Nutritional evidence suggests that a vegan diet is the most adaptive one for humans. An ethical principle based on following our biological nature (naturalistic ethic) could therefore provide additional support for a vegan diet. However, some argue that humans in the natural world could not eat a vegan diet, since it relies on supplements, particularly vitamin B12. This leads to the conclusion that humans are naturally omnivores, and therefore our natural diet should include small amounts of animal products.

Three approaches to this conclusion are discussed. The first rejects a naturalistic ethic in favour of normative principles based on animal sentience. The second expands the definition of what is natural and argues that there is nothing unnatural about taking supplements.

The third approach maintains a stronger naturalistic claim that the vegan diet is both completely natural and is the most adaptive for optimising human health. This can be used as the basis for vegan advocacy. It can also encourage a research programme to fill the gaps in our nutritional knowledge.