This is just too simplistic. Eating a vegan diet really doesn't automatically = malnourishment any more than driving a car means you'll die in a car accident. Any person who is not eating a properly balanced diet is going to eventually feel like shit because of that? Whilst that is possible if a person decides to go vegan and just achieves that by cutting out whole food groups (usually proteins and fats) without replacing them with anything. This is actually common enough especially if the motivation is really weight loss rather than a longer term lifestyle change. But the vegan diet isn't an issue if it is balanced, the average silverback gorilla is hardly lacking in testosterone despite almost entirely plant based diet is it? And there is a huge list of elite athletes who eat this way. If you just make sure you get the balance of good fats, proteins, carbohydrates and nutrients for your lifestyle and activity levels right you'll be fine. If you do that and choose plants then you get to be even more than fine - you'll be helping avoid environmental disaster, missing out on all those nasty diseases that vegans just don't get and becoming a more compassionate being.