In a four-month study at the University of Alberta presented in March at a scientific meeting, obese rats fed a diet enriched with vaccenic acid -- a naturally occurring trans fat found in milk and yogurt -- had significant reductions in total cholesterol, LDL (or "bad") cholesterol and triglycerides.

The researchers reported that a key benefit of vaccenic acid is its ability to reduce the production of chylomicrons -- small particles of fat, protein and cholesterol formed in the gut that transport fats to various tissues of the body.

Like humans, obese rats produce too many chylomicrons, which raises lipids in the bloodstream. After 16 weeks of consuming vaccenic acid-enriched chow, however, the levels of chylomicrons dropped by more than half.

It is not clear what this finding means for humans. First, the study was done in rats -- the researchers say they are planning some human clinical trials with vaccenic acid supplementation. Second, because the study diets were supplemented with vaccenic acid, the amounts the rats ate relative to their body weight was more than we would naturally eat in our usual diet.

The study is in line with other reports that natural trans fats have different effects on the body than the industrially created ones.

Most of the trans fats that consumers eat -- by far -- come from partially hydrogenated vegetable oils, produced from liquid oils by industrial processing to create a firmer fat. Others occur naturally in milk products, formed in the rumen (or first stomach) of ruminant animals such as cows, goats, sheep and yaks when they are fed a grass-rich diet.

Several studies of large populations have looked at the link between trans fatty acid intake and risk of developing atherosclerosis, and all have shown that the risk goes up only with the intake of "industrial" trans fatty acids, not the natural ones. Several clinical trials -- in which people were fed special diets for weeks or months -- have shown that manufactured trans fats raise LDL cholesterol levels to the same degree as saturated fats, and also lead to lower levels of the good, or HDL, cholesterol. It has been estimated that it takes only about 12g of manmade trans fats to see this effect.

Trans-fat-free foods are big business, and today, the majority of margarines, cookies, snack cakes and chips are devoid of the stuff. The change was fueled by the fact that, two years ago, it became law that food labels disclose industrial trans fat content.

Even if all the partially hydrogenated vegetable oil disappeared from our foods, consumers would still consume about 1-5% of our calories from naturally occurring trans fatty acids, mostly vaccenic acid.

At this point, it is not known how much vaccenic acid consumers would need to consume to reap benefits. However, in the meantime, anyone wanting to increase their natural-trans-fat intake might want to develop a taste for exotic cheese.

A study published in February in the Journal of Agriculture and Food Chemistry,/i> reported that yak cheese, from animals grazing in alpine grassland, contains more than four times the vaccenic acid of conventional cheddar cheese from grain-fed dairy cows. (The study did not investigate the levels in cheese from grass-fed cattle.) It also contains three times more beneficial omega-3 fatty acids. The authors conclude that a daily serving of three ounces of yak cheese might promote health.

Yak cheese is not easy to find -- but the bottom line seems to be that the fatty acid composition of milk, cheese and yogurt from grass-fed animals may be more healthful than we knew -- and perhaps, when the clinical trials are done, vaccenic acid-rich milkfat may join the ranks of other healthful fats along with those found in fish oil and nuts.