BETHESDA, MARYLAND. There is considerable evidence that fish and fish oils are beneficial to heart health, reduce the risk of cancer, and benefit mental health. The “active” components of fish oils are eicosapentaenoic acid (EPA), a polyunsaturated fatty acid with 20 carbon atoms in its backbone, and docosahexaenoic acid (DHA), a polyunsaturated fatty acid with 22 carbon atoms. Both are members of the omega-3 group of essential fatty acids. EPA and DHA are found exclusively in marine animals; fatty fish such as herring, sardines, salmon and fresh tuna are the best sources.
Alpha-linolenic acid (ALA) is another omega-3 fatty acid found in flaxseed and flaxseed oil. ALA has 18 carbon atoms in its backbone and can be converted to EPA in the body (in the liver) by the addition of two carbon atoms. EPA, in turn, can be converted to DHA. Because the typical American diet is relatively low in fish intake ALA becomes a crucial source of the EPA and DHA required for optimum health.
Researchers at the National Institutes of Health have just completed a study designed to determine just how much ALA is actually converted to EPA in the body. Their study included eight healthy subjects who were fed a standard diet for three weeks and then given one gram of ALA labeled with an isotope tracer. The diet was beef-based in order to avoid extraneous sources of EPA and DHA. The researchers measured blood plasma concentrations of ALA, EPA and DHA 8, 24, 48, 72, 96 and 168 hours after ingestion of the labeled ALA.
The results show that only about 0.2 per cent of the ALA (2 mg) was actually converted to EPA. In contrast, about 23 per cent of the EPA was available for conversion to DHA. The researchers also noted that the half-life (the time it takes to reduce initial concentration by 50 per cent) of ALA in blood plasma was quite low at about one hour. In comparison, the half-life of EPA was 67 hours and that of DHA 20 hours.
The researchers conclude that ALA is not a viable source of EPA and DHA and cannot replace fish and fish oils in the diet.
Pawlosky, Robert J. Physiological compartmental analysis of alpha-linolenic acid metabolism in adult humans. Journal of Lipid Research, Vol. 42, August 2001, pp. 1257-65