Once again, a study offers evidence that one of the keys to good nutrition is avoiding excess.

Theodore M. Brasky, PhD, of the Ohio State University Comprehensive Cancer Center in Columbus, and colleagues recently reported in the Journal of the National Cancer Institute that men who had the highest intake of omega-3 fatty acids had a 44% and 71% increased risk of low- and high-grade prostate cancer (PCa), respectively, compared with men who had the lowest intake.

Intake of omega-3 fatty acids, either by eating fish rich in these fatty acids (such as salmon or mackerel) or by taking fish oil supplements, have been widely advocated as a way to lower the risk of cardiovascular disease and cancer. But the new study underscores that the human body needs nutrients in the right amounts for proper functioning, and exceeding these amounts either offers no additional health benefit or could even be harmful.

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Vitamins are a good (but hardly the only) example. The study by Dr. Brasky’s group looked at men who participated in SELECT (Selenium and Vitamin E Cancer Prevention Trial), which previously had shown that men who took vitamin E supplements had an elevated PCa risk compared with those who took placebo after seven years of follow-up. Earlier investigations had suggested that vitamin E had a protective effect.

And consider niacin, a water-soluble vitamin needed for proper metabolic functioning. It is used to lower levels of low-density lipoprotein and raise levels of high-density lipoprotein, and has been shown to decrease cardiovascular event risk. Within a certain dose range, niacin keeps the body healthy. But too much niacin can cause liver damage and other adverse effects.

Then there is magnesium, a mineral vital to a number of bodily functions. Excessive intake of magnesium supplements can cause nausea, muscle seizures, and abdominal spasms.

In the U.S., there seems to be a widespread perception that, with respect to nutrition, if something is good for you, then more of it is even better or at least will not do any harm. In light of these recent studies, clinicians who counsel patients about embracing a healthy lifestyle may want to issue a caution about excessive intake of any food or supplement, even those high on the list of healthy items.