, these chemokines are necessary for normal immune function. However, a high production of these chemokines during an inflammatory condition is believed to exacerbate the inflammatory response.
Laxation
Few reports have discussed the effects of yogurt and LAB on laxation. In the studies published, however, both significant effects (G Wilhelm, unpublished observations, 1993; 69) and no effects (70) of yogurt or LAB on laxation and gastrointestinal transit time were described.
Strandhagen et al (69) reported that the transit time for 50% (t50) of gastric content was significantly greater for ropy milk, an L. bulgaricus - and S. thermophilus-fermented milk product indigenous to Sweden, than for unfermented milk. Another study showed that milk fermented with L. bulgaricus and S. thermophilus reduced intestinal transit time in human subjects with habitual constipation (G Wilhelm, unpublished observations, 1993). In the same study, subjects consuming fermented milk also had improved bowel function. The number of defecations increased from 3/wk during a control period to 7/wk when fermented milk was consumed. When milk fermented with L. acidophilus was consumed, the number of defecations increased further to 15/wk.
Studies were conducted of the effects of a commercially available yogurt fermented with B. animalis on orofecal gut transit time (71, 72). In a double-blind, randomized, crossover design, B. animalis reduced the colonic transit time in a group of healthy women aged 18-45 y (72). Likewise, in a group of elderly subjects experiencing lengthy orofecal gut transit time but otherwise free of any gastrointestinal pathology, B. animalis intake provided led to a significant reduction in transit time (71). Thus, the effect of LAB ingestion on orofecal gut transit time appears to be dependent on the bacterial strain used and the population being studied.
YOGURT AND DISEASES OF THE GASTROINTESTINAL TRACT
Lactase deficiency and lactose maldigestion
Lactase deficiency among adults is the most common of all known enzyme deficiencies. More than half of the world's adult population is lactose intolerant. In developmental terms, this may not necessarily be considered abnormal, because humans are the only known mammal in whom lactase activity in the small intestine is sustained after weaning. In the case of lactose maldigestion, undigested lactose remains in the intestinal lumen, and, as it reaches the colon, it is fermented by colonic bacteria. Byproducts of this process include short-chain fatty acids such as lactate, butyrate, acetate, and propionate. These fatty acids associate with electrolytes and lead to an osmotic load that can induce diarrhea. Furthermore, fermentation of lactose by colonic bacteria produces methane, hydrogen, and carbon dioxide. These gases may stay in the lumen and eventually will both be excreted as flatus, diffusing into the circulation, and be exhaled via the lungs. Exhaled hydrogen after a lactose load has been used as an indirect but measurable indicator of lactose maldigestion. In addition to lactose, some sources of dietary fiber and other unabsorbed carbohydrates can serve as substrates for colonic fermentation that results in increased hydrogen production.
Inability to digest lactose varies widely among ethnic and geographic populations (73, 74). In the United States, the prevalence of primary lactose intolerance in adults is 53% among Mexican Americans, 75% among African Americans, and 15% among whites. The prevalence among adults in South America and Africa is >50% and that in some Asian countries is close to 100%. Lactose intolerance varies greatly between European countries, from 2% prevalence in Scandinavian adults to 70% among Southern Italian adults (74).
Lactose maldigestion may develop secondary to inflammation or as a result of functional loss of the small intestinal mucosa (14), which can result from conditions such as Crohn disease, celiac sprue, short bowel syndrome, or bacterial and parasitic infections. In addition, lactose maldigestion may develop as a consequence of severe protein calorie malnutrition. The disorder is clinically expressed by symptoms of abdominal cramps, diarrhea, and flatulence after milk ingestion. However, most persons who have symptoms of lactose intolerance can endure small amounts (2-10 g) of lactose in a meal without becoming symptomatic (14).
It is well known that, for many lactose-intolerant people, fermented milk products are better accepted than are unfermented milk products. There may be more than one reason for this. During fermentation of milk, lactose is partially hydrolyzed, which results in a lower lactose content in yogurt than in milk (2). However, this reduction in lactose may not be significant, because milk solids are usually added during processing. The greater tolerance of lactose from yogurt than of that from milk among lactose-intolerant subjects may be due to the endogenous lactase activity of yogurt organisms (13, 15, 75). Kolars et al (15) used a series of breath hydrogen tests as well as a subjective assessment to ascertain whether subjects who were identified as lactose-intolerant digested and absorbed lactose in yogurt better than they digested and absorbed lactose in milk. The area under the curve for breath hydrogen was smaller after yogurt consumption than after consumption of milk or lactose in water, which indicates better digestion and absorption of lactose from yogurt than of that from either milk or lactose in water. Subjective assessment by the subjects in the study of Kolars et al also indicated that lactose in yogurt was better tolerated than the same amount of lactose from milk or in water. Using breath hydrogen measurement, Savaiano et al (75) investigated the effects of 3 varieties of cultured milk products on the digestion of lactose by 9 lactase-deficient human subjects. When yogurt, cultured milk (buttermilk), and sweet acidophilus milk were compared, yogurt had the most beneficial effect on lactose digestion in these subjects. Lactase activity and the number of surviving LAB were significantly reduced when the yogurt was pasteurized.
The enzyme activity of lactase is generally stable in response to environmental factors. For example, it was shown that the lactase activity of yogurt was preserved and even increased when the yogurt was subjected to an environment that simulated the temperature and low pH values of the gut (15). As suggested by the authors, this study supports the notion that lactose in yogurt is autohydrolyzed once it is in the jejunal environment. Other studies reported that lactase activity is less stable in response to acidic environment. Pochart et al (76) reported that lactase activity in yogurt decreased by >80% at a pH of 5.0 in an in vitro model.
However, heating yogurt does significantly decrease lactase activity, which indicates that yogurt that has been heat treated is not as beneficial for lactose-intolerant persons is yogurt containing live and active cultures. Thus, there is a growing body of evidence that yogurt containing live and active cultures is better tolerated by lactose malabsorbers than are heat-treated fermented milks (50). During the fermentation process, the amount of lactose present in yogurt is reduced. The lactose content also varies with the duration of storage after fermentation. In addition, the bacterial lactase activity corresponds with the survival time of lactobacilli after ingestion. The enhanced digestion of lactose is explained partly by the improved lactase activity after yogurt ingestion and partly by other enzymatic functions, such as the activity of the lactose transport system (permease) that allows lactose to enter the probiotic cell (77, 78). Furthermore, animal studies have suggested that LAB may induce lactase activity of the gut intestinal endothelial cells (79).
A study by Martini et al (80) supports the microbial mediation of lactase activity in the gastrointestinal tract. Those authors showed that lactase activity in yogurt was stable at pH 4.0, but that microbial cell disruption resulted in 80% loss of lactase activity and a twofold increase in lactose malabsorption in a group of lactose maldigesters.
Although the organisms that make up the live cultures in yogurt are recognized as having functional lactase activity and as contributing to the digestion of lactose, their survival in the gastrointestinal tract is short. On average, significant numbers survive for <1 h after ingestion (15, 50). Regardless of this somewhat limited survival time, the beneficial effect of LAB on lactose digestion in those suffering from lactose intolerance is now widely accepted.
Diarrheal diseases
Diarrhea is a common problem among children worldwide and has been reported to contribute substantially to pediatric physician visits and hospitalizations in the United States (81). Since the early 20th century, it has been hypothesized that live bacterial cultures, such as those used for the fermentation of dairy products, may offer benefits in preventing and treating diarrhea (4).
A recent meta-analysis of randomized, controlled studies by Van Neil et al (82) found that therapy using Lactobacillus strains offered a safe and effective means of treating acute infectious diarrhea in children. Both the duration and frequency of diarrheal episodes were reduced when compared with those in control subjects. The benefit of Lactobacillus therapy was seen in diarrheal diseases caused by various pathogens. The effect of supplementing formula with B. bifidum and S. thermophilus on preventing the onset of acute viral diarrhea in infants was examined in a double-blind, placebo-controlled trial (83). The infants receiving bacterial therapy developed diarrhea and shed rotavirus less than did the infants fed the control formula. Evidence of the beneficial effect of LAB on the occurrence of diarrhea of bacterial origin is more contradictory because both benefits (84, 85) and no effects (86, 87) of feeding LAB were reported.
Several studies investigated the effects of probiotic bacteria on diarrhea associated with the use of antibiotics. The most likely cause of diarrhea associated with antibiotic use is the negative influence of antibiotics on the bacterial steady state of the intestines (88). Most cases of antibiotic-associated diarrhea are mild, and they end shortly after antibiotic therapy is discontinued. A less common but more serious type of antibiotic-associated diarrhea is due to antibiotic-mediated overgrowth of pathogenic bacterial species such as Clostridium difficile that is associated with pseudomembranous colitis (89).
A recent meta-analysis evaluated the ability of several different probiotic LAB species to prevent antibiotic-associated diarrhea (90). Of the 9 studies that were included in the analysis, 4 used Lactobacilli strains or a combination of Lactobacilli and Bifidobacteria (91-94). Of those 4 studies, 2 showed a significant benefit of probiotic use in comparison with placebo (93, 94). The authors concluded that probiotic bacteria supplied in capsules or as yogurt-based products may be useful in preventing antibiotic-associated diarrhea. However, none of these studies provide evidence for a role of probiotic bacteria in the treatment of such diarrhea.
The mechanisms by which LAB may provide a beneficial effect against some forms of diarrheal disease are unknown. It has been suggested that the beneficial effect may stem from the ability of LAB to reestablish the intestinal microflora, to increase the intestinal barrier by competing with pathogenic bacteria for adhesion to the enterocytes, or to increase mucosal IgA response to pathogens.
Colon cancer
According to the National Cancer Institute, cancer of the colon is the second leading cancer diagnosis among both women and men in the United States (95). Colon cancer is also the second most common cause of cancer death. Risk factors for colorectal cancer include both genetic and environmental factors, and several reports have suggested that interactions between dietary factors, colonic epithelium, and intestinal flora are central to the development of colon cancer.
The role of diet in the etiology of cancer has been given greater attention in recent years. Although the relation between colon cancer and certain food constituents, such as fiber and fat, generated the most interest, the possibility that fermented dairy products may protect against tumor formation in the colon was also investigated. Epidemiologic evidence suggests a negative correlation between the incidence of certain cancers, including colon cancer, and the intake of fermented dairy products (96). Moreover, fermented dairy products or the bacteria used for milk fermentation were shown to have an effect on colon cancer and certain other tumors in murine models of carcinogenesis (97-100). However, a number of animal studies investigating the effect of various strains of LAB on colon carcinogenesis showed inconsistent results.
Wollowski et al (100) investigated the protective effect of several strains of LAB, traditionally used for milk fermentation, against 1,2-dimethylhydrazine (DMH)-induced colon carcinogenesis in rats. Oral treatment with L. bulgaricus for 4 d protected against DMH-induced DNA damage in the colon. In contrast, there was no protective effect when S. thermophilus was administered. The authors did not ascertain the mechanisms of protection by L. bulgaricus, but they speculated that thiol-containing breakdown products of proteins that result from the proteolytic activity of L. bulgaricus may have produced the effect.
In a previous study using a similar DMH-induced colon cancer model in rats, Shackelford et al (99) showed that milk fermented with L. bulgaricus resulted in greater survival than did nonfermented milk. However, in contrast to the findings of Wollowski et al (100), L. bulgaricus-fermented milk did not reduce the number of rats that developed colon tumors, whereas S. thermophilus-fermented milk did do so (99). In a study using azoxymethane to induce aberrant crypt foci in the colon of rats, no significant effects were seen with either B. longum or L. casei (101). Those authors did, however, observe a protective effect of L. acidophilus and inulin, but only when the total fat content of the diet was increased.
Using a colon carcinoma cell culture system, Ganjam et al (102) isolated a yogurt fraction that decreased cell proliferation, as ascertained with the use of thymidine incorporation. Cell proliferation was not inhibited in response to a similarly isolated milk fraction or to lactic acid.
Elevated activity of several bacterial fecal enzymes, some of which are involved in the metabolism of genotoxic nitrates, was associated with an increased risk of colon cancer (103, 104). The activity of these enzymes can be altered by diet or antibiotic intake (10, 105). L. acidophilus (106) and L. gasseri (43) were shown to reduce the fecal enzyme activity of nitroreductase, azoreductase, and ß-glucuronidase in humans, with a reduction by 50% or 75% in the activities of these enzymes during a period of Lactobacilli feeding. Likewise, Guerin-Danan et al (46) reported that 10-18-mo-old infants fed yogurt fermented with S. thermophilus, L. bulgaricus, and L. casei had lower fecal ß-glucuronidase activity than did a similar group of infants fed milk or yogurt not fermented with L. casei.
The mechanism by which LAB may have an effect on colon carcinogenesis is currently unknown. Some of the mechanisms that may be involved include enhancement of the host's gut immune response, suppression of harmful intestinal bacteria, sequestration of potential mutagens, production of antimutagenic compounds, reduction of pH concentrations in the colon, and alteration of other physiologic conditions (107). Furthermore, it was shown by Pedrosa et al (43) that the feeding of yogurt or Lactobacillus reduced fecal enzymes, which convert procarcinogens to carcinogens, such as azoreductase and nitroreductase.
Inflammatory bowel disease
Inflammatory bowel disease (IBD) is a term used for certain chronic immune-mediated conditions of the intestinal tract. These chronic diseases include Crohn disease and ulcerative colitis, conditions that have comparable symptoms but that affect the digestive tract in very different ways (66). Ulcerative colitis involves inflammation of the colon and rectum and not that of the upper gastrointestinal tract, whereas Crohn disease can affect the upper intestinal digestive tract and thus can lead to malabsorption of both macronutrients and micronutrients. The etiologies of these diseases are unknown, but studies suggest that the intestinal microflora play a crucial pathogenic role (108). This notion is supported by animal models of Crohn disease, in which the presence of intestinal microflora is absolutely required for the development of disease.
Proinflammatory cytokines, particularly TNF-, have also been recognized as playing a central role in the pathogenesis of Crohn disease. However, despite earlier hopes, the results from studies using TNF- antagonists were disappointing, and there were some reports of severe complications (109). Nevertheless, reducing the production or effect of TNF- (or both) in Crohn disease patients is belived to be beneficial. Bourrel et al (63) reported that, when inflamed intestinal mucosa from a group of Crohn's disease patients was cocultured in the presence of L. casei or L. bulgaricus, expression and release of TNF- by intraepithelial lymphocytes were reduced.
Normally, a healthy mucosal barrier provides a first defense mechanism against both the intestinal microflora and invading pathogens. It has been suggested that the proportions of different intestinal microflora are altered in patients with IBD. For example, colonic biopsy specimens have shown lower concentrations of Lactobacillus and lower fecal concentrations of both Lactobacillus and Bifidobacterium species in patients with Crohn disease than in healthy subjects (110). This disturbance in intestinal flora may increase the opportunity for colonization of pathogens and bring about a subsequent proinflammatory response.
In the case of IBD, a defective mucosal barrier allows for increased uptake of antigens and proinflammatory mediators originating from luminal bacteria. It has been reported that patients with IBD have diminished mucosal protection as a result of changes in the composition and thickness of the mucosal layer and alterations in the glycosylation status of mucosal glycoproteins (111). These changes in the intestinal mucosa are also associated with decreased intestinal IgA activity and increased IgG activity, which coincides with reduced state of protection and a proinflammatory condition. With weakened mucosal barrier and thereby increased adherence of bacterial pathogens to the mucosa, sustained inflammation results, and that leads to further damage to the gut mucosa. In recent years, immunosuppressive and immunomodulating therapies, such as the steroids used since the 1960s, have become more and more frequent in the treatment of these conditions. Although efficacious, these types of drugs can increase the prevalence of opportunistic infections as well as the severity of any underlying infection that may be present (112). Other side effects of these treatments may include hepatotoxicity, fibrosis, lymphoma, and pathologic suppression of bone marrow function.
The role of beneficial intestinal microflora in the prevention of intestinal inflammation was investigated by using gene-targeted IL-10 knockout (IL-10–/–) mice (113, 114). These IL-10-deficient mice spontaneously develop ileocolitis with many similarities to Crohn disease in humans. Furthermore, affected mice respond favorably to immunosuppression or immunomodulatory drugs that are similar to those used to treat human IBD. The immunoregulatory activity of IL-10 has been studied extensively. It is now well established that IL-10 plays a role in down-regulating both the synthesis of inflammatory cytokines and the presentation of antigens. Thus, IL-10 has been suggested for use as an immunomodulator for the treatment of Crohn disease. Targeted in vivo delivery of IL-10 to the affected intestinal epithelium by using genetically engineered Lactococcus lactis has shown great promise in 2 mouse models of IBD (114).
Madsen et al (113) found that IL-10–/– mice had increased adherence of luminal bacteria to the mucosal layer in the colon that preceded the development of colitis. This occurred in parallel to decreased numbers of luminal Lactobacillus. When the concentrations of Lactobacillus in the gastrointestinal lumen were restored by rectal delivery of Lactobacillus reuteri or by oral lactulose therapy, colitis was attenuated. The concentrations of adherent and translocated bacteria in the mucosal wall also were reduced.
Another benefit of LAB in Crohn disease may be due to the stimulation of the IgA response. A report by Malin et al (115) suggests that oral bacteriotherapy using L. casei can restore antigen-specific IgA immune response in persons with Crohn disease. In a previous study from the same laboratory (116), oral administration of L. casei to patients with viral gastroenteritis promoted antigen-specific IgA responses and shortened the patient diarrhea.
Although experimental evidence exists indicating beneficial effects of LAB on Crohn disease and ulcerative colitis, the exact mechanism through which LAB species antagonize the progression of these diseases is poorly understood. The exact etiology of IBD is also unknown, but it is likely that, in susceptible persons, IBD results from an ongoing inflammatory response, which may be due to a defect in both the regulation of the mucosal proinflammatory response and the function of the intestinal epithelium. Currently, evidence suggests that yogurt and LAB have modest clinical benefits and are safe for use in patients with these conditions. Further studies are required to ascertain whether yogurt is beneficial as a prophylactic or a therapeutic regimen for IBD (or both) and to establish exactly which mechanisms are involved.
Helicobacter pylori
It has only been 20 y since Helicobacter pylori, a gram-negative, spiral-shaped bacterium that is found in the gastric mucous layer or adherent to the epithelial lining of the stomach, was discovered (117). H. pylori relies on the ammonia-producing surface protein urease for adherence and colonization to the gastric epithelium. Urease allows H. pylori to survive by neutralizing the acidic gastric environment (118). H. pylori produces catalase, which may play a role in protecting the bacteria from free radicals that are released by activated leukocytes. H. pylori infection is associated with a massive infiltration of neutrophils into the gastric wall and local production of IFN-, proinflammatory cytokines—eg, TNF-, IL-1ß, and IL-6—and the chemokine IL-8.
Infection with H. pylori is now known to play a role in peptic ulcer disease, chronic gastritis, gastric adenocarcinoma, and mucosa-associated lymphoid tissue lymphoma. The association between duodenal ulcer disease and H. pylori is also well documented: H. pylori infection is reported in >90% of duodenal ulcer patients (119). Treatment of this infection involves the use of proton pump inhibitors, often in combination with antibiotics. However, the use of antibiotics to treat H. pylori infection has been associated with adverse effects and frequently leads to resistance to antibiotic therapy.
Several in vitro and animal studies have shown reduced viability of H. pylori and less adhesion of the bacteria to human intestinal mucosal cells after treatment with various Lactobacillus strains (120). In series of in vitro assays, Midolo et al (121) showed that the growth of H. pylori was inhibited by lactic acid in a pH-independent manner. They also found that 6 strains of L. acidophilus and L. casei inhibited the growth of H. pylori, whereas B. bifidus and L. bulgaricus did not. The inhibitory effect correlated with the concentrations of lactic acid produced by the LAB examined. In another study, Coconnier et al (122) reported that conditioned media from L. acidophilus reduced the viability of H. pylori in vitro, independent of lactic acid concentrations. In addition, the adhesion of H. pylori to human mucosecreting HT-29 cells decreased. Several in vitro studies were conducted to ascertain whether the effects of LAB on H. pylori survival and function are due to lactic acid or to other antibacterial products generated by LAB, such as bacteriocins. Of the several bacteriocins tested, lacticins produced by Lactoc. lactis were shown to have the greatest anti-Helicobacter activity when used against several strains of H. pylori (123).
Studies that indicate promising inhibitory effects of LAB on H. pylori survival and function in vitro were extended to in vivo studies using human patients. Armuzzi et al (124) reported that, when 120 asymptomatic subjects who were positive for H. pylori infection received an L. casei strain GG supplement over a 14-d period in addition to a standard 1-wk antibiotic therapy regimen, the eradication of H. pylori was faster than that in control subjects.
Although promising results have been reported, the effects of LAB on H. pylori infection in humans remain ambiguous. For example, L. acidophilus and L. gasseri were both shown to decrease H. pylori infection, as indicated by reduced [13C] urea breath test values (125, 126), and therapy with L. acidophilus was shown to reduce gastric mucosal inflammation (125). However, gastric biopsies did not show eradication of H. pylori. Similarly, Cats et al (127) reported that viable L. casei was required to inhibit the growth of H. pylori in vitro, but only a slight nonsignificant trend was observed toward an in vivo suppressive effect of an L. casei-supplemented milk drink.
Allergic reactions
The effects of yogurt and LAB on allergic reactions in the gastrointestinal tract have received some interest (128, 129). It was reported that a delay in the development of Bifidobacterium and Lactobacillus in the gastrointestinal microflora is a general finding in children with allergic reactions (128). Isolauri (130) reported data suggesting that Lactobacillus GG can be used to prevent food allergies.
Heat treatment was suggested as a way of reducing the ability of milk proteins to cause allergic reactions, which would make milk a more suitable source of protein for persons with an immunologic sensitization to cow milk protein (131). However, Kirjavainen et al (129) used a randomized double-blind design to investigate in a recent study the effects of heat-inactivated and viable L. rhamnosus GG on infants with atopic eczema and cow milk allergy. Milk formula supplemented with viable but not heat-inactivated L. rhamnosus GG significantly improved atopic eczema and subjective symptoms of cow milk allergy in subjects in comparison with the control group. These results suggest that, in persons with cow milk allergy, the presence of viable LAB may provide benefits that outweigh the possible detrimental effects that undenatured milk proteins may have on milk allergy. Furthermore, the immunologic response to native milk proteins may differ from that to heat-denatured milk proteins. A recent study using a rat model showed that heat-denaturated ß-lactoglobulin induced a local mucosal inflammatory response, whereas native ß-lactoglobulin induced an IgE-mediated systemic response (132). Heat denaturation is likely to result in conformational changes that expose or hide (or both) epitopes and lead to the activation of different subpopulations of immune cells and thus to different end results.
The mechanisms of the protective effects of LAB on allergic reactions are not known. A proinflammatory response in the gut mucosa that is induced by food allergens may impair the function of the intestinal barrier. It is possible that LAB may prevent allergic reactions by having a protective effect on the function of the intestinal barrier, although the mechanism of such an effect is poorly understood. A more direct link between the function of GALT and allergic responses is also possible. One of the primary mechanisms of active cellular suppression of proinflammatory events in the gut after antigen-specific triggering is the secretion of suppressive cytokines, such as transforming growth factor ß and IL-10. Transforming growth factor ß is produced by both CD4+ and CD8+ GALT-derived T cells and is an important mediator of the active suppression component of oral tolerance. Furthermore, IL-4-mediated isotype switching of immunoglobulin from IgM to IgE and IgE-dependent degranulation of mast cells has been shown to be involved in the pathogenesis of food allergy-related enteropathy (133).
Yogurt's LAB are known to enhance the production of IFN- (62, 134), which acts to inhibit isotype switching to IgE. IgE-mediated hypersensitivity reaction, also known as type 1 allergy, is triggered by the cross-linking of antigens with IgE antibodies that are bound to Fc receptors on mast cells. It was reported that L. casei inhibited antigen-induced IgE production by mouse splenocytes (135). In addition, production of the immunosuppressive cytokine IL-10 is induced by LAB (60).
A combination of enhancing and suppressive effects is the most likely mechanism by which LAB may have their effects. However, the ways in which LAB or other components of yogurt influence the production of these immunoregulatory cytokines in the gut remain to be elucidated, as do the possible mechanisms of LAB-mediated protection.
SAFETY
Although the safe use of nonsporing anaerobic LAB in fermented foods is widespread and has a long history, there have been occasional reports associating LAB with clinical infections (53, 136) because benign microorganisms have been shown to be infective when a patient is severely debilitated or immunosuppressed (137, 138). Some of the diseases that have been associated with LAB infection include septicemia, infective endocarditis, and dental caries.
Very rarely, cases of lactobacillemia have been reported in patients with severe underlying illness, many of whom received a prior antibiotic therapy that may have selected-out for the organism (139, 140). Moreover, Husni et al (141) reviewed the cases of 45 patients with clinically significant lactobacillemia and reported that 11 of the patients were receiving immunosuppressive therapy and 23 had received antibiotics. In none of these reports was a definitive link made between the consumption of fermented milk products and infection.
In addition, rare cases of endocarditis have been associated with L. rhamnosus, a LAB indigenous to the human gastrointestinal tract (142-144). However, as with lactobacillemia, no reports to date have been able to identify a connection between LAB from fermented milk and infection in humans. In most of these cases, the origin of the Lactobacillus is most likely the host. There is also a hypothetical risk of the transfer of antimicrobial resistance from LAB to other microorganisms with which LAB might come in contact, but this has not yet been described in the literature.
In the past, Lactobacilli isolated from infections were habitually dismissed as contaminants or secondary invaders. However, recent evidence suggests that they might function as opportunistic pathogens in a small number of severely immunosuppressed persons. Even in these patients, this is a very rare event, and it has not yet been reported in a large group of immunosuppressed persons, such as the elderly or persons with AIDS. LAB have a long history of safe use in foods and also in products that have been tested in clinical trials. However, as with any new food ingredient, the safety of a new strain of LAB must be clearly established before it is introduced into fermented dairy products.
CONCLUDING REMARKS AND RECOMMENDATIONS FOR FUTURE STUDIES
It has long been believed that the consumption of yogurt and other fermented milk products provides various health benefits. Recent studies of the possible health benefits of yogurt in gut-associated diseases substantiate some of these beliefs. Of particular interest are the reduction—by yogurt, yogurt bacteria, or both—in the duration of diarrheal diseases in children, the preventive or therapeutic (or both) effects on IBD and colon cancer as suggested by epidemiologic evidence and animal studies, and the possible beneficial effects in increasing the eradication rate of H. pylori as indicated by in vitro and preliminary human studies. In addition, there is ever-increasing evidence of the beneficial effect of yogurt containing live and active cultures on the digestion of lactose in persons with lactose intolerance.
These findings are interesting and should encourage future studies to 1) substantiate or extend these findings by using animal models and clinical trials; 2) ascertain whether these effects are age-specific or can be observed across all age groups: eg, ascertain whether yogurt would have effects similar to those observed in children on attenuation of the incidence or duration of diarrheal diseases in elderly people, a group that has high morbidity and mortality from these infections; and 3) investigate the mechanisms through which yogurt exerts its effects and ascertain the critical components of yogurt involved in its mechanisms of action. Finally, in recent years, yogurt has been touted as improving "gut health." In the absence of a universally accepted definition or any definition of "gut health," it is difficult to substantiate these claims. Studies focused on determining the characteristics of a healthy gut would be extremely helpful in evaluating the effect of yogurt on gut health.
ACKNOWLEDGMENTS
All 3 authors participated in the literature review and the development of the manuscript outline, and SNM and RMR determined the areas to be discussed. OA conducted the literature search and organized and wrote the manuscript. SNM provided corrections. RMR revised the manuscript.
This review was prepared in response to a request from the National Yogurt Association for a critical and objective review, for which the authors received an honorarium.
REFERENCES