Conjugated linoleic acid

Conjugated linoleic acids (CLA) (brand name Tonalin) are a family of at least 28 isomers of linoleic acid found mostly in the meat and dairy products derived from ruminants. CLAs can be either cis- or trans-fats and the double bonds of CLAs are conjugated and separated by a single bond between them.

CLA is marketed as a dietary supplement on the basis of its supposed health benefits. There is however no convincing evidence that taking CLA supplements has any benefits for human health.

History
The biological activity of CLA was noted by researchers in 1979 who found it to inhibit chemically-induced cancer in mice. In 2004, one of those researchers noted that the scientific literature was "growing at a phenomenal rate".

The United States Food and Drug Administration categorizes CLA as generally recognized as safe (GRAS) status for certain food categories, including fluid milk, yogurt, meal replacement shakes, nutritional bars, fruit juices and soy milk. With GRAS status, food companies are able to add CLA to products in these food categories.

Biochemistry
Most studies of CLAs have used a mixture of isomers wherein the isomers c9,t11-CLA (rumenic acid) and t10,c12-CLA were the most abundant. More recent studies using individual isomers indicate that the two isomers have very different health effects.

Conjugated linoleic acid is both a trans fatty acid and a cis fatty acid. The cis bond causes a lower melting point and ostensibly also the observed beneficial health effects. Unlike other trans fatty acids, it may have beneficial effects on human health. CLA is conjugated, and in the United States, trans linkages in a conjugated system are not counted as trans fats for the purposes of nutritional regulations and labeling. CLA and some trans isomers of oleic acid are produced by microorganisms in the rumens of ruminants. Non-ruminants, including humans, produce certain isomers of CLA from trans isomers of oleic acid, such as vaccenic acid, which is converted to CLA by delta-9-desaturase.

In healthy humans, CLA and the related conjugated linolenic acid (CLNA) isomers are bioconverted from linoleic acid and alpha-linolenic acid, respectively, mainly by Bifidobacterium bacteria strains inhabiting the gastrointestinal tract. However, this bioconversion may not occur at any significant level in those with a digestive disease, gluten sensitivity and/or dysbiosis.

Health
CLA is marketed in dietary supplement form for its supposed anticancer benefit (for which there is no evidence) and as a bodybuilding aid.

A systematic review of the evidence found that,, it did not convincingly show CLA to have a useful benefit for overweight or obese people, as it had no long-term effect on body composition.

A 2004 review of the evidence said that while CLA seemed to benefit animals, there was a lack of good evidence of human health benefits, despite the many claims made for it.

CLA has shown an effect on insulin response in diabetic rats, but there is no evidence of this effect in people.

Dietary sources
Kangaroo meat may have the highest concentration of CLA. Food products from grass-fed ruminants (e.g. mutton and beef) are good sources of CLA, and contain much more of it than those from grain-fed animals. In fact, meat and dairy products from grass-fed animals can produce 300-500% more CLA than those of cattle fed the usual diet of 50% hay and silage, and 50% grain.

Eggs from chickens that have been fed CLA are also rich in CLA, and CLA in eggs has been shown to survive the temperatures encountered during frying.

Some mushrooms, such as Agaricus bisporus and Agaricus subrufescens, are rare nonanimal sources of CLA.