In the UK, the official Reference Nutrient Intake tables, issued by the Department of Health, suggest a range of anywhere from 1:1 to 3:1 of omega-6 to omega-3.

In Canada, daily intake ratios range from 4:1 to 10:1 (Scientific Review Committee, Nutrition Recommendations, Ottawa: Minister of National Health and Welfare Canada, 1990). The joint committee of the Food Agriculture Organization/World Health Organization recommends a ratio between 5:1 and 10:1 (Nutr Rev, 1995; 53: 202-5).

The problem of determining sensible recommendations is, of course, further confounded by the fact that essential fatty acid needs may vary according to lifestyle as well as climate. A physically active person in a cold climate may require more fats of all kinds than a sedentary person living in a warmer climate.

Nevertheless, what is clear is that our diets are dangerously overbalanced towards omega-6, a problem that sets up a vicious cycle. Omega-6 and omega-3 fatty acids compete in the body for the same binding sites. The more omega-6, the less of the already meagre omega-3 in the diet will be absorbed (Am J Clin Nutr, 2000; 71: 179-88).

Problems of overabundance There is good evidence to suggest that EFAs are important for heart health, relief from allergies, optimal immunity and even protection from cancer. But equally good evidence links diets containing an excess of omega-6 in relation to omega-3 not only to heart disease, but also cancer and autoimmune disorders (Lancet, 1989; ii: 757-61; Nutr Cancer, 1987; 9: 205-16; Br J Rheumatol, 1997; 36: 513-5). A relative excess of omega-6 has also been linked to depression (Lipids, 1996; 31: S157-61).

Omega-6 fatty acids contribute to the production of prostaglandins, which can contribute to, among other conditions, inflammation of the airways. Indeed, a recent study showed that toddlers who consume large quantities of margarine and fried foods may be twice as likely to develop asthma as their peers who eat less of these foods (Thorax, 2001; 56: 589-95).

While we think of saturated fats as killers, a recent Swedish study involving 60,000 participants found that it was the consumption of vegetable oil, not saturated fat, that was most strongly linked with the development of breast cancer (Arch Intern Med, 1998; 158: 41).

Even more disturbing, given the myth of heart protection that has sprung up for polyunsaturated oils, atheromas - the first step toward atherosclerotic plaques - have been found to be made up of 74 per cent polyunsaturated fats and only 26 per cent saturated fats (Lancet, 1994; 344: 1195-6).

Omega-3s are generally considered more healthful; supplementation can benefit a range of inflammatory conditions such as Crohn’s, rheumatoid arthritis, kidney diseases and chronic obstructive pulmonary diseases (Gastroenterology, 1991; 100: A228; Arthritis Rheum, 1995; 38: 1107-14; J Am Soc Nephrol; 1999; 10: 1772-7; N Engl J Med, 1994; 331: 228-33). Omega-3s can also help Raynaud’s disease, immune function and cardiac arrhythmias (Am J Med, 1989; 86: 158-64; Nutrition, 1998; 14: 627-33; J Nutr, 1997; 127: 383-93). Fish oil can prevent cancer (Nutr Cancer, 1995; 24: 151-60), and low blood levels of omega-3 have been found in people with depression (Psychiatry Res, 1999; 85: 275-91).

But while fish oils may appear beneficial over the short term, longer-term studies have revealed a possible paradoxical effect.

In a small-scale study of 40 people given either placebo or increasing doses of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), early results showed the expected healthful effects - a rise in blood and red-blood-cell levels of both these fatty acids, and a decline in the more harmful arachidonic and linoleic acids (see box above).