24.Conjugated linoleic acids (CLA)

In my last post (BLOG 23) I showed there is convincing evidence that the fat present in milk is linked with specific health benefits. In recent years there has been growing interest in the CLAs which is the term used to describe the positional and geometric isomers of linoleic acid (LA). These have either one or both of the double bonds in the cis (c) or trans (t) conformation which are separated by a simple carbon-carbon linkage. CLA isomers are naturally found in dairy and meat products of ruminant origin such as cows, sheep and goats. About 30 years ago it was discovered that CLA was an effective inhibitor of the mutagens that were produced in hamburgers when they were grilled. Since then evidence has emerged which demonstrate that CLA can reduce the risks of developing heart disease, inflammation, diabetes, bone density loss and immune dysfunction. The two major CLA isomers are rumenic acid (c9, t11 (C18:2)) and the t10, c12 isomer. Typically the CLA in cow’s milk contains 85-90% rumenic acid. Experiments with small animals have shown that diets with a relatively high content of rumenic acid are associated with improving cardiovascular health, anti-carcinogenic properties and improved immune function. The evidence also shows that the HDL cholesterol is raised while the triglycerides are lowered(1).

The relationship between the consumption of high-fat dairy products and the incidence of colo-rectal cancer was studied in the Swedish Mammography Cohort. Data were collected from 60,708 women on 2 occasions separated by a period of almost 15 years. During this time there were 798 cases of colorectal cancer diagnosed in the participants. It was found that total high-fat dairy food consumption was significantly and inversely associated with the risk of colorectal cancer .Compared with the women who consumed one serving of high fat dairy foods/day, the women who consumed 4 servings/day had a multivariate RR of 0.59(ie a reduction in risk of 49%). Additional adjustments for alcohol consumption, family history of colorectal cancer, smoking, physical activity, and the use of multivitamin supplements, aspirin, oral contraceptives, and postmenopausal hormones did not significantly change the results. Also, these findings remained after allowing for the effect of supplementation with calcium and vitamin D. After exclusion of cancer cases that occurred during the first 3 years of follow-up to eliminate the effect of any factors prior to the beginning of the study, the multivariate RR of colorectal cancer was effectively the same with a value of 0.57. When the effect of individual high-fat dairy foods was examined it was shown that the lowest risk of colorectal cancer was observed in those with the highest consumption of cheese. Although this study does not prove that the CLA are directly responsible for the reduction in the incidence of colorectal cancer the results are consistent with other approaches involving experiments with small animals. Even if the reduction in colorectal cancer is not entirely due to the CLA, it is very likely that other constituents in the milk fat contribute to this effect(2).

TABLE 1 CONSUMPTION OF HIGH FAT DAIRY FOODS AND THE INCIDENCE OF COLORECTAL CANCER

In a study conducted at the Harvard School of Public Health the CLA status of participants based in Costa Rica was assessed by analysis of a sample of adipose tissue. It was found that those with the highest level of CLA reduced their risk of a heart attack by almost a half when compared with those with the lowest level(3).

It has been proposed that the optimum required daily intake of CLA is about 3g. However the actual consumption has undoubtedly declined considerably in recent years because of the recommendations to reduce the intake of saturated and the concerted efforts to promote low fat milk and dairy products.

CLA is produced by the action of micro-organisms in the rumen which utilise dietary linoleic and linolenic acids as substrates.

The level of CLA in the fat present in the milk and meat is dependent on a range of factors which includes lactation number, stage of lactation, region, season, breed and especially diet. This means that there can be a very wide variation in the amount available.

Various methods to alter the diet have been tried in order to raise the level of CLA in the milk fat. These include supplementation with fish oils, animal fat, plant oils and seeds, ionophores and forage. Oils rich in linoleic acid which include cottonseed, soybean, sunflower, safflower, corn, as well as linseed which is rich in linoleic acid, have been used successfully to boost the production of CLA in cows. There is general agreement that the supplementation of a basal diet of concentrates and conserved forage with plant-derived oils can substantially increase the concentration of CLA in milk fat. The most successful results have been obtained with those oils which contain linoleic acid.

The synthesis of CLA in the rumen is facilitated if the seeds are subjected to processes such as grinding and crushing because this increases the surface area which is available to the micro-organisms. In addition the oils are released which means that the micro-organisms have improved access to the substrates. Processes that involve heating can result in partial hydrolysis of

bound fatty acid which makes them more available to the micro-organisms.

Diets of cows which have been supplemented with fish oils or with marine algae have also been successful in achieving substantial levels of CLA in milk fat.

A comparison between countries where cows have access to pasture (eg Ireland, New Zealand) and those which do not (eg  large feedlots in the USA) confirms that when there is access to fresh pasture, the concentration of CLA is about three times higher than when there is not.

In the USA the CLA content of milk from cows grazing pasture was found to be 5 times higher than in milk from cows fed conserved forage and grain(4).

CONCLUSION

The information is further evidence of the critical importance of milk fat in the human diet and it also emphasises the particular benefits which are linked to milk produced by cows that are fed grass.

REFERENCES

1. Catherine Stanton, Personal communication
2. S C Larsson et al (2005) American Journal of Clinical Nutrition 82 (4) pp 894-900
3. L A Smit(2010) American Journal of Clinical Nutrition 92 (1) pp 34-40
4. T R Dhiman(1999) Journal of Dairy Science 82 pp2146-2157