No-fault fat: more praise for fish oil.

No-fault fat: More praise for fish oil

Studies have suggested that underfed animals live longer (SN: 8/27/88, p.142) and suffer less autoimmune disease. Now researchers are eliciting similar benefits without the belt-tightening. In fact, their well-fed mice eat a high-fat diet. Their only prescription is that the fat the mice eat must be fish oils rich in omega-3 fatty acids.

Immunologist Gabriel Fernandes, at the University of Texas Health Science Center in San Antonio, conducted the studies using hybrid mice with a genetic susceptibility to lupus or rheumatoid arthritis -- inflammatory diseases in which antibodies attack the body's own tissues. One group of each strain of mice received a restricted diet (60 percent the normal calories with 5 percent corn oil as its fat). Similar groups of about 25 animals were allowed to eat as much as they wanted. The only difference in their diets was the source of the high (40 percent of calories) fat levels: lard, corn oil or fish oil.

Mice on the lard and corn-oil diets died slightly earlier than normal for these strains. And before they died, the lard-fed animals developed inflamed blood or lymph vessels, thickened blood vessels and accelerated kidney disease. Mice on the fish-oil and calorie-restricted diets not only lived twice as long as normal, but produced half the normal levels of harmful autoantibodies and showed lower-than-normal inflammation. They also were free of kidney disease -- which normally afflicts all of these mice. Moreover, the fish-oil diet yielded blood cholesterol levels just half of normal -- and even lower than those in the low-fat, calorie-restricted animals.

The studies, reported last week at the American Chemical Society national meeting in Los Angeles, indicate that for maximum efficacy, the fish oils must not undergo oxygen-mediated chemical reactions. Preventing oxidation can pose a challenge, Fernandes explains, because omega-3 fatty acids are extremely susceptible to it.

One way to limit oxidation is to keep the fats in the fish. At the University of Massachusetts' Marine Science Station in Gloucester, Herbert Hultin measured omega-3 oxidation in Atlantic mackerel and cod that had been refrigerated for up to 10 days, frozen up to eight months, pan fried, broiled or baked. In no case did measurable levels of their lipids oxidize, he reports. However, once the fish was minced, oxidation occurred rapidly. Hultin says this suggests that oxidation-promoting chemicals are sequestered in fish tissue until physically liberated, as in mincing.

Though it's widely assumed that fatty oceanic fish provide the best source of omega-3 fatty acids, food scientist Paul Addis has identified comparable fresh-water alternatives. A cold environment is one factor causing fish to accumulate omega-3s in their tissue, notes Addis, who is at the University of Minnesota in St. Paul. Because Lake Superior "borders on being classified an arctic lake," Addis says, he investigated omega-3 levels in its inhabitants.

He studied lean lake trout, deep-water lake trout, whitefish, chub, smelt, burbot, sucker and lake herring. All but the burbot and smelt were as rich in these fatty acids as the Chinook salmon, a fish renowned for its omega-3 levels. Far and away the leader was the deep-water lake trout. Depending on size, this species contained up to three times as much omega-3 oil as the Chinook salmon.