CLA is a unique fatty acid from ruminant animal sources that, in animal studies, exhibits powerful anti-carcinogenic and other beneficial health effects at relatively low dietary levels. Research findings from animal studies need to be replicated in humans.
News about conjugated linoleic acid (CLA) has stirred a lot of interest in the cattle industry. Scientific interest in the fatty acid began in 1988 when Michael Pariza, a University of Wisconsin food science researcher, discovered CLA had cancer-fighting properties in rats fed fried hamburger.
While a host of commercial CLA nutritional supplements is available, dietary beef and dairy products represent the primary “natural” sources of CLA.
The cattle industry would dearly love to cash in on the health benefits associated with CLA. But, the jury is still out on just how this compound benefits humans.
First - The Pros
“Animal studies have shown CLA is a powerful anti-carcinogen that can be administered safely through diet to achieve cancer protection,” says Mary Young, executive director for nutrition strategy and research at the National Cattlemen's Beef Association. “CLA in milk fat may be a protective factor against breast cancer and coronary heart disease.”
Pariza's initial work on CLA as an anti-carcinogen has been extended to many other areas of human health. The purported benefits include anti-atherogenic and anti-diabetic properties, enhanced immune response and positive effects on energy partitioning, fat deposition and muscle growth.
Estimates from USDA's “Survey of Food Intake of Individuals” suggest that 36% of total CLA intake comes from beef and 52% from dairy products. Pork, seafood, most poultry products and vegetable oils are not notable sources of CLA.
Most research to date has been with animals and found that as little as 0.5% CLA in the diet has reduced tumor burden by more than 50%. But, can a reasonable diet containing beef provide beneficial levels of CLA for humans, considering that the minimal effective dose response is still unknown?
“At present, the answer would be ‘probably,’” says Young. “But, more studies in humans are needed to determine effective levels of CLA.”
A study published in the December 2000 Journal of Nutrition showed CLA reduces fat and preserves muscle tissue. An average reduction of 6 lbs. of body fat was found in the group that took CLA compared to a placebo group. The study found that about 3.4 g. of CLA/day is the level needed to obtain beneficial effects on body fat. CLA is generally found at a level of 4-7 mg. of CLA/g. of fat.
Pariza reported in August 2000 to the American Chemical Society that: “CLA doesn't make a big fat cell get little. It keeps a little fat cell from getting big.”
In a Purdue University study, CLA was found to improve insulin levels in two-thirds of diabetic patients, and it moderately reduced the blood glucose level and triglyceride levels.
Some Of The Cons
J.C. Stanley, MD, of Great Britain's Lincoln Edge Nutrition is a CLA critic. He says he can find only one scientific study describing a human health benefit of CLA — that it can lower blood glucose levels. He says more studies are needed to establish a reliable value of CLA content of foods.
Stanley, other nutritionists and physicians question if the benefits shown in laboratory animals will extend to humans. “Similar sounding claims were made for dietary fiber in the 1970s,” he says.
An Agricultural Research Service (ARS) interpretive summary concludes that it's not known if CLA in human diets will benefit immune response. Work by ARS research chemist Darshan Kelley at the Western Human Nutrition Research Center, Davis, CA, shows CLA does not appear to have any beneficial or adverse affects on human immune status.
In other studies, ARS's Nancy Keim and Marta Van Loan found CLA didn't reduce volunteers' body fat or help them build muscle. In addition, CLA didn't lower blood-fat levels or improve any of the other health indicators the research team examined.
Kelley says the results might have been different if volunteers had consumed more CLA over a longer period of time. Also, a different mixture of CLA components might have led to a different outcome.
All Fats Are Not Equal
Americans have been warned about too much fat in the diet. But, Kent Erickson, researcher at the University of California-Davis, says not all fats are created equal.
Each type of fat has a slightly different characteristic or function in the human body, he says. “The real question is: As Americans, do we consume too much fat or do we consume the right kind of fats in our diet?” Erickson asks.
While Erickson's research shows incredibly promising results, he admits most work to date has focused on animals. He also thinks humans probably can't get the protective benefits of CLA from food alone.
Cattle Diets And CLA
Animal management can alter the amounts of CLA found in the fat components of meat and milk. Wisconsin research shows that dairy cows grazing grass had five times more CLA in their milk than those fed silage or hay.
Researchers were able to boost CLA content of milk from confined cows by adding extracted whole soybean and linseed oils to a corn-alfalfa diet. Other work in the U.S. and Canada has shown that adding plant oils such as safflower, sunflower and canola to cattle rations can increase the level of CLA in beef and dairy fat.