I looked seriously at veganism after reading a WHO report that attributes 13% of world greenhouse gas emissions to animal farming. [Strike that. The figure is 18%, and the source is a 2006 FAO study sponsored by the World Bank and the European Union, among others.] I built a database from the USDA's and whatever credible recommendations for daily micronutrition I could find, and then I built a diet around that database. All recommended micronutrients can be had from non-meat, non-dairy foods except two: it is not possible to be a healthy vegan without also taking choline and vitamin B12 supplements.
An athlete would face extra challenges. Tudor Bompa cites research showing athletes tend to be deficient in important micronutrients compared to the general population, likely because of the stresses of training. And though the position paper on sports nutrition published by the world's most important scientific journal on nutrition recommends that athletes consume up to 1.4g of protein per 1kg of body mass per day, for a vegan athlete this means consuming approximately 1kg to 2kg of beans a day, or even more if the food is less protein-dense than beans. It's hard to eat that many beans.
Sources:
Livestock's Long Shadow: Environmental issues and options, p.112, Food and Agriculture Organization, 2006
United States Department of Agriculture Food Composition Databases
Tudor S. Bompa, Periodic Training for Sports, 2015
"Nutrition and Athletic Performance", Dietitians of Canada, the Academy of Nutrition and Dietetics, the American College of Sports Medicine, December 2016
An athlete would face extra challenges. Tudor Bompa cites research showing athletes tend to be deficient in important micronutrients compared to the general population, likely because of the stresses of training. And though the position paper on sports nutrition published by the world's most important scientific journal on nutrition recommends that athletes consume up to 1.4g of protein per 1kg of body mass per day, for a vegan athlete this means consuming approximately 1kg to 2kg of beans a day, or even more if the food is less protein-dense than beans. It's hard to eat that many beans.
Sources:
Livestock's Long Shadow: Environmental issues and options, p.112, Food and Agriculture Organization, 2006
United States Department of Agriculture Food Composition Databases
Tudor S. Bompa, Periodic Training for Sports, 2015
"Nutrition and Athletic Performance", Dietitians of Canada, the Academy of Nutrition and Dietetics, the American College of Sports Medicine, December 2016