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Linoleic acid (LA) is a naturally occurring fatty acid found predominantly in beef and dairy products. LA is one of the two essential fatty acids (the other is linolenic acid). Linoleic acid is an omega-6 fatty acid, meaning that it is unsaturated, with a double bond occurring at the sixth carbon atom from the omega end of the molecule. Conjugated linoleic acid (CLA) is an isomer of LA - which refers to a slight rearrangement of the molecular structure (conjugation) - resulting in a fatty acid with altered chemical functions. The rearrangement in this case is a conjugated double bond occurring at carbons 10 and 12 or at carbons 9 and 11. Linoleic acid is found in the diet in vegetable oils, whereas the conjugated variety, CLA, is found primarily in meat and dairy products. The form of CLA found most commonly in dietary supplements is manufactured from vegetable oils such as sunflower oil or safflower oil. One of the leading brands of CLA (TonalinTM), and the one on which the majority of studies has been conducted, is derived from safflower oil.
The anti-tumor/anti-cancer properties attributed to CLA may be due to an antioxidant effect or to an undefined interaction between CLA and various carcinogens. CLA is also theorized to modulate the production of prostaglandins, which are derived from fatty acid molecules and have been linked to an elevated synthesis of growth hormone. Increased growth hormone levels are viewed as beneficial to both athletes and dieters as a way to promote enhanced muscle growth, strength and fat loss. Some prostaglandins may also increase blood circulation to the muscles and adipose tissue - an effect that has been suggested to improve muscle function and fat mobilization. Perhaps the more tantalizing effects of CLA when it comes to supplements, however, are those reported for the induction of weight loss, body fat loss and increased caloric expenditure.
The majority of research on dietary intake of CLA has been conducted in animals. Several studies have indicated an anti-tumor effect of CLA in normal doses (1-4 grams) - close to what an average person might consume daily from a "typical" intake of meat and dairy products. The potential anti-cancer effects of CLA (most notable the cis-9/trans-11 isomer) have been attributed to several possible mechanisms including its actions as an antioxidant.
The positive effects of CLA on body composition (less fat and more lean) have been shown in numerous animal studies (pigs, mice, rats, chicks), but the evidence in humans has been somewhat controversial. The joke around the scientific meetings has always been that CLA is a great weight loss supplement for mice, but not so good for humans. This view, however, is rapidly beginning to change based on recent results from several research groups.
In animals, adding CLA (primarily the trans-10/cis-12 isomer) to the diet consistently leads to the supplemented animals gaining less body fat, but more lean body mass (muscle), compared to control animals. As such, many of the studies show no change in total body weight - but that weight is made up of less fat and more muscle - good stuff! - but remember, these results are primarily being shown in rodents and livestock (very few of which are reading these web pages). In livestock studies (cattle, pigs, chickens), supplemental CLA has been shown to promote growth and prevent muscle wasting, whereas body fat accumulation and energy expenditure increased - so here you get a leaner stronger animal (isn't that what we're all looking for in terms of weight loss?). As a nice side benefit, CLA feeding also appears to reduce LDL cholesterol and triglyceride levels in rabbits with elevated cholesterol. So what does this picture look like? - A Happy Barnyard! The cattle, pigs and chickens are all low-fat, high-muscle machines - sort of like a barnyard version of Muscle Beach. Likewise, the CLA-supplemented lab animals (rats, rabbits, and mice) are slim and trim and are the envy of the guinea pigs who only seem to get recruited for the studies on carcinogens and tobacco-related studies.
But what does this all mean for you (the non-lab-rat human looking to shed a few pounds)? Based on a handful of recent studies, it means good news (maybe). Two recent studies have shown that CLA supplementation (3-4 grams/day) promotes a loss of body fat (2-4 lbs. extra in overweight subjects over 12 weeks) and reduces abdominal fat (by about 1 inch) in obese men. So does this mean that CLA is the answer to your weight loss prayers? Probably not - but it might help (keep reading).
Despite these recent positive findings on CLA, there are numerous earlier (but small) studies that have found no benefits of CLA for fat loss. One small study of weight lifters found no differences on measures of body weight, fat mass or fat-free mass following a month of CLA supplementation, but this study looked at subjects who already had a fairly low body fat percentage (14%). Another small study followed 10 subjects consuming 3-4 grams of CLA each day for three months and compared them to 10 subjects consuming a placebo. Results showed no difference in weight loss between CLA and placebo, but those talking CLA dropped somewhat more body fat (a good thing). In another study of 17 healthy women, CLA supplements (3 grams/day) or a sunflower oil placebo for 64 days, resulted in no change in body weight, fat-free mass, fat mass, or percentage of body fat. Likewise, CLA had no significant effect on energy expenditure, fat oxidation, or respiratory exchange ratio at rest or during exercise.
Bottom line=the scientific evidence is overwhelmingly positive in the animal studies, but about evenly split between positive and no-effect studies of humans (see more in the Value section).
No adverse side effects are reported with CLA supplementation - but at least one of the more recent human trials reported that about 30% of subjects reported gastrointestinal symptoms associated with recommended doses (3-5 grams/day).
Confused yet? What we're left with is a supplement that has scientific evidence on both side of the fence (like almost every supplement) - which side of the CLA fence should you be on? In considering the overall "value" of CLA for fat loss, we need to consider some of the metabolic differences between animals (where almost all of the CLA data is positive for lowering body fat and increasing lean tissue) and humans (where we have a split). Rodents, for example, have a metabolic rate that is about 7 times higher than that of a human - which translates, after normalizing rodent and human data to the same scales, to a reduction of body fat that is 7 times greater in rodents than in humans following CLA supplementation. In the rodent studies, CLA supplementation has also been shown to increase daily energy expenditure by nearly 8% - but in humans this effect may only be a bit over 1% (too small to be detected in anything but the largest study using the most sensitive equipment). So this leaves you to make our own judgement call - to supplement with CLA or not. Our recommendation is that if you're looking for a non-stimulant method for slightly increasing energy expenditure and promoting body fat loss, then CLA is worth a try (perhaps combined with green tea extract).
Most people ingest less than 1 gram per day from meat and dairy foods. Typical dosage recommendations are 3-5 grams per day and the 2 most recent studies on CLA have shown benefits using doses of 3.4g and 4.2g per day. Because most of the studies showing a positive effect of CLA have used the TonalinTM brand of CLA (a 50/50 blend of the cis-9/trans-11 and trans-10/cis-12 isomers), SupplementWatch recommends that you look for products that include TonalinTM.
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