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1 From Agriculture and Agri-Food Canada (AAFC), Lethbridge, Canada (PSM, TAM, and ZM); AAFC, Brandon, Canada (SS); AAFC, Lacombe, Canada (JA, VB, and DM); AAFC, Nappan, Canada (EC); Alberta Agriculture, Food and Rural Development (AAFRD), Edmonton, Canada (LG); AAFRD, Lacombe, Canada (JB); the University of Alberta, Edmonton, Canada (EO); and the University of Lethbridge, Lethbridge, Canada (RJW)
2 Presented at the workshop "The Role of Conjugated Linoleic Acid in Human Health," held in Winnipeg, Canada, March 13–15, 2003. 3 Supported by grants from the Beef Industry Development Fund, Pioneer Canada, the Round-Up 80 Ranch, the Alberta Livestock Industry Development Fund, the Alberta Cattle Commission, the Agriculture and Agri-Food Canada–Matching Investments Initiative, and Washington State University. 4 Address reprint requests to PS Mir, 5403 1st Avenue S, PO Box 3000, Lethbridge, AB, Canada T1J 4B1. E-mail: mirp{at}agr.gc.ca.
ABSTRACT
Canadian beef consumption is 31 kg per annum, or a third of all meats consumed. Beef is a nutrient-rich food, providing good quality protein, vitamins B-6 and B-12, niacin, iron, and zinc. However, animal fats have gained the reputation of being less healthy. The identification of the anticarcinogenic effects of beef extracts due to the presence of conjugated linoleic acid (CLA) has heightened interest in increasing the amount of CLA deposited in beef. Beef cattle produce CLA and deposit these compounds in the meat; thus, beef consumers can receive bioformed CLA. Beef contains both of the bioactive CLA isomers, namely, cis-9, trans-11 and trans-10, cis-12. The relative content of these CLA isomers in beef depends on the feeds consumed by the animals during production. Feeding cattle linoleic acid–rich oils for extended periods of time increases the CLA content of beef. Depending on the type and relative maturity of the pasture, beef from pasture-fed cattle may have a higher CLA content than beef from grain- or silage-fed cattle. In feedlot animals fed high-grain diets, inclusion of dietary oil along with hay during both the growth and finishing phases led to an increase in CLA content from 2.8 to 14 mg/g beef fat, which would provide 77 mg CLA in an 85-g serving of beef. The CLAs appear to be concentrated in intramuscular and subcutaneous fat of beef cattle, with the CLA trans-10, cis-12 isomer being greater in the subcutaneous fat.
Key Words: Conjugated linoleic acid • beef • diets • linoleic acid • meat quality
INTRODUCTION
The formation of conjugated dienes in the rumen during biohydrogenation of lipids in feed was observed previously (1); however, the anticarcinogenic effect of beef extracts was first demonstrated by Pariza et al (2, 3). It was later identified that the anticarcinogenic effect was due to the presence of conjugated linoleic acid (CLA) in the beef extracts. It is now known that many such conjugated dienes are formed in the rumen, depending on the composition of the lipid in the feed. These bioconverted fatty acids are deposited in the tissues and can be available to consumers in ruminant products such as milk and beef. These CLAs appear to have a wide range of beneficial effects in animal models and were suggested as being potentially capable of providing health benefits for various metabolic disorders in humans. These health benefits include anticarcinogenesis in mice (4), enhancement of immunity (5), alleviation of allergies and asthma (6), decreased blood cholesterol in hamsters (7), antiatherosclerotic effects in rabbits (8), decreased obesity in mice (9), and enhanced insulin sensitivity in Zucker obese rats (10). These effects appear to be mediated by 2 isomers of CLA, and the 2 biologically active isomers are the CLA cis-9, trans-11 and the CLA trans-10, cis-12.
CONJUGATED LINOLEIC ACID IN MILK
Research indicated that the CLA cis-9, trans-11 is found in milk and beef to a greater extent than the CLA trans-10, cis-12 (11, 12). The CLA cis-9, trans-11 is more abundant because there appear to be 2 routes of formation of this fatty acid: one route a ruminal route, and a second route, in which the precursor fatty acid trans-vaccenic acid (18:1 trans-11) is converted to CLA cis-9, trans-11 by oxidation (removal of 2 hydrogens) at the ninth carbon of the fatty acid by the enzyme 9-desaturase, present in the mammary gland and perhaps muscle. As a result, under appropriate feeding management the yield of CLA from milk can be as high as 16 g/d, because concentration of CLA can be elevated to 20 mg/g fat in milk (12).
CONJUGATED LINOLEIC ACID IN BEEF
Although very small amounts of CLA trans-10, cis-12 occur in beef, there is a greater opportunity for this compound to be found in beef than in milk because it is a rumen product (13) and appears to be formed in cattle fed greater proportions of grain with soybean oil (14) or high-oil corn (15). A linear increase in CLA trans-10, cis-12 was observed with the increase of dietary soybean oil (14). However, the amount of the CLAs found in milk and meat are small, relative to the recommended daily intake for appreciation of health benefits in humans, which is 3500 mg/d (16). Moloney et al (17) summarized the amount of CLA observed by different researchers in beef, and the values range from 1.2 to 12.5 mg/g fat (Table 1). Some other observed values have been added to the list provided by Moloney et al (17). This variation in concentration of CLA depends on the system used for production of the beef and the diet used to finish (feed fed to animals to get them ready for slaughter) the animals. For example, factors that affect CLA content of beef include pasture-compared with feedlot-finished, nature of diet in feedlot, whether the diet contained oil or oilseed, the fatty acid composition of the oil, and the other dietary components in the feed, such as proportion of grain and silage compared with hay.
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TABLE 1. Conjugated linoleic acid (CLA) concentrations in uncooked beef
Rule et al (30) compared CLA concentrations in 3 different muscles of pasture- or feedlot-finished cattle and found that CLA concentrations of both isomers in fat extracted from muscle were greater in pasture-finished cattle in the longissimus and supraspinatus muscles but not in the semitendinosus muscle. The CLA concentrations differed among the muscles investigated. However, pasture-finished cattle generally had lower total fatty acid concentrations than feedlot-finished cattle; therefore, when CLA content of the beef was calculated, the differences were less pronounced between pasture- and feedlot-finished cattle. Thus, it is important to consider the net CLA yield to the consumer rather than merely the concentration on per unit weight of fat. Duynisveld et al (31) found that the CLA yield (mg/100 mg meat) from pasture-finished cattle was greater than from cattle fed preserved silage:grain (60:40)–based diets, which concurred with the observation of French et al (27), yet the provision of a supplement of crushed soybean for the pasture-finished cattle further increased the CLA and the total fat content of the meat.
The inclusion of sunflower oil in the diets (80% barley, 20% barley silage) of finishing cattle at 0%, 3%, or 6% increased the CLA content of the beef by 75% (Table 2) when cattle were fed 6% sunflower oil, but the yield per 100 g beef increased 100% from 10.5 to 19.5 mg (32). These animals were fed barley silage and barley grain–based diets, and Ivan et al (33) observed only small increases in muscle CLA content when lambs were fed similar barley grain and barley silage–based diets. In contrast, when weaned lambs were fed a 1:1 barley grain:alfalfa pellet with safflower oil at 6% of dry matter, the CLA deposited in diaphragm, leg muscle, adipose tissue, and liver increased 2- to 4-fold (34) (Table 3).
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TABLE 2. Effect of feeding sunflower oil during fattening phase on beef conjugated linoleic acid (CLA) content1
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TABLE 3. Effect of dietary supplementation with safflower oil on conjugated linoleic acid (CLA) content in various lamb tissues1
Although supplementation with oil or oilseed increases CLA content in muscle, the inclusion of linoleic acid–rich oil or oilseeds, such as safflower or sunflower, in the diet of ruminants appears to be most effective (35). In cattle, the effect of sunflower oil supplementation at 6% of dry matter on muscle CLA concentration was determined in steers fed diets containing barley grain and pea hay (36). The progressive deposition of CLA was monitored by obtaining biopsies from the semitendinosus muscle of the steers. These biopsies were obtained a month after initiation of feeding of the weaned steers and before initiation of the finishing diet in which the grain component of the diet was increased substantially (from 35% to 78% of total diet dry matter). Finally, on slaughter a sample of the muscle was procured for determination of the CLA concentration. The provision of dietary oil increased the CLA concentration in the muscle, and the values were 4.5 and 10.4 mg/g fat in the first biopsy and 4.1 and 16.3 mg/g fat in the second biopsy in control and oil-fed steers, respectively (PS Mir, M Ivan, and TA McAllister, unpublished observations, 2001). The concentration in the muscle at slaughter was 2.1 and 14.8 mg/g fat for control and oil-fed steers, respectively.
As indicated earlier, aside from the concentration of CLA, the total fat content of the muscle will affect the net available CLA for the consumer. The study described previously was conducted with steers from 3 breed types, namely the Wagyu (Japanese high-marbling beef breed), Wagyu/Limousin crossbred, and Limousin breeds. As expected, the fat content of the muscle of the Wagyu steers was greater than that of the crossbred steers and that of the Limousin steers (36). Because the CLA concentration was increased to the same extent in all breeds, the animals that had higher fat content had higher content of CLA per 100 g meat (Table 4). As a result, Wagyu cattle receiving dietary oil had the highest CLA content (134 mg/100 g meat).
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TABLE 4. Breed effects on fat and conjugated linoleic acid (CLA) contents of beef from the longissimus muscle of steers fed control or oil-containing (sunflower oil, 6% of dry matter) diets1
CONJUGATED LINOLEIC ACID AND MEAT QUALITY AND POSSIBLE EFFECT IN CONSUMERS
There are concerns that feeding oil to meat-producing animals can affect meat quality. It was suggested that CLA could have antioxidant properties (37). Although this claim is disputed, it is recognized that CLA does inhibit the production of lipid peroxidation products; thus, the effect of increasing CLA by way of inclusion of 6% oil in the diet on the shelf life of beef was examined. It was found that even after 4 d of exposure in the retail case, the deepening of the color was restricted (32) and that meat from steers fed 6% sunflower oil maintained better retail acceptability scores than beef from control or steers fed only 3% sunflower oil. Dietary oil did not appear to affect the tenderness or palatability scores for the beef. Duynisveld et al (31) also failed to find any differences in meat acceptability from pasture-fed compared with silage and grain–fed steers and suggested that the lack of difference was due to the differing levels of CLA in the meat.
It was hypothesized that fatty acids esterified to the sn-2 position of the triacylglycerol would be retained preferentially and perhaps participate in providing some protection against peroxidation. Investigation into the location at which the CLA occurred on the triacylglycerol seemed to indicate that they were esterified at the sn-2 position (32). The digestion, absorption, and ultimate metabolic fate of fatty acids in this position was suggested to be at variance from that of fatty acids in the sn-1/3 position. Provision of meat from animals that had relatively higher concentration of CLA to weaned rats caused the adipocyte number to be decreased in both the retroperitoneal and inguinal fat pads relative to that in rats fed meat with low CLA concentrations. Differences in adipocyte number between rats fed the CLA-enriched meat diet or a diet with synthetic CLA, included at 1.1% of diet dry matter, were not significant (38) (Table 5), but only inclusion of synthetic CLA led to decreases in retroperitoneal fat pad weight and concurs with published reports. The effectiveness of low concentrations of bioformed CLA in decreasing adipocyte number in comparison to the synthetic CLA is intriguing and needs further investigation.
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TABLE 5. Adipocyte numbers in retroperitoneal and inguinal fat pads of rats fed synthetic or meat-containing diets1
In general, CLA concentrations in beef can be increased, yet the extent of increase may not be sufficient to meet current recommended dietary intakes. These suggested intakes are based on extrapolation of required intakes of the synthetic compounds in animal models to observe particular effects. It is possible that these suggestions have to be revised after taking into account consumption over a lifetime as opposed to set short durations. One has to consider the extent of benefit that would not be realized by consuming CLA in amounts lower than the current recommended intakes. If modest CLA intake imparts benefits greater than no CLA consumption at all, then there are many advantages to producing foods such as milk and beef with enhanced levels of bioformed CLA because the consumption of these foods is high. On the basis of the per capita beef consumption of 31 kg per annum, the daily beef intake is 85 g. The CLA intake from beef (based on calculations in Table 4) would range from 10.2 to 113.9 mg/d, depending on the marbling fat content of the beef and whether the animals were fed diets with oil supplements to deposit CLA in their tissues.
INCREASING CONJUGATED LINOLEIC ACID IN BEEF
To increase the CLA yield in beef it is essential to provide cattle an appropriate substrate for formation of CLA. The provision of a source of dietary linoleic acid appears to increase the CLA concentration to the greatest extent. Dietary forage such as grass or legume hay appears to facilitate the establishment of the microflora that enhance the formation and deposition of CLA in the tissues. The provision of modest amounts of grain is more conducive to CLA synthesis rather than high levels of grain. Certain breeds of cattle that have a tendency to deposit high amounts of fat in muscle will deliver a greater amount of CLA to the consumer.
ACKNOWLEDGMENTS
We thank BM Pink and B Linderman for their technical help.
REFERENCES