Revealing the finicky functions of fish oil.

Revealing the finicky functions of fish oil

Fats in fish oil exert varied and often conflicting effects on heart-disease risk factors ranging from clotting to cholesterol. At this week's meeting in New Orleans of the Federation of American Societies of Experimental Biology, scientists offered explanations for several of the oil's more peculiar properties.

In a study of fish oil's anti-clotting action, scientists have now demonstrated in rats that the effect depends on the dosage of fish oil in relation to other kinds of polyunsaturated fats -- not the absolute amount of fish oil consumed. If confirmed in humans, the finding may lead to recommendations on how much of the different kinds of polyunsaturated fats people should consume, say Prithiva Chanmugam, Daniel H. Hwang and Mary Boudreaux, nutrition scientists at Pennington Biomedical Research Center in Baton Rouge, La.

In their work, the researchers looked at the interaction between omega-3 fatty acids -- the active ingredient in fish oil -- and omega-6 fatty acids, in this case derived from safflower oil. Research by others had shown that omega-3s may decrease the risk of artery-blocking clots by suppressing the synthesis of clot-promoting chemicals such as arachidonic acid (an omega-6 fatty acid) and its metabolites. And Chanmugam says she and her co-workers had demonstrated in rats that higher doses of omega-3s enhance the anti-clotting effect if dietary omega-6 levels are held constant. In the group's most recent study, groups of rats ate diets containing three different levels of omega-3s for 16 weeks while their omega-6 intake was altered to maintain a constant dietary ratio of the two fatty acid types. Chanmugam found that the rats' blood platelets made similar amounts of anti-clotting chemicals. She suggests it may be easier to obtain a favorable ratio by controlling intake of both fatty acid types.

Other work presented at the meeting begins to explain some of fish oil's odd effects on fat and cholesterol synthesis. Past experiments have shown that while fish oil seems to markedly decrease the amounts of triglycerides and very-low-density lipoprotein (VLDL) cholesterol in human blood, it does not decrease the level of low-density lipoprotein (LDL) cholesterol, the type linked to coronary artery disease. In fact, it seems to increase LDL cholesterol levels in some patients (SN: 11/28/87, p. 342). This has puzzled scientists because LDL particles are derived from VLDL particles. But new data suggest an explanation, reports nutritional biochemist William Harris of the University of Kansas Medical Center in Kansas City.

His study indicates that fish oil speeds LDL synthesis by changing the size and chemical composition of certain VLDL particles made by liver cells. Liver cells make VLDL particles of varying sizes, and the smaller ones seem more readily converted to LDL. "We knew that fish oils changed the [overall] output of VLDL and we hypothesized that they might change the size," Harris says.

He and co-worker Masahiro Inagaki gave six men with high triglycerides 12 fish oil capsules per day for four weeks, measuring lipid levels before and after treatment. The fish oil not only prompted a marked decrease in the patients' blood triglycerides and VLDL cholesterol, but also changed the size and composition of VLDL particles. Using a vertical column that separates particles by size, the researches found that fish oil treatment caused liver cells to make fewer large VLDL particles and more small ones. Further chemical analysis revealed that the particles shrank because their lipid content had dropped by more than 50 percent, while their protein content stayed the same. Harris hypothesizes that LDL levels rise because the liver makes more of the smaller, protein-rich VLDL particles, which convert to LDL more readily than do larger, lipid-rich VLDL particles.