Contradicting previous findings, California researchers say the manner in which fish oil protects against heart disease apparently does not hinge on lowering the amount of cholesterol in the blood.

Drs. Dennis Davidson and Kurt Gold of the University of California, Irvine, studied 70 medical students and found low doses of fish oil had no effect on the levels of cholesterol or triglycerides in their blood.

"We cannot exclude other . . . effects of low-dose fish-oil consumption, but we do conclude that one or two capsules (of typical commercial concentration) do not significantly alter the lipid and lipoprotein levels in healthy, normal adults," the researchers wrote in a letter published recently in The New England Journal of Medicine.

In the study, students either received no fish oil, one capsule or two capsules of fish oil and had their blood tested for the levels of overall cholesterol, triglycerides, low-density lipoproteins and high-density lipoproteins.

Dr. Alexander Leaf of Massachusetts General Hospital in Boston noted in an accompanying letter that the beneficial effects of the fatty acids in fish oil may be to prevent changes in the walls of blood vessels that lead to fatty buildups.

In another letter, meanwhile, Dr. Colin Granger of Warner-Lambert Co., in Morris Plains, N.J., wrote that an analysis of the types of fatty acids found in the types of fish Americans eat most commonly indicates the fish contain too little of the fats believed to be beneficial. "Although it is possible that small amounts of . . . fatty acids consumed over long periods may afford the same biologic effects as high doses consumed over short periods, one must seriously question whether in the absence of a fish-oil supplement, the average North American is ingesting sufficient quantities of . . . fatty acids from fish to have any important and realistic health benefits," he wrote.