Vegans need to supliment a lot of their diet, especially vitamin B12 and protein

Vegans don't need to supplement protein. It's easy to get enough protein as long as you eat enough.

There are hardly any vegan athletes.

There are hardly any vegans, hence there are hardly any vegan athletes. Some of the world's top athletes in their sports have been vegan, such as Carl Lewis in track and Lewis Hamilton in auto racing.

Also, what about people who eat nothing besides meat? Like the Masaai and the Inuits?

The Inuit have lifespans 12 to 15 years shorter than the average Canadian's, which is thought to be influenced by factors such as their diet.