Every week we see new articles in the press and on the Internet, touting the benefits of a new "miracle food supplement," one that cures or prevents the diseases we're most afraid of. All of them seem to start with, "A recent study proves ," and go on to cite the supposedly "scientific" evidence for this new miracle supplement. One week it's Vitamin E to prevent heart attacks, and the next week it's Vitamin X to prevent disease X. The result is that stores selling vitamins and food supplements are now the size of supermarkets, and Americans spend over 20 billion dollars a year on them.
Is the scientific evidence for health supplements really scientific?
The problem with all of this is that these "scientific" studies more often than not don't "prove" anything. If you read the studies themselves, you find that their authors state very clearly that their studies were preliminary, possibly with inadequate controls or lasting for too short a period to make conclusive statements. But somehow these statements never make it into the articles you read in the press. Instead, their authors jump on phrases like "suggests a reduced risk of disease X" and turn it into "Vitamin X prevents disease X."
A comprehensive study of supplements undertaken by the U.S. Preventive Task Force found that there was "insufficient evidence to recommend taking them." Despite many of the claims in the press and by the companies selling the supplements, the "preliminary findings" remain preliminary, and no direct causal relationships can be proved for the efficacy of almost all of the supplements. Almost all. There is one supplement that stands out, and seems to have a legitimate body of scientific evidence backing its claims of being able to prevent certain diseases fish oil.
Why fish oil what makes it different?
The claims for the benefits of fish oil, which contains the omega-3 fatty acids eicosapentaenoic (EPA) and docosahexaenoic (DHA), has repeatedly been shown, in study after study, to reduce inflammations that may lead to heart attacks and stroke. The body of evidence on fish oil is so large that unlike other supplements it presents a "totality of evidence" that is pretty much undeniable. The American Heart Association considers the evidence conclusive enough that it recommends the consumption of one gram of fish oil daily, especially if your blood contains high levels of triglycerides, which increase your risk of heart attack and stroke.
Ideally, you could get these fish oils by eating fresh fish several times a week, but many people have concerns about mercury and other toxins being increasingly found in fresh fish. Fortunately, these toxins tend to accumulate in the tissue of the fish, and not in its oils; studies that measured over-the-counter brands of fish oil found that they contained negligible amounts of these substances. So taking a fish oil capsule every day is possibly better for you than eating fresh fish every day.
The strongest evidence for the benefits of fish oils is in the area of cardiovascular (heart) health. Other studies have strongly suggested (but not proved as of yet) that DHA is associated with a reduced risk of breast cancer, that EPA reduces the risk of depression, schizophrenia, and suicide, and that DHA may help to prevent cognitive problems associated with Alzheimer's disease. Some of the strongest research indicates that fish oil supplements may be particularly beneficial during pregnancy, both for the mother and the child.
What's the bad news?
There actually isn't very much. Fish oil isn't particularly fattening only 13 calories per gram, and taking up to 3 grams of it a day is safe for most people. You should not take fish oil supplements if you're allergic to fish or seafood, and you should consult your doctor before taking it if you have liver disease, diabetes, bipolar disorder, or are receiving chemotherapy treatments.
With most vitamins and food supplements, the most that science can really say about them definitively is that taking them in reasonable amounts may not help you very much, but they won't hurt you. With fish oil, taking it in reasonable amounts can definitely help.