What are omega-3 fatty acids?

Omega-3 fatty acids are long-chain polyunsaturated fatty acids (18–22 carbon atoms in chain length) with the first of many double bonds beginning with the third carbon atom (when counting from the methyl end of the fatty acid molecule). The fish-based and fish-oil–based omega-3 polyunsaturated fatty acids (also referred to as n-3 PUFA) consist of EPA (20 carbon atoms, 5 double bonds) and DHA (22 carbon atoms, 6 double bonds). Whereas plant foods and vegetable oils lack EPA and DHA, some do contain varying amounts of the n-3 PUFA alpha-linolenic acid (ALA), which has 18 carbon atoms and 3 double bonds.

Many vegetable oils are greatly enriched in omega-6 fatty acids (mainly as linoleic acid in corn, safflower, sunflower and soybean oils), but canola oil (nonhydrogenated), ground flaxseed and walnuts are rich sources of ALA. The typical North American diet provides about 1–3 g of ALA per day but only 0.10–0.15 g of EPA plus DHA per day. The very high intake of n-6 PUFA, mostly as linoleic acid (LA) in our diet (12–15 g/day) from common vegetable oils (corn, safflower, soybean) and other sources, yields an overall n-6:n-3 dietary ratio (total omega-6 fatty acids in the diet: total omega-3 fatty acids in the diet) of about 8:1. Health Canada has recommended that this ratio be as low as 4:1 to reduce the competitive influence of high LA intakes on ALA metabolism to its longer chain products (such as EPA and DHA).

Although high intakes of LA can provide some modest blood cholesterol lowering, experimental studies in animals have raised concerns regarding the enhancing effect of these high intakes on certain cancers. This association has not been established in human studies.

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