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1 Private-practice nutrition consultant, Kelowna, Canada (BCD), and from the Department of Nutritional Sciences, Pennsylvania State University, University Park (PMK-E).
2 Presented at the Fourth International Congress on Vegetarian Nutrition, held in Loma Linda, CA, April 8–11, 2002. Published proceedings edited by Joan Sabaté and Sujatha Rajaram, Loma Linda University, Loma Linda, CA. 3 Address reprint requests to PM Kris-Etherton, Department of Nutritional Sciences, Pennsylvania State University, University Park, PA 16802. E-mail: pmk3{at}psu.edu.
ABSTRACT
Although vegetarian diets are generally lower in total fat, saturated fat, and cholesterol than are nonvegetarian diets, they provide comparable levels of essential fatty acids. Vegetarian, especially vegan, diets are relatively low in -linolenic acid (ALA) compared with linoleic acid (LA) and provide little, if any, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Clinical studies suggest that tissue levels of long-chain n-3 fatty acids are depressed in vegetarians, particularly in vegans. n-3 Fatty acids have numerous physiologic benefits, including potent cardioprotective effects. These effects have been demonstrated for ALA as well as EPA and DHA, although the response is generally less for ALA than for EPA and DHA. Conversion of ALA by the body to the more active longer-chain metabolites is inefficient: < 5–10% for EPA and 2–5% for DHA. Thus, total n-3 requirements may be higher for vegetarians than for nonvegetarians, as vegetarians must rely on conversion of ALA to EPA and DHA. Because of the beneficial effects of n-3 fatty acids, it is recommended that vegetarians make dietary changes to optimize n-3 fatty acid status.
Key Words: Essential fatty acids (EFAs) • vegetarians • vegans • eicosapentaenoic acid (EPA) • docosahexaenoic acid (DHA) • -linolenic acid (ALA) • cardiovascular disease
INTRODUCTION
It is widely recognized that overall morbidity and mortality are lower in vegetarians compared with omnivores (1). The dietary patterns of vegetarians as well as their healthful lifestyle practices are thought to at least partly explain these differences. One notable difference relates to the type and amount of fat in the diet. Vegetarian diets are slightly lower in total fat than omnivorous diets (28–32% for vegans, 30–34% for lactoovovegetarians, and 34–36% for omnivores) (2). However, vegetarians eat about one-third less saturated fat (vegans about one half) and about one-half as much cholesterol (vegans consume none) as omnivores (3–5). Intake of trans fatty acids is highly dependent on the amount of processed foods in the diet. Limited research comparing trans fatty acid intake of omnivores and vegetarians suggests that vegetarians consume slightly smaller amounts, with more pronounced differences for vegans eating whole-foods diets (3). One study found that vegetarians had a markedly lower serum trans fatty acid level than nonvegetarians (0.03% compared with 0.50% of total fatty acids), a finding that presumably reflects differences in intake (6).
With respect to essential fatty acid (EFA) intake and balance, vegetarian diets appear to offer no advantages over omnivorous dietary patterns. Some have suggested that vegetarians could be at a significant disadvantage, as consumption of -linolenic acid (ALA) is low relative to linoleic acid (LA), resulting in limited conversion of ALA to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) (6). In addition, vegetarians consume very little EPA and DHA (7).
The purpose of this paper is to review the EFA intake and status of vegetarians; compare the biological effects of ALA and its long-chain metabolites, EPA and DHA; and explore the practical implications of this knowledge as it relates to optimal EFA status in this population. In addition, the scientific justification for ensuring adequate EFA status with respect to cardiovascular disease (CVD) will be reviewed.
OPTIMIZING ESSENTIAL FATTY ACID STATUS
Essential fatty acid conversion
There are 2 EFAs, both polyunsaturated fats: LA (the parent n-6 fatty acid) and ALA (the parent n-3 fatty acid). Humans are able to convert LA and ALA to more physiologically active fatty acids through a series of elongation and desaturation reactions [LA to arachidonic acid (AA) and ALA to EPA and DHA]. The resulting highly unsaturated fatty acids are necessary for cell membrane function, the proper development and functioning of the brain and nervous system, and the production of eicosanoids (thromboxanes, leukotrienes, prostaglandins, and prostacyclins). While conversion of LA to AA is typically very efficient, conversion of ALA to EPA and DHA is much less so. In healthy individuals, 5–10% of ALA is converted to EPA, and 2–5% to DHA (8–10). In a recent study conducted with women, estimated net fractional conversion of ALA to EPA was 21% and 9% for DHA (11). The long-chain members of the n-6 and n-3 families generally are not considered essential because humans have the capacity to make them from "parent" fatty acids. n-6 And n-3 fatty acids compete for enzymes responsible for their conversion; because they are not interchangeable, humans must consume both. This takes on special importance in people with diabetes or other metabolic disorders where conversion enzymes may be compromised, as well as in those who have inherited a limited ability to produce these enzymes (12).
Recommended essential fatty acid intake
Recently, the National Academies released the Dietary Reference Intakes Report for Energy and Macronutrients (13). Adequate intakes (AIs) have been set for LA and ALA.
The AI for LA is 17 and 12 g/d for men and women aged 19–50 y, respectively. The AI for ALA is 1.6 and 1.1 g/d for men and women aged 19 to > 70 y, respectively. In 1999, the National Institutes of Health (NIH) sponsored an international workshop on the essentiality and recommended dietary intakes for n-6 and n-3 fatty acids. The NIH Working Group proposed AIs of 2–3% of total calories for LA, 1% of total calories for ALA, and 0.3% of total calories for EPA and DHA. The Working Group further recommended intakes of EPA and DHA of =" BORDER="0"> 650 mg/d and a minimum of 300 mg DHA/d during pregnancy and lactation (14). Health Canada suggests a minimum of 3% of energy from n-6 fatty acids and 0.5% from n-3 fatty acids or 1% for infants who do not receive a preformed source of EPA and DHA (15). The United Kingdom recommends that 1% of energy be from ALA and 0.5% from EPA and DHA combined (16). While there are no official recommendations for vegetarians and vegans, it is not possible for this population to achieve the NIH Working Group’s proposed AIs for EPA and DHA. Even with the use of DHA-enriched eggs, some seaweed, and/or DHA supplements, the best vegetarians could do is to meet the recommended intakes for DHA. Some experts suggest that vegetarians (and others receiving no direct sources of EPA and DHA) at least double the recommended intakes for ALA (S Innis, A Simopoulos, and B Holub, personal communication, 2002). This would suggest an intake of ALA in the range of 1–2%.
The ratio of n-6 to n-3 fatty acids is often used to assess the balance between EFA in the diet, although there is some controversy as to its practical significance. For vegetarians and others who consume little, if any, EPA and DHA, the n-6-to-n-3 ratio is of greater relevance than for individuals who consume significant daily sources of EPA and DHA. A number of recommendations have been made on the basis of the ratio of n-6 to n-3 fatty acids. The World Health Organization/Food and Agriculture Organization suggests a ratio of 5:1–10:1 (17), Sweden recommends a ratio of 5:1 (18), Canada recommends 4:1–10:1 (15), and Japan recently changed its recommendation from 4:1 to 2:1 (19). On the basis of the proposed AIs, the NIH suggests a ratio of 2:1–3:1 (14). One study found that a ratio of 4:1 allows for adequate conversion to DHA in healthy vegetarians (20). Another research group suggested that the optimal ratio to maximize the conversion of ALA to DHA is 2.3:1 (21). Given the rate of conversion of ALA to EPA and DHA, it has been suggested that a safe and adequate ratio for the vegetarian and vegan populations would be in the range of 2:1–4:1 (22). This can best be achieved by increasing ALA in the diet and decreasing LA, if indicated (see below).
Essential fatty acid intake and status of vegetarians
Total n-3 fatty acid intakes are similar for vegans, vegetarians, and omnivores (< 1–3 g/d, with the current average being 1.1–1.6 g/d) (2, 18). However, the intakes of very-long-chain n-3 fatty acids (EPA and DHA) vary appreciably. Vegans consume negligible amounts of EPA and DHA, while vegetarians consume minimal EPA (< 5 mg/d) and varying amounts of DHA depending on egg consumption (a source of DHA, averaging < 33 mg/d) (7). Consumption of EPA and DHA in omnivores varies according to fish and egg intake, with average intakes in the 100–150-mg/d range. n-6 Intakes are significantly higher in vegan and vegetarian populations than in omnivorous populations, ranging from a low of about 5–7% of calories in omnivores to a high of about 10–12% of calories in vegans. As a result, the n-6-to-n-3 ratio is generally considered to be elevated in vegans (< 14:1–20:1) and lactoovovegetarians (< 10:1–16:1) compared with omnivores (< 10:1).
Because the consumption of long-chain n-3 fatty acids is low in vegetarian populations and LA intakes are relatively high, one would expect decreased levels of EPA and DHA in body stores. Reports to date suggest that this is indeed the case but that it is significantly more pronounced in vegans than in vegetarians. In 1978 Sanders et al (23) first noted that the plasma and total erythrocyte lipid levels of vegans were significantly decreased. EPA levels were only 12–15% and DHA levels were 32–35% of those of nonvegetarians (23). Another study found that vegan plasma EPA levels were only 22% of those of omnivores and DHA levels were 38% of those of omnivores, although AA levels were similar (24). In 1994, Reddy et al (25) again demonstrated that vegetarians (who appeared to be vegan, based on zero intake of EPA and DHA) had reduced EPA in plasma phospholipids (37% of nonvegetarians), and DHA (52% of nonvegetarians). Krajcovicova et al (26) compared the fatty acid status of vegans, lactoovovegetarians, semivegetarians, and omnivores between the ages of 11 and 15 y. While the levels of EPA and DHA were similar in vegetarians and nonvegetarians, they were significantly lower in vegans (62–65% of nonvegetarians’ values). Interestingly, while long-chain n-3 fatty acids were reduced in vegan children, they were not as severely decreased as reported in vegan adults. The higher EPA and DHA levels in vegan children may be the result of reserves from extended breast-feeding, or from better conversion in this age group (26).
Ågren et al (27) compared the serum lipid levels of vegans eating uncooked food with those of nonvegetarians. The proportions of EPA and DHA in vegans were only 29–36% and 49–52% those of nonvegetarian controls, respectively, while the levels of AA were similar, indicating no difficulty with the n-6 conversion (27). Sanders and Reddy (28) compared the EFA content of vegan, vegetarian, and nonvegetarian human milk. Milk from vegan mothers had over double the LA and ALA of the nonvegetarian mothers’ milk but less than half of the DHA. The EFA status of the infants (as determined by erythrocyte lipids) reflected the levels in the milk they received. Vegan infants had < 30% of the EPA and DHA of omnivorous infants (28). Melchert et al (29) examined the fatty acid levels in the serum of vegetarians and nonvegetarians and found that DHA levels were < 40% lower in vegetarians than nonvegetarians. Two other groups comparing EPA and DHA status of vegetarians and nonvegetarians found little difference in their levels (30, 31). Conquer and Holub (31) suggest that it is possible that Canadian vegetarians have elevated ALA intakes because of the high consumption of canola oil in their diets and, thus, better DHA status. These studies did not assess EFA status in vegans.
Implications of reduced essential fatty acid status
Highly unsaturated fatty acids are essential for health (32). AA, dihommo--linolenic acid (DGLA), and EPA all serve as precursors for eicosanoids. Eicosanoids formed from AA are very potent, increasing blood pressure, inflammation, platelet aggregation, thrombosis, vasospasm, and cell proliferation, while those formed from EPA and DGLA are less potent in these responses. While the eicosanoids formed from AA are important, their overproduction relative to eicosanoids formed from EPA has been associated with elevated risk for numerous disease states, including heart disease, cancer, diabetes, osteoporosis, and numerous immune/inflammatory disorders.
Docosahexaenoic acid is not a precursor for eicosanoids but is an important structural component of the gray matter of the brain, the retina of the eye, and specific cell membranes and is found in high levels in the testes and sperm. Low levels of DHA have been associated with several neurological and behavioral disorders, such as depression, schizophrenia, Alzheimer’s disease, and attention deficit hyperactivity disorder (7). In addition, low levels of DHA are linked to suboptimal visual acuity and reduced brain development in infants (33). Thus, while these long-chain fatty acids are not technically "essential" nutrients, it is important to ensure that there are sufficient levels by relying on conversion from "parent" fatty acids or by consuming them directly.
Scientific evidence for increasing n-3 fatty acids in vegetarian diets
There is a rapidly expanding database that demonstrates remarkable effects of n-3 fatty acids on primary and secondary prevention of CVD. Both epidemiologic and randomized controlled clinical studies have evaluated the effects of marine and plant sources of n-3 fatty acids from food sources as well as supplements. These studies have established that both marine- and plant-derived n-3 fatty acids have cardioprotective effects. These findings have prompted great interest in unraveling the biological mechanisms that explain these reported benefits. It is apparent that multiple mechanisms of action mediate the effects of n-3 fatty acids. Collectively, advances from epidemiologic, clinical, and biochemistry/molecular biology research will be useful in refining dietary guidance to further decrease the risk of CVD.
Epidemiologic studies
-Linolenic acid.
Recent epidemiologic studies have shown that dietary ALA is associated with a lower risk of CVD in men and women. In the EURAMIC Study, the relative risk for myocardial infarction (MI) was 0.42 (P for trend = 0.02) in individuals with the highest quintile of adipose tissue ALA, a long-term measure of ALA intake (34). However, after other CVD risk factors were adjusted for, this association was nonsignificant. In a 10-y follow-up of the Nurses’ Health Study, Hu et al (35) reported a dose-response relationship between ALA intake and relative risk of fatal ischemic heart disease. Risk was reduced by 45% in the highest quintile of ALA intake (P for trend = 0.01). However, ALA was not related to nonfatal myocardial infarction. In the Health Professionals Follow-up Study, conducted with men, a 1% increase in ALA was associated with a 0.41 relative risk for acute MI (P for trend = 0.01); however, there was no association with fatal coronary artery disease (CAD) (36). The highest quintile of ALA intake was 1.4–1.5 g/d. In a recent cross-sectional study (the National Heart, Lung, and Blood Institute Family Heart Study) with 4584 men and women, ALA was inversely related to coronary artery disease (37). ALA intake in the top 3 quintiles was 0.65, 0.76, and 0.96 g/d, respectively; the prevalence odds ratio of coronary artery disease in these 3 quintiles was reduced < 40% for men and 50–70% for women.
In contrast to epidemiologic studies that have demonstrated a beneficial association between ALA intake and CVD, some studies have found the contrary. In the Zutphen Elderly Study, a prospective epidemiologic study with 667 men aged 64–84 y, there was no beneficial effect of ALA intake on incidence of 10-y coronary artery disease (38). The results of this study, however, were complicated by an association between ALA and trans fatty acid intake (37) as well as limitations in the collection of the dietary data (estimated on the basis of only food tables and dietary recollection) (39). ALA intake still had no effect on coronary artery disease risk even when foods that did not contain trans fatty acids were evaluated.
On the basis of available epidemiologic evidence from within population studies, ALA has been shown to have a protective effect on coronary disease. These findings provide strong support for conducting well-controlled intervention trials to assess the cause-and-effect relationship between ALA and incidence of CVD.
Marine-derived n-3 fatty acids.
There has been controversy in the literature about whether fish consumption decreases coronary artery disease CAD mortality. To resolve this controversy, Marckmann and Gronbaek (40) systematically reviewed studies of fish or n-3 polyunsaturated fatty acid intake and CAD death (11 studies with 116 764 subjects). This analysis found that individuals at high risk for CAD appeared to benefit in a dose-responsive manner from increased fish consumption. The optimum dose for fatal CAD prevention in high-risk populations is 40–60 g/d. In contrast, increased fish consumption does not appear to protect against CAD in individuals at low risk of CAD and with healthy lifestyles.
There have been various explanations for the conflicting data from epidemiologic studies of fish consumption and CAD risk. These explanations reflect many differences in the experimental designs employed, including definition of sudden death (41), endpoint, experimental design, fish intake estimation method, and study population (42). Another consideration relates to the type of fish consumed; fatty fish but not lean fish consumption was associated with a lower CAD mortality (43). In addition, the possible adverse effects of the methylmercury content of fish may attenuate the health benefits of n-3 fatty acids (44).
The available evidence supports a conclusion that fish consumption is protective against coronary disease. Because of this, the American Heart Association Dietary Guidelines recommend 2 servings of fish per week, preferably fatty fish, to reduce the risk of coronary disease (45).
Randomized controlled clinical trials
The effects of n-3 fatty acids have been evaluated in clinical trials with subjects either with existing CVD or at high risk for CVD using both marine-derived EPA and DHA and plant-derived ALA. Collectively, these studies have shown a beneficial effect of both sources of n-3 fatty acids.
The Diet and Reinfarction Trial (DART) reported a 29% reduction in all-cause mortality after 2 y in male MI survivors who were counseled to increase their intake of oily fish (200–400 g fatty fish/wk, which provided an additional 500–800 mg n-3 fatty acids/d) (46). A fish oil supplement (900 mg EPA and DHA/d) was given to patients who would not eat fish, and a subsequent analysis demonstrated that n-3 fatty acids accounted for the protective effect (47). Singh et al (48) studied patients with suspected acute MI who were randomized to either fish oil capsules (containing 1.8 g EPA and DHA/d), mustard oil (20 g/d providing 2.9 g ALA), or placebo. Total cardiac events were lower in the fish oil and mustard oil groups (25% and 28%, respectively) compared with the placebo group (ie, 35%; P < 0.01). In the Gruppo Italiano per lo Studio della Sopravvivenza nell’Infarto Miocardico-Prevenzione (GISSI-Prevention Study) (49), 11 324 patients with preexisting CAD who were receiving conventional cardiac pharmacotherapy were randomly assigned to receive 850 mg n-3 fatty acid ethyl esters (as EPA and DHA), 300 mg vitamin E, both, or neither. The n-3 fatty acid intervention group experienced a 20% reduction in all-cause mortality (P = 0.01) and a 45% reduction in sudden death (P < 0.001) compared with the control group. No additional benefit was conferred by vitamin E.
In contrast, a recent study did not find an effect of 3.5 g DHA and EPA/d compared with corn oil on cardiac events in post-MI patients (n = 300) after 1.5 y of intervention (50). The lack of response to marine-derived n-3 fatty acids may have been due to the high habitual fish intake in western Norway, beyond which supplemental n-3 fatty acids would not be of benefit.
The effects of ALA on coronary events have been evaluated in 4 studies. Three studies reported beneficial effects of ALA, and one did not. The Indian Experiment of Infarct Survival (48) (see above) reported a significant decrease in total cardiac events in the mustard seed oil (a source of ALA) group that was quite comparable to that of the EPA and DHA treatment group. The Lyon Diet Heart Study, a seminal study, was designed to evaluate the effects of a blood cholesterol–lowering, Mediterranean-type diet (including increased amounts of ALA: 1.5 g/d compared with 0.5 g/d for the control group) on recurrence rates of cardiac events compared with rates for a prudent Western diet (51, 52). Subjects in the experimental group were instructed to adopt a Mediterranean-type diet that contained more bread, more root and green vegetables, more fish, fruit at least once daily, less red meat (replaced with poultry), and less butter and cream (replaced with margarine high in ALA supplied by the study). This diet provided 30% of calories from fat, 8% from saturated fat, 13% from monounsaturated fat, 4.6% from polyunsaturated fat (0.84% ALA), and 203 mg cholesterol/d. Despite a similar coronary risk factor profile (plasma lipids and lipoproteins, systolic and diastolic blood pressure, body mass index, and smoking status), subjects following the Mediterranean-style diet had a 50–70% lower risk of recurrent heart disease. The Mediterranean -Linolenic Enriched Groningen Dietary Intervention (MARGARIN) study reported a trend toward fewer CVD events in subjects (n = 124 men and 158 women) with multiple CVD risk factors who used a margarine high in ALA versus LA (1.8% compared with 5.7%; P = 0.20) (53). However, the 10-y estimated ischemic heart disease risk decreased similarly in both groups (2.1% and 2.5%, respectively).
The Norwegian Vegetable Oil Experiment (54) did not report a beneficial effect of ALA. In this study, over 13 000 men aged 50–59 y with no history of MI were randomly assigned to consume 5.5 g ALA/d (from 10 mL linseed oil) or 10 mL sunflower seed oil for 1 y. In each treatment group there were 27 cases of new CAD or sudden death, and 40 compared with 43 deaths from any cause in the control and linseed oil groups, respectively.
In summary, evidence from randomized controlled trials shows a beneficial effect of dietary and supplemental n-3 fatty acids, including both EPA/DHA and ALA, on CAD (55). Thus, there is scientific evidence that adequate intake of n-3 fatty acids will elicit cardioprotective effects. From this, it follows that n-3 fatty acids are an integral component of a diet designed to maximally reduce risk of CVD.
Achieving optimal essential fatty acid status in vegetarians
There are 2 important steps vegetarians can take to improve their EFA status. 1) Maximize the conversion of ALA to EPA and DHA. 2) Provide a direct source of EPA and DHA.
For those with increased needs for EPA and DHA (eg, pregnant and lactating women) or at greater risk for poor conversion (persons with diabetes, those with neurological disorders, premature infants, the elderly), it may be prudent to ensure that there is a direct source of EPA and DHA. While it is not common, it is possible to overconsume n-3 fatty acids. If a person minimizes n-6 fatty acids and uses large amounts of n-3 fatty acids [ie, > 2 tbsp (28 g) flax oil/d], resulting in an n-6-to-n-3 ratio of < 1:1, insufficient LA conversion to AA can occur. Elongase and desaturase enzymes preferentially convert n-3 fatty acids, when compared with n-6 fatty acids. A balance of 2:1–4:1 (n-6:n-3) appears optimal for vegetarians and others who do not receive preformed EPA and DHA.
Maximizing conversion of n-3 fatty acids
While conversion of EFAs to longer-chain fatty acids is, at least in part, dependent on genetics, age, and overall health, several dietary factors also have a significant impact on the conversion process. First, it is important to ensure that the diet is nutritionally adequate, as poorly designed diets can impair the conversion process. Insufficient energy or protein decreases the activity of conversion enzymes, as can deficiencies of pyridoxine, biotin, calcium, copper, magnesium, and zinc (56, 57). Excessive intakes of trans fatty acids can also depress conversion enzymes. In addition, alcohol inhibits the activity of -5 and -6 desaturase and depletes tissues of long-chain n-3 fatty acids (58). High n-6 fatty acid content can have a profound effect on n-3 fatty acid conversion, reducing it as much as 40% (59).
In the context of current guidelines, total polyunsaturated fatty acid intake should be < 7% of calories, with 10% being the maximum. Vegetarians currently consume about 8–12% of their calories from polyunsaturated fatty acid. Given a total polyunsaturated fatty acid intake of 7–10% of calories, to achieve an n-6-to-n-3 ratio of 4:1, < 1.5–2% of calories should be obtained from n-3 fatty acids and, as a result, 5.5–8% of calories from n-6 fatty acids. Alternatively, LA can be decreased; however, given the recognized health benefits of LA, a marked reduction is not recommended. To achieve reduced n-6 intakes, oils rich in n-6 fatty acids should not be used as primary cooking oils. Cooking oils with the greatest n-6 fatty acid content include safflower oil (75% n-6), grapeseed oil (70% n-6), sunflower oil (65% n-6), corn oil (57% n-6), cottonseed oil (52% n-6), and soybean oil (51% n-6). Processed foods, convenience foods, and snack foods also are significant contributors to n-6 intake; thus, their use should be moderate. n-6-Rich whole foods such as sunflower seeds, pumpkin seeds, sesame seeds, walnuts, wheat germ, and soy foods need not be avoided, as they tend to be relatively minor contributors to overall n-6 intake. These foods also provide a myriad of beneficial dietary components, including phytochemicals, fiber, B vitamins, vitamin E, and trace minerals.
The primary fat in the diet should come from foods and oils rich in monounsaturated fat. When monounsaturated fats predominate, saturated fats, trans fatty acids, and n-6 fatty acids are kept in check and the ratio of n-6 to n-3 fatty acids improves. Monounsaturated fats are high in nuts (except for walnuts and butternuts), peanuts (a legume), olive oil, olives, avocados, canola oil, high-oleic sunflower oil, and high-oleic safflower oil. Whole foods rather than oils are better sources of monounsaturated fats because they contribute many other nutrients to the diet.
Finally, it is important to ensure that there are sufficient amounts of ALA, which is necessary for the production of EPA and DHA. Most healthy vegetarians would be well advised to double their intake of ALA, providing =" BORDER="0"> 1% of energy from n-3 fatty acids or 1.1 g/1000 cal. For those with increased needs or decreased capacity to convert, an intake of 2% of energy or 2.2 g/1000 cal may be necessary. The primary sources of ALA are selected seeds, nuts, and legumes (flaxseed, hempseed, canola, walnuts, and soy) and the green leaves of plants, including phytoplankton and algae. Table 1 provides a list of common ALA-rich plant foods, the percentage of fat as ALA, the n-6-to-n-3 ratio, and the total grams of ALA per serving.
View this table:
TABLE 1 . -Linolenic acid (ALA) and linoleic acid (LA) content of selected plant foods1
Providing a direct source of eicosapentaenoic and docosahexaenoic acids
The primary sources of EPA and DHA are fish and seafood. Thus, for vegetarians, increasing consumption of these long-chain n-3 fatty acids can be a challenge. For lactoovovegetarians, eggs provide a reasonable amount of DHA (< 50 mg/egg) but very little EPA. Most supermarkets also sell DHA-rich eggs, providing 2–3 times the DHA of conventional eggs. Eggs from chickens fed flax generally provide 60–100 mg DHA/egg, while those from chickens fed microalgae contain 100–150 mg DHA/egg.
The only plant sources of long-chain n-3 fatty acids are plants of the sea—microalgae and seaweed. There is a great deal of confusion about the EFA content of plants. While they are the original sources of EPA and DHA (fish do not produce long-chain n-3 fatty acids), most are not concentrated sources because of their extremely low total fat content. An important exception is a DHA-rich microalgae that provides 10–40% DHA by dry weight and is currently available in supplement form. When supplementing with a direct DHA source, 100–300 mg/d is recommended. Blue-green algae (spirulina and Aphanizomenon flos-aquae) are low in long-chain n-3 fatty acids. Spirulina is rich in -linolenic acid (GLA, n-6), while A. flos-aquae is more concentrated in ALA. Though blue-green algae is not a significant source of EPA or DHA, some research indicates that it has a very high conversion rate in comparison to other plants (R Kushak et al, unpublished observations, 1999). Macroalgae, otherwise known as seaweed, is even lower in fat than most vegetables (< 1–14% of calories from fat), although it does contain small amounts of long-chain fatty acids. A 100-g serving provides, on average, 100 mg EPA but little DHA. Seaweeds do not contribute significantly to EPA intakes in the Western world but are important sources where people use large quantities of seaweed on a daily basis (eg, Japan and other parts of Asia). Thus, while vegetarians can rely on eggs and/or microalgae supplements for DHA, most consume little if any EPA. However, < 10–11% of DHA is retroconverted to EPA; thus, if sufficient ALA and DHA are consumed, total EPA production would be expected to be adequate (61).
Practical guidelines
Practical guidelines for achieving optimal EFA intake in vegetarians are as follows. 1) Make a wide variety of whole plant foods the foundation of the diet. 2) Get most fat from whole foods—nuts, seeds, olives, avocados, and soy foods. 3) If using concentrated fats and oils, select those rich in monounsaturated fats, such as olive, canola, or nut oils. n-3-Rich oils can also be used but should not be heated. Moderate use of n-6-rich oils is recommended. 4) Limit intake of processed foods and deep-fried foods rich in trans and n-6 fatty acids. 5) Reduce intake of foods rich in saturated fat and cholesterol. 6) Include foods rich in n-3 fatty acids in the daily diet. Aim for 2–4 g ALA/d. 7) Consider using a direct source of DHA. Aim for 100–300 mg/d.
ACKNOWLEDGMENTS
The authors had no conflict of interest.
REFERENCES