Troll the medical literature, and you'll come up with study after study showing that fish and fish oil are good for us, especially for our hearts but maybe also for our moods and immune systems. Various epidemiologic investigations have found that people who eat fish regularly are less likely to have heart attacks, suffer strokes, or die from sudden cardiac arrest. The definition of "regularly" varies, but it usually means at least a couple of times a week.
Fish, and especially fish oil, have also been the subject of clinical trials, most involving people with existing heart conditions. In large amounts (several grams a day), fish oil has been shown to nudge various cardiac risk factors ("good" HDL cholesterol, triglycerides, blood pressure) in the right direction. Smaller amounts (a gram a day) work against irregular heart rhythms — particularly atrial fibrillation. The evidence doesn't line up perfectly, however. A review published in 2006 in the British Medical Journal came to the conclusion that fish oil doesn't have much benefit after all.
Getting fish oil into your diet can be difficult. Eating fish will certainly do it — if you feast on salmon, trout, mackerel, and other oily species. A three-ounce serving of those fish supplies about a gram's worth. Fish oil supplements may be more practical. But the capsules are large, and side effects are a problem.
There's also mercury contamination to think about. Mercury accumulates in the food chain, so some of the heaviest concentrations are found in some of the most desirable from the standpoint of fish-oil consumption. Fortunately, the supplements haven't been implicated as a source of mercury exposure.
The third omega-3
Flaxseed oil is being heavily promoted as an alternative to fish oil. The health benefits of fish oil are believed to derive principally from two omega-3 fats, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Flaxseed oil contains a third, plant-based omega-3, alpha-linolenic acid (ALA). Other foods (especially walnuts) and oils (canola and soybean, for example) contain ALA. But at about 7 grams per tablespoon, flaxseed oil is by far the richest source.
The main problem with ALA is that to have the good effects attributed to omega-3s, it must be converted into EPA and DHA. As a result, only a small fraction of it has omega-3's effects . So in terms of omega-3 "power," a tablespoon of flaxseed oil is worth about 700 milligrams (mg) of EPA and DHA. That's still more than the 300 mg of EPA and DHA in many 1-gram fish oil capsules, but far less than what the 7 grams listed on the label might imply.
A serving is six capsules
A better comparison might be between the fish oil and flaxseed oil capsules. The first thing to notice about flaxseed oil is the serving size. For the Spectrum brand, it's six capsules! A single fish oil capsule will give you the same amount of omega-3.
The bottom line
Flaxseed oil will give your diet a nice little omega-3 boost in the form of alpha-linolenic acid. But it's a backup, not a substitute, for the omega-3s in fish and fish oil because of the conversion factor. If you're in need of omega-3s but are concerned about mercury, fish oil capsules might be a good choice.
November 2006 Update