"I think the two biggest public health messages are to supplement calcium with diet and keep your weight down, " said Dr. Rosen, explaining that excess weight can drive down vitamin D levels.

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Omega-3 fatty acids are the fourth most common supplement after multivitamins, calcium and vitamin C, according to the Council for Responsible Nutrition, an industry trade group.

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Rodriguez said people choosing a vegetarian diet have to be extra diligent to supplement their diet with iron, zinc and calcium.

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