Kaiser Permanente
Kaiser Permanente, the largest HMO in the United States, is now recommending that people follow a plant-based diet.
http://share.kaiserpermanente.org/wp-content/uploads/2015/10/The-Plant-Based-Diet-booklet.pdf
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.
http://www.cnpp.usda.gov/sites/default/files/dietary_guidelines_for_americans/PolicyDoc.pdf
They also have an extensive list of nutrition resources.
http://www.choosemyplate.gov/sites/default/files/printablematerials/VegetarianNutritionResourceList.pdf
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.
http://www.ncbi.nlm.nih.gov/pubmed/19562864
Other organizations from around the world have affirmed that vegan diets are healthy
The Mayo Clinic
Dietitians of Canada
The British National Health Service
http://www.nhs.uk/Livewell/Vegetarianhealth/Pages/Vegandiets.aspx
The British Nutrition Foundation
http://www.nutrition.org.uk/publications/briefingpapers/vegetarian-nutrition
The Dietitians Association of Australia
http://daa.asn.au/for-the-public/smart-eating-for-you/nutrition-a-z/vegan-diets/