Even Gaullier noted that caloric intake had decreased from the beginning of the study to the end by about 250 per day, which required a significant change in lifestyle even if it did not affect the results of the study. Before you run out and fill your pantry with supplements intended to promote weight loss, make sure you are committed to making it a permanent lifestyle change!
Following up to the previous post about green tea extract, I'd like to stick to the "New Year's Resolution Weight Loss" feature on another popular fat loss supplement.
Conjugated Linoleic Acid
Conjugated linoleic acid is a supplement that has been proposed to increase metabolic rate, increase fat utilization, and thus, with these combined, result in weight loss.
Conjugated linoleic acid, also known as "CLA", is a group of positional and geometric isomers; that is, they are positioned differently as if putting them in front of a mirror, of a fatty acid known as linoleic acid. Linoleic acid, as well as linolenic acid, are fatty acids both required by our bodies. CLA is found primarily in dairy products (not in skimmed dairy products) and meat, specifically, beef and lamb. The typical dietary intake is around 212 mg/day for men and 151 mg/day for women.
The predominant isomer that is found in cattle and other ruminant animals is "Cis-9, Trans-11-CLA" (this simply refers to the chemistry of the CLA), which is appears to be the most biologically active form. Researchers often call this type of CLA "Rumenic Acid" because it is the "natural" form from ruminant animals. Commercial CLA preparations are those found in supplements, and are often in a 1:1 ratio of the Cis-9, Trans-11-CLA and Trans-10, Cis-12-CLA.
CLA has been studied for various effects on health including: effects on the immune system, cancer prevention, blood lipid and glucose levels, and body composition / weight loss. The research on CLA and body composition in animals has been extremely positive.
For example, it has resulted in leaner pigs, mice, rats, and cows. However, these positive results have not been so consistent with humans. Most researchers have reported no change in body composition with CLA supplementation in humans. Furthermore, CLA may potentially have negative effects on blood lipid levels and glucose levels; though these results have been mixed as well.
A recent meta-analysis published in the American Journal of Clinical Nutrition reviewed the few human studies available on CLA. This meta-analysis only included studies that were published as full length articles.
It was noted that the dosages in the studies reviewed ranged from as low as 1.4 g/day (approximately 6 times that of "normal" dietary intake for men) up to 6.8 g/day (approximately 30 times that of the normal dietary intake for men). Out of the eight studies considering weight loss as an outcome reviewed in this meta-analysis, none found a significant reduction in body weight and only 2 showed a significant, but small decrease in body fat.
The authors drew the conclusion that "the results of the studies in humans indicate that the effect of CLA on body fat is considerably less than that anticipated from mice studies..."
It is important to note, however, that because there are various isomers of CLA, some speculate there may be a difference among the available isomers, contributing to the lack of consistency in results.
For example, although the CLA contained naturally in the diet is primarily the cis-9 trans-11 isomer, the majority of studies have instead utilized the trans-10, cis-12 isomer, potentially resulting in mechanistic differences.
Therefore, because of this potential variation among CLA isomers, a group recently published their work comparing two different isomers on body fat mass in overweight humans.
This particular 24-week study included 88 overweight, but otherwise healthy men and women. Subjects were randomly split into 5 groups and were randomly assigned to either 3 g high oleic acid sunflower oil (placebo), 1.5 or 3 grams cis-9, trans-11 CLA and 1.5 grams or 3 grams of trans-10, cis-12 CLA.
Body fat mass and lean body mass were assessed at baseline and again at the end of the 24-weeks. Dietary intake was also collected to ensure there were no significant differences among groups.
At the end of the study, the authors noted there were no significant differences among any of the groups for fat loss, body weight loss or dietary intake.
The authors suggest several potential differences when comparing their research to others; they suggest their study may have been too short or the doses of CLA too low to observe changes. These facts alone demonstrate the need for more research.
Another noticeable difference when comparing the animal data and human data is the doses are inconsistent. If considering the dose of CLA as a percentage of total energy, animal studies that have demonstrated a positive effect from CLA have provided approximately 0.70 g/kg body mass, whereas human studies (such as that described above) provide approximately 0.04 g/kg.
For humans to take the same dose of CLA as used in the animal studies, they would need to consume over 50 g of CLA/day (which would of course vary according to bodyweight). This dose is not cost-effective and may in fact have negative effects in humans because of the pro-oxidant effect of CLA that have been suggested.
Although the data in mice seems promising, the exiguous data in humans suggests that more research is required in humans where energy intake, energy expenditure, overall nutrient intake, CLA type, and CLA amount are all better controlled. This will allow researchers to truly identify if CLA plays a consistent role in decreasing body weight in humans.
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