Conjugated Linoleic Acid-rich Chicken Eggs as a Functional Food
Gita Cherian, Ph.D.
Cardiovascular disease (CVD) continues to be the principal cause of death in the United States. Because the current U.S. diet is rich in saturated fatty acids, cholesterol, and linoleic acid, it is proatherogenic, prothrombotic, proinflammatory, and proaggregatory, leading to CVD. A diet-based approach offers great potential to reduce CVD, as about 80% of heart attacks can be avoided by diet modification and lifestyle changes. Conjugated linoleic acids (CLA) are isomers of linoleic acid (a polyunsaturated omega-6 fatty acid) that have the same chemical composition but different spatial orientations that affect their physiological roles. The major isomer is the cis-9, trans-11 form. CLA have received considerable attention for their antiatherogenic, anticancer, and body fat-reduction properties. Dietary CLA is contributed by ruminant foods, such as dairy and beef. Based on animal data, it is estimated that approximately three grams per day of CLA would be required to produce beneficial effects in humans, but the current consumption of CLA is less than 600 milligrams per day. As Americans opt for low-fat dairy products and choose more poultry than beef, it is possible that the dietary supply of CLA from ruminant foods will be further limited. Chicken eggs, due to their high content of nutrients, low cost, and versatility, are a popular food item for all cultures. Incorporation of CLA in eggs could lead to alternate sources of CLA for humans. However, chicken eggs are also rich in saturated fatty acids and cholesterol, and the consumption of CLA-rich eggs on human and animal health needs to be investigated.
My current project addressed the following issues: 1) the effect of feeding CLA to hens on the fatty acid profile, cholesterol, and vitamin E content of egg yolks, and 2) the effect of feeding CLA-rich egg yolk powder to hamsters on lipid metabolism, lipid peroxidation, and antioxidant enzyme activities.
I fed CLA oil to hens, which resulted in a significant incorporation of CLA isomers in eggs laid by these hens. One serving (two eggs) of CLA-enriched eggs could provide over 560 mg of dietary CLA. No effect of CLA was observed on egg cholesterol or quality or on hen performance, but storage of CLA-enriched eggs over three weeks reduced the vitamin E content (alpha- and gamma-tocopherol).
I also investigated the effect of CLA-enriched yolk powder on lipid metabolism in hamsters. Eggs were collected from hens fed diets containing 1% (low CLA) or 2% (high CLA) CLA-enriched diets. Hamsters were fed a diet containing 20% of CLA-enriched yolk powder that provided about 30% of fat energy as recommended by national dietary guidelines. Except for CLA, the composition of the dietary fatty acids and cholesterol resembled that in a typical Western diet.
The high-CLA diet decreased total fat in hamster livers. A significant increase in CLA fatty acid incorporation was observed in the liver and spleen with a concomitant reduction in omega-6 fatty acids, including arachidonic acid, the precursor of proinflammatory eicosanoids. The incorporation of monounsaturated fatty acids was lower in the spleen and liver of hamsters fed low- and high-CLA diets compared with regular egg yolk or casein control diets. CLA-rich yolk feeding also resulted in an increase in saturated fatty acids in tissues. Hepatic triglycerides were lower in hamsters fed the high CLA diet.
The changes in lipid content and liver fatty acid profiles were associated with a decrease in lipid oxidation products, such as malondialdehyde, which was lower in the livers of hamsters fed CLA yolk-based diets. No difference was observed in plasma prostaglandin E2 concentration or F2-isoprostanes, another marker of lipid oxidation. I then measured changes in the antioxidant defense system by assessing activities of superoxide dismutase, glutathione peroxidase, and glutathione reductase. The activities of these endogenous antioxidant enzymes were increased in the livers of hamsters fed CLA-rich egg yolk. Fecal total lipids and total cholesterol were higher in hamsters fed high CLA. Finally, there was no change in aortic plaque formation associated with feeding the hamsters CLA-rich egg yolks.
The results of this study indicate that in hamsters, CLA from a non-ruminant source like eggs can be increased without any adverse effects. We observed a significant increase in antioxidant activity associated with CLA feeding, which may suggest an antioxidant action of CLA. Fecal lipid and cholesterol excretion also suggest that egg yolk CLA modulates lipid and cholesterol metabolism. However, the exact mechanism of dietary CLA in lipid metabolism needs to be further explored.
CLA consumption in Western diets may be increased by CLA-enriched eggs used in foods like mayonnaise, pasta, salad dressings, baked goods, or other poultry meat products. The success of such CLA-modified egg products will depend upon acceptable sensory characteristics and stability during cooking, storage, and processing. These factors have yet to be investigated in detail.
Based on these preliminary results obtained through research supported by an LPI pilot project grant, I plan to submit a grant proposal to the USDA to investigate the functional food attributes of CLA-enriched chicken eggs in humans. These findings will generate new fundamental knowledge about functional food strategies to prevent the progression of chronic diseases like heart disease.
Last updated November, 2006