Like many people, I take omega-3 fatty acids every day. I began doing so more than 20 years ago when I was completing my residency in internal medicine at the Massachusetts General Hospital of Harvard Medical School. Dr. Alexander Leaf, who was then chief of medicine and an inspiring mentor, had conducted pioneering research documenting the extraordinary health benefits of these fatty acids.

In the past few months, however, new studies have raised questions about whether the benefits are universal. The latest research is indicating that, for some people, the omega-3 fatty acids may actually be harmful, perhaps even lethal. Omega-3 fatty acids are found in cold-water fish (mackerel, herring, salmon, trout, sardines and albacore tuna), as well as oils from canola, soybean, flaxseed and walnuts. (In contrast, olive oil does not contain much of the omega-3 fatty acids.) In smaller concentrations, they are present in dark green leafy vegetables like kale and collard greens.

Dr. Leaf's studies, confirmed by those of others, found that consuming omega-3 fatty acids on a regular basis may reduce the risk of sudden cardiac death (which is as bad as it sounds) by 42 percent to as much as 90 percent by stabilizing the rhythm of your heart. Because of this, the American Heart Association recommends at least two servings of fish per week.
The omega-3 fatty acids may reduce triglycerides (a form of fat), lower blood pressure and decrease inflammation (thereby reducing arthritis and other inflammatory illnesses), as well as autoimmune diseases such as lupus. They can help to prevent excessive blood clots from forming which, in turn, may decrease the risk of a heart attack and stroke. They may help prevent irregular heartbeats such as atrial fibrillation. 

When given to pregnant women and lactating mothers, omega-3 fatty acids (which are an important part of your brain) may actually increase your baby's IQ by six points or more and may reduce the incidence of allergic disease in the offspring. They also may reduce depression and may help prevent dementia. Some studies suggest that the omega-3 fatty acids may even reduce the risk of prostate cancer, breast cancer, and colon cancer.

Given these amazing benefits, I've been taking fish oil for many years and have been advising just about everyone else to do the same. I take fish-oil capsules (3 grams per day) in which pollutants such as mercury, dioxin and PCBs that are often found in fish have been removed. This gives all the benefits of the omega-3 fatty acids without the extra fat, calories and pollutants that come from eating fish.

However, like many wonderful things in life, there is also a dark side to the omega-3 fatty acids. A recent British Medical Journal analysis of nearly 100 studies of omega-3 fatty acids found mixed benefits. In most people they were beneficial, but not in everyone. Some people actually got worse.