Conjugated linoleic acid is found primarily in the flesh and dairy products of ruminant animals such as cows and sheep. CLA is produced by micro-organisms within the rumen of these animals.
Animals that are grass-fed, such as those who are able to graze in a pasture, possess more CLA than those who are fed only grain. Grass-fed beef may contain 300 to 500 percent more CLA than grain-fed beef; the latter includes cattle who receive a diet of 50 percent grain and 50 percent hay .
Conjugated lineolic acid is also found in eggs and kangaroo meat. Kangaroo meat may possess the highest concentration of CLA. The CLA in eggs remains intact when encountering high frying temperatures.
CLA is a family of positional and geometric isomers composed of two double bonds formed from linoleic acid and linolenic acid . Most studies focus on a combination of the isomers, usually with c9, t10, c12-CLA and t11-CLA being most prevalent.
Technically, CLA is a trans fat. Trans fats are notorious for clogging and hardening arteries; decreasing HDL, or good, cholesterol; and drastically increasing LDL, or bad, cholesterol. However, CLA is more widely accepted as beneficial rather than harmful. Trans fats in a conjugated system are not considered as genuine trans fats when it comes to food labeling and dietary guidelines in some countries.
A daily dose of 3.2 g of CLA has been shown to reduce body fat . This dosage has also shown decreased glucose uptake and changes in serum total lipids. It's important to note that this dosage can only be achieved by taking supplements as opposed to solely supplementing the diet with grass-fed animal products.
CLA also possesses anti-tumoral and antioxidant properties. Studies involving rodents show a reduction in tumor growth in the skin, mammary gland and colon. However, this may not be attainable in humans with safe oral doses.
A Norwegian study found that taking CLA supplements over the long term reduces body fat mass and increases lean body mass .
The study involved 118 participants with varying body compositions: healthy, overweight and obese. Two groups received either 3 to 4 g daily of CLA or a placebo for six months. Body fat mass in the group receiving CLA decreased significantly at months three and six. Fat reduction occurred most readily in the legs, the waist-hip ratio and in females. Lean body mass also increased in the CLA group. Diet and physical activity did not play a role in the changes noted.
Scandinavian scientists realised similar results when studying obese adults...
They found that CLA supplements reduce body fat mass in obese adults over the long-term . Most notably, CLA effectively facilitated body fat mass reduction at the onset, and overall weight control during the long term. This long-term study was designed to gauge changes in body composition: body fat mass, lean body mass, bone mineral mass, body weight and body mass index. The study involved 157 participants for one year, and 134 of the original participants for another year. Two groups received 3.4 g of CLA daily as either a triglyceride or a free fatty acid, and the placebo group received olive oil. Body composition changes were unrelated to diet or physical training.
Likewise, researchers at the University of North Carolina found that CLA may increase lean body mass in obese adults . Obese adults who were otherwise healthy received CLA supplements for 12 weeks; the placebo group received 8 g of safflower oil. The control group received either 3.2 grams daily or 6.4 grams daily of CLA. Of 48 study participants, 35 were female and 13 were male. Changes in body fat mass and lean body mass were measured; bodily energy expenditure was measured at a resting state. Lean body mass increased by 0.64 kg in the group receiving 6.4 g of CLA daily after 12 weeks.
One of the purported benefits of CLA is that it increases the body's energy expenditure, even during resting periods.
Researchers at the University of Wisconsin found that CLA supplements positively alter energy expenditure and fat oxidation during sleep . Study participants received either 4 g daily of 78 percent active CLA isomers or 4 g daily of safflower oil. Energy expenditure and dietary fat oxidation were measured before and after six months of receiving CLA supplements. The CLA group exhibited a shift in fat utilization during sleep after receiving CLA supplements every day for six months; also, the percentage of energy derived from protein was decreased during sleep, and a change in energy expenditure during sleep was noted.
However, it's important to note that a study conducted by the U.S. Department of Agriculture involving 17 healthy adult women found that receiving 3 g daily of CLA for 64 days did not significantly affect energy expenditure or body composition . Participants received either 3 g daily of CLA or sunflower oil for 64 days following a baseline of 30 days. All participants' diets and exercise activity were monitored to ensure consistency during the 94-day study. Changes in the overall percentage of body fat, fat mass, fat-free mass and body weight did not change significantly while receiving CLA supplements.
This contrasts findings from previous studies involving rodents and pigs. CLA reduces body fat, and it may do so by altering the body's energy expenditure. Participants' energy expenditure, respiratory exchange ratio and fat oxidation were measured in a resting state and while walking during the baseline period, and again at week four and week eight after receiving supplements. None of the factors was altered with CLA supplements either at rest or during moderate physical activity.
This may lead one to conclude that CLA works best for adults who are overweight rather than those who are already fairly fit.
It can be tricky to ascertain CLA's benefits with conflicting study results. Fortunately, the University of Wisconsin School of Medicine and Public Health conducted a meta-analysis of human studies involving CLA.
Researchers identified 18 studies wherein randomized, double-blind and placebo controlled methods were implemented, which assessed body composition utilizing validated methods. In analyzing and summarizing results from these studies, researchers found that CLA initiates a modest loss of body fat if taken at a minimum dosage of 3.2 g daily .
There's some concern that overweight adults who take CLA supplements may become insulin-resistant, increasing their risk of diabetes.
Swedish researchers found that the trans-10, cis-12 isomer may predispose overweight men who exhibit metabolic Syndrome X toward developing type II diabetes . Sixty overweight men, all of whom showed signs of Syndrome X, received either 3.4 g of CLA as blended isomers, 3.4 g of CLA as the trans-10, cis-12 isomer, or a placebo. Participants who received the trans-10, cis-12 isomer became 19 percent more insulin-resistant. This increase of insulin may warrant caution against using high dosages of CLA over the long term.
However, the Norwegian study referenced earlier found that diabetogenic markers remained within the normal range . The same held true for blood lipids and inflammatory markers. Also, bone mineral content remain unchanged. Adverse events reported amongst the control and placebo groups were similar.
It's interesting to note, though that researchers at the University of North Carolina found that CLA may increase markers of inflammation in the short term . One reason for undertaking this particular study was to ascertain clinical safety measures in obese adults and to examine clinical laboratory values. The control group, obese adults who were otherwise healthy, received 3.2 or 6.4 g of CLA daily for 12 weeks. In the 6.4 CLA group, significant decreases of serum sodium, HDL cholesterol, hemoglobin and hematocrit were noted. Significant increases of serum white blood cells, alkaline phosphatase, C-reactive protein and IL-6 were also noted. However, all noted changes were within the parameter of being normal. Although participants reported mild gastrointestinal discomfort, no severe adverse reactions were noted.
The Scandanavian study found that CLA is well-tolerated by healthy and overweight adults . The second year of the long-term study was undertaken to ascertain the safety, conduct clinical chemical analyses, and to note any reported any adverse events. Researchers noted no change in fasting blood glucose. A significant increase of aspartate amino trasferase was noted, but not for alanine amino transferase. Overall cholesterol and LDL cholesterol levels were reduced; HDL cholesterol and triglycerides, however, remained the same. Reported adverse events decreased during year two.
Initial studies of CLA involved mice and pigs. Studies involving humans are relatively recent, and only a few long-term studies have been conducted. Given the study results above, it's possible to conclude that that CLA may work best for overweight adults rather than adults who are already fairly fit.
And for overweight adults who are trying to lose weight, CLA may be most helpful with body fat mass reduction at the onset of a weight loss program, and help control weight over the long term.
However, certain overweight individuals should be cautioned against using CLA over the long term to avoid an increased risk of developing insulin resistance or diabetes.