As you know, vegans consume only plant foods. Not only are meat and fish off limits for vegans, but also eggs, dairy, honey, gelatin, and any and all other foods that involve animals in any way. It’s a very restrictive diet, but at least you can eat your spinach cooked.
Slightly more sustainable than strict veganism for the average person is a lacto-ovo vegetarian diet, which allows egg and dairy consumption. But are lacto-ovo vegetarians less healthy than vegans? In some ways yes, in others now. Studies have shown that vegans tend to be leaner than lacto-ovo vegetarians, but they also have lower bone mineral density and less muscle mass in relation to bodyweight, which is less desirable for athletes. Overall, it’s a wash.