Omega-3 fatty acids are the good fats. Found in fish, walnuts, and flaxseed, they boost brain power, stave off depression, and decrease inflammation. While the typical American diet lacks omega-3s, it's loaded with omega-6s, which are found in vegetable oils and processed foods. In fact, nutritionists agree that our omega-6-rich diet may be to blame for the dramatic rise in inflammatory diseases such as asthma, heart disease, and cancer. While most omega-6s-including the infamous trans fats that come from the partial hydrogenation of vegetable oil-are unhealthy, an increasing body of evidence shows that a lesser-known member of the omega-6 family called conjugated linoleic acid (CLA) is surprisingly beneficial. It may promote weight loss, build lean muscle, boost immunity, and halt cancer growth.
CLA is a fatty acid found only in the meat and milk of ruminants such as cattle. Research shows that CLA levels are much higher if these animals are fed on lush pastures- a rarity in today's industrial farms and dairies, where cows munch mostly on grains. A 1999 study, published in the Journal of Dairy Science, found that milk from grass-fed cows contained up to 500 percent more CLA than milk from cows fed a diet of 50 percent grain.
The word fat here is crucial: Because CLA is found only in the fat of ruminants and their milk, you won't find it in skim milk or non-fat yogurt. "The higher the fat-cream, butter, ice cream, cheese-the more CLA there is," says Dale Bauman, Ph.D., a professor in the Department of Animal Science at Cornell University who studies the effect of diet on cows' CLA levels. If you don't like full-fat milk, or don't drink milk at all, you can take CLA as a supplement.
CLA's potential to spur weight loss and increase lean body mass has been demonstrated in many animal studies. Studies on humans have been less consistent: One of the most promising was published last August in the International Journal of Obesity. Over a six-month period that included the calorie-laden holiday season, 40 overweight men and women took a daily 3.2-gram CLA supplement or a placebo. Those who took CLA lost an average of 1.3 pounds, while those in the placebo group gained 2.4 pounds.
That's a difference of just a few pounds, but it could be enough to halt middle-age weight gain, says Vicki March, medical director of the Weight Management Program at the University of Pittsburgh Medical Center. According to March, people tend to gain a full pound over the holidays. "But that's weight we tend not to lose," she says. "If there were a way to avoid gaining that one pound each year, you might stave off the weight creep that people tend to get in middle age."
The study's lead author, Dale Schoeller, Ph.D., whose metaanalysis of recent studies on CLA and weight management will be published this fall in the American Journal of Clinical Nutrition, says CLA appears to improve fat metabolism. According to Schoeller, fat loss was statistically significant in seven out of the 18 human studies he reviewed; 11 others showed a more modest trend toward fat loss. "Most of the studies that didn't show 'statistically significant' weight loss were done for too short a time at too low a dose," says Schoeller. When he adjusted the data for these two factors across all 18 studies, he found that weight loss was significant. In general, Schoeller says, people taking a daily supplement of CLA would lose an average of five pounds over two years.
Fighting allergies, asthma, and cancer
Researchers are also investigating CLA's potential to boost immunity and reduce symptoms of inflammatory disorders such as allergies and asthma. Animal and test-tube studies have already established this, and human evidence is mounting. A 2005 study in the European Journal of Clinical Nutrition found that volunteers who took CLA supplements for 12 weeks had increased levels of bacteria- and virus-fighting immunoglobulins, and decreased levels of the antibodies that cause allergic reactions and of proteins that cause inflammation.
Another human trial, conducted recently at St. Paul's Hospital at the University of British Columbia in Vancouver, showed that CLA may reduce asthma symptoms such as wheezing and coughing. Thirty subjects with a history of mild to moderate allergic asthma took either 4.5 g of CLA a day for 12 weeks or a placebo. After 12 weeks, those taking CLA showed significant improvement in lung function. "They still had asthma but they had markedly better control over it," says lead author Delbert Dorscheid, M.D., Ph.D., adding that the study will soon be published in a major medical journal.
"CLA is anti-inflammatory- there's no question about it," says Mark Cook, Ph.D., an animal scientist at the University of Wisconsin-Madison, who has conducted dozens of studies on CLA and immune response in animal models. Cook says scientists in his department have seen so many positive responses in animal studies that they're convinced. "Even my immunologist takes CLA for his allergies now," says Cook.
In the 80s, Michael Pariza, director of the University of Wisconsin- Madison Food Research Institute, discovered CLA's anti-carcinogenic properties when his team found that partially purified beef extracts could inhibit tumor growth in mice. Since then, more than 100 in vitro and animal studies have shown that CLA halts cancer growth-especially in breast, skin, and colon cancers. However, because human tumors are measured in decades, not years, cancer studies on people are both impractical and prohibitively expensive, says Cook.
Milk vs. supplements
So, should you drink more milk or take a supplement to get the most out of CLA? It depends on the health benefits you're seeking.
There are 28 different isomers in CLA, but the greatest health benefits seem to be concentrated in just two of them, known as t10c12 and c9t11. There's strong evidence you need both isomers to lose weight and enhance overall immune function, and milk is rich in only one of them: c9t11. So if you're looking to shed pounds, supplements make the most sense. When shopping for a supplement, look for the name Tonalin or Clarinol on the label-these are branded CLA formulations that contain an equal mix of the two isomers and have been used most often in scientific studies. Check the label for dosage, and keep in mind that most studies have involved a daily dose of 2 to 5 g of CLA.
However, if you're looking primarily for anti-cancer benefits, milk may be a smarter choice. Ninetytwo percent of the CLA in milk from grass-fed cows is the cancer-inhibiting c9t11 isomer, and some studies have shown that the two isomers in CLA supplements may cancel each other out in terms of anti-cancer protection. If you decide to go the milk route, you should aim to get three daily servings of full-fat dairy products from grass-fed cows. That's a rough equivalent of the minimum dose shown to be beneficial in animal studies.