I wrote this as a letter to the editor of Mother Earth News, which is a generally good publication that sometimes publishes bad dietary advice:
In The Fats You Need for a Healthy Diet (August/September 2011 of Mother Earth News), Oscar H. Will, III, provides dangerously misleading dietary advice. Saturated fat does not “do a body good.” You don’t need to get any saturated, monounsaturated, or trans fatty acids whatsoever from the diet. Only two fatty acids are essential, which means that they must come from the food. One is an omega-6 fatty acid called linoleic acid. The other is an omega-3 fatty acid called alpha-linolenic acid. However, the dietary requirement for these fatty acids is so small that you can find true cases of deficiency only in extreme situations, such as people who were being fed nothing but sugar intravenously. For those patients, the requirement for essential fatty acids could be met by rubbing a small amount of vegetable oil on the skin. Fat deficiency is practically nonexistent because even a diet based on low-fat grains and vegetables provides enough of the essential fatty acids
The usual problem is that the person is eating too much fat. Excess fat of any kind promotes obesity, atherosclerosis, diabetes, and other chronic diseases. The omega-3 fatty acids tend to have a blood-thinning effect, which offsets some of the effect of their contribution to atherosclerosis. An excess of the polyunsaturated fatty acids (omega-3 and omega-6) has been linked to an increased risk of cancer, possibly because of their effect on the immune system.
Even vegetarians and vegans often eat too much fat, and they tend to eat a disproportionately large amount of omega-6 relative to omega-3 fatty acids because of a large intake of nuts and oils. The obvious solution to this problem is to restrict the overall fat intake and add a small amount of ground flaxseed to the diet. Flaxseed is an excellent source of the relatively scarce omega-3 fatty acids.
Conjugated linolenic acid is found almost exclusively in foods of animal origin. Yet nutritional epidemiology studies show that those foods promote the sorts of diseases that the dairy industry is claiming that conjugated linolenic acid is supposed to help prevent.