Kaiser Permanente, the largest HMO in the United States, is now recommending that people follow a plant-based diet.
The United States Department of Agriculture
The United States Department of Agriculture (USDA) has guidelines for vegan diets on p. 82 of their Dietary Guidelines for Americans.
They also have an extensive list of nutrition resources.
Academy of Nutrition and Dietetics (formerly known as American Dietetic Association)
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.