Published in the American Journal of Clinical Nutrition, the study included 3,623 adults from Costa Rica, where cattle are more commonly raised on grass rather than on grain than they are in the U.S. Half of the adults had suffered one nonfatal heart attack, while the other half were healthy. The researchers collected information on diet and physical activity, and they took samples of adipose tissue that they later analyzed for a fatty acid found primarily in dairy products called conjugated linoleic acid (CLA). Levels of CLA are higher in milk from grass-fed cows, says the study's lead author Hannia Campos, PhD, senior lecturer in the department of nutrition at Harvard School of Public Health. "With grass-fed cows, their stomachs maintain the right acidity for enzymes [that produce CLA] to work normally," she says. "But when they're fed corn, the corn increases the acidity of their stomachs, so enzymes in their gut don't work properly."
More from Rodale News: It's Official: Organic Milk Is Better for You
People who had the highest concentrations of CLA in their tissue had a 36 percent lower risk for heart attacks than people with the lowest concentrations, even after the researchers took into account other lifestyle behaviors, such as smoking and alcohol intake, and family history of the disease.
Prior studies have found that milk from grass-fed cows can contain as much as five times more CLA than conventional milk, and Campos says the protective effects of CLA are high enough to offset the harms of saturated fat in milk, even whole milk. "We looked at the effects of dairy intake on cholesterol [which raises with saturated fat intake] to see if we could explain our results, but we didn't see any correlation," she adds. Whole milk has the highest levels of CLA, because "CLA is a type of fat," she says, "so when you start removing fat, you start removing the CLA." Fortunately, she says, grass-fed milk has lower levels of saturated fat than conventional milk (for instance, whole grass-fed milk has less saturated fat than whole conventional milk), so you can still get the benefits of CLA, which is a polyunsaturated fatty acid, even if you want to watch your saturated-fat intake.
To make sure you're getting the highest levels of CLA, make sure you're getting truly organic milk:
• Look for the seal. The U.S. Department of Agriculture (USDA) recently revised its feeding requirements for dairy cattle so that now all organically raised cows must be released to actual grass pasture for the entire grazing season; previously its definition of "pasture" was vague and didn't actually require that organic dairy farmers allow their cattle any grass at all. The new requirement also includes a stipulation that an organic cow's diet be made up of 30 percent grass.
• Befriend a farmer. Even organic cattle can eat corn and soy, so long as it's certified organic, and those grains can reduce their milk's levels of CLA. To get the most heart-healthy CLA out of your glass of milk, find a farmer that feeds his or her cattle nothing but grass. You might find one at your local farmer's market, which you can locate at Local Harvest or the Rodale Institute's Farm Locator.
• Buy American Grassfed. The American Grassfed Association certifies dairy farmers that feed their cattle nothing but grass. However, the certification program launched just a few years ago, and currently, there are only four dairy producers they've certified. But keep your eye out for the "American Grassfed" seal as it becomes more widely used. If you see the words "grass-fed" without an official seal, the term is generally meaningless. In that case, try to track down the dairy farmer and ask what it means for their cows.
• "Hormone-free," "rBST-free," "rBGH-free," or "antibiotic-free" don't equal USDA Organic certification. It's good that so many milk producers are raising cattle not treated with growth hormones or antibiotics, but that doesn't necessarily mean the cows are fed grass. These labels also aren't independently verified, so any producer can slap them on a milk carton without having the evidence to back up the claims.