The protein synthesis process required to build muscle mass depends on the presence of certain amino acids in particular BCAAs, Arginine and Lysine. While veganism is a healthy practice for detoxing, as well as fighting cancer and other diseases, it can be detrimental to the objective of bodybuilders as protein rich vegan foods lack the aforementioned amino acids forcing the muscles to eat themselves and shrink in size.
Another discouraging aspect of a vegan based diet that would concern men in particular is one of the most dominant and prominent vegan protein sources: soybean! Soybean promotes higher estrogen levels. While estrogen (the female hormone) and testosterone (the male hormone) exist in both males and females, the amounts of both hormones as well as their ratio to one another influence sexuality, appearance and gender specific health.
Elevated estrogen levels suppress the production of testosterone that is required for healthy muscle building other than the undesired effect it has on sexual performance including erectile dysfunction as well as appearance in males, such as enlarged breasts and flaccid penis.
Females who want to trim down or stay in shape can benefit much more than males from a vegan-based diet as derived from above.