Boning up on fish oil.

For decades, nutritionists have extolled the virtues of fish oils and other foods rich in omega-3 fats as a means of keeping the heart healthy. Studies in animals now suggest that this type of fat also aids bone, add the same holds for humans, physicians may one day add omega-3 fats to the dietary regimen they prescribe to help stave off osteoporosis.

Most polyunsaturated fats fall into one of two groups: the omega-6 fats found in corn oil and the omega-3s found in ocean fish, flaxseed, green vegetables, and some nuts.

In recent years, the omega-6 content of Western fare has been climbing--often at the expense of omega-3, which now constitutes around 7 percent of polyunsaturated fats in the typical diet, notes Bruce A. Watkins of Purdue University in West Lafayette, Ind. At an international omega-3 conference in Bethesda, Md., last month, he reported on his studies of chicks and rats. As omega-3 dropped from 50 percent of polyunsaturates to some 12 percent, bone formation also fell--by about 20 percent.

The fat's apparent stimulation of bone growth seems to arise from its effect on at least two hormonelike compounds. When omega-3 makes up only 12 percent of polyunsaturates in the diet, production of prostaglandin E ([PGE.sub.2]), a "local hormone," is high, limiting bone growth, Watkins says. Increasing the omega-3 share of polyunsaturates to 50 percent halved the bone-making cells' production of [PGE.sub.2], bringing it to a level more conducive to bone formation.

The same dramatic increase in omega-3 fats enhanced the activity of insulinlike growth factor (IGF) by 50 percent and increased significantly the production of a protein that ferries IGF around the body, Watkins' data show. Production of IGF, which fosters continual growth and remodeling of bone, typically falls in old age, precipitating the bone loss underlying osteoporosis.

"I think the message here," Watkins told Science News, "is that when the diet supports more responsive remodeling, you can shape the bone to be stronger."