Human beings evolved consuming a diet that contained approximately equal amounts of omega-6 and omega-3 essential fatty acids. Over the past 100-150 years there has been an enormous increase in the consumption of omega-6 fatty acids due to the increased intake of vegetable oils from seeds of corn, sunflower, safflower, cotton and soybeans. Today, in Western diets, the ratio of omega-6 to omega-3 fatty acids ranges from 10 to 20:1 instead of the traditional range of 1-2:1. Studies indicate that a high intake of omega-6 fatty acids shifts the physiologic state to one that is prothrombotic and proaggregatory, characterized by increases in blood viscosity, vasospasm, and vasoconstriction and decreases in bleeding time, whereas omega-3 fatty acids have anti-inflammatory, antithrombotic, anti-arrhythmic, hypolipidemic, and vasodilatory properties. These beneficial effects of omega-3 fatty acids have been shown in the secondary prevention of coronary heart disease and hypertension, as for example, in the Lyon Heart Study, the GISSI Prevenzione Trial, and in the The Dietary Approaches to Stop Hypertension Study. Most of the studies have been carried out with fish oils (eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)). However, alpha-linolenic acid (ALA), found in green leafy vegetables, flaxseed, rapeseed, and walnuts, desaturates and elongates in the body to EPA and DHA and by itself may have beneficial effects in health and in the control of chronic diseases. The present paper identifies multiple sources of ALA from plants, legumes, nuts and seeds and emphasizes the importance of the ratio of omega-6 to omega-3 fatty acids for proper desaturation and elongation of ALA into EPA and DHA. alpha-linolenic acid is not. equivalent in its biological effects to the long-chain omega-3 fatty acids found in marine oils. Eicosapentaenoic acid and DHA are more rapidly incorporated into plasma and membrane lipids and produce more rapid effects than does ALA. Relatively large reserves of linoleic acid in body fat, as are found in vegans or in the diet of omnivores in Western societies, would tend to slow down the formation of long-chain omega-3 fatty acids from ALA. Therefore, the role of ALA in human nutrition becomes important in terms of long-term dietary intake. One advantage of the consumption of ALA over omega-3 fatty acids from fish is that the problem of insufficient vitamin E intake does not exist with high intake of ALA from plant sources.
