The American Dietetic Association is the world’s largest organization of nutrition professionals. This month, they released an updated version of their position paper on vegetarian diets.
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Here’s an excerpt. You can read the entire paper online.

It is the position of the American Dietetic Association that appropriately planned vegetarian diets, including total vegetarian or vegan diets, are healthful, nutritionally adequate, and may provide health benefits in the prevention and treatment of certain diseases. Well-planned vegetarian diets are appropriate for individuals during all stages of the life cycle, including pregnancy, lactation, infancy, childhood, and adolescence, and for athletes.