The idea that conjugated linoleic acids (CLAs) could protect against certain cancers, as well as heart disease is attractive but the evidence to date, while encouraging, is still far from conclusive.
CLAs are natural components of animal foods derived from linoleic acid. They are found in milk fat, dairy foods and meats derived from ruminant animals. Recently, there has been a surge of interest in the CLA content of the diet because of evidence, based largely on animal studies, that suggests potential health benefits. Since current dietary recommendations include eating less animal fat, there are questions being raised about a possible decline in dietary CLA consumption and the implications for health.
Research into CLAs has been undertaken in several areas including cancer, heart disease, diabetes and weight control. The evidence in all areas is far from conclusive.
Protection against cancer
Early research with animal models demonstrated that CLAs could inhibit cancer formation and growth, and there is now some evidence to suggest that CLAs may help to protect against certain types of cancer. Most of the evidence investigating the effects of CLAs on breast as well as skin, liver and colon cancers has, however, come from animal and human tissue studies. Although clinical studies are limited, recent work has found that a low risk of breast cancer is associated with high intakes of CLAs, high consumption of cheese and high blood levels of CLAs. Associations do not, however, prove cause-effect relationships and further human studies are needed to follow up these promising results.
One of the major risk factors for coronary heart disease is abnormal levels of fats in the blood, particularly high levels of low-density lipoproteins (LDL) cholesterol (or "bad" cholesterol). Speculation on the potential benefits of CLAs has been fuelled by results from animal models but the limited number of human studies have not provided consistent evidence for beneficial effects of CLAs on blood fats and atherosclerosis (hardening of the arteries).
The incidence of type II diabetes (the type of diabetes normally associated with overweight) in Europe is increasing dramatically. There is some evidence that CLAs may have the ability to normalise glucose metabolism. Although the research is far from conclusive, further work is encouraged.
CLAs have been shown to alter body composition in growing mice, causing an increase in energy expended, increased body muscle and reduced body fat. Similar effects in humans have not been demonstrated and much more work needs to be carried out.
Sources of CLAs
CLAs are found in fatty foods from ruminant animals such as full fat milk and dairy products and fatty meats. It is possible to further increase the CLA content of these foods by increasing the amount of plant oils high in linoleic acids (such as sunflower and soyabean oils) in cow feed. This has been shown to increase the CLA content of milk. In addition, cows grazing pasture increases the CLA content of milk, especially when the grass is at an early growth stage.
More research needed
While the research appears promising, no firm conclusions can be made regarding the ideal CLA content of the diet and there are as yet no dietary recommendations for CLA intake. One of the areas that remains to be addressed is an evaluation of upper levels of intake. As with many other discoveries in the dynamic area of nutrition, more research is needed.
- Cannella C and Giusti AM (2000) Conjugated linoleic acid: a natural anticarcinogenic substance from animal food. Ital. J Food Sc, 12:123-27
- Lawson, RE, Moss, AR & Givens, DI (2001) The role of dairy products in supplying conjugated linoleic acid to man's diet: a review. Nutrition Research Reviews 14, 153-172
- Stanley, J & Hunter, K (2001) The wonder nutrient. Chemistry and Industry, 19th November, 729-731