Just tell them that humans are OMNIVORES! We are DESIGNED to eat both plant and animal substances.
I have a friend who was a vegetarian for many years. She had all kinds of stomach issues during that time. When she got pregnant, she started eating meat. She's healthy as a horse now.
I don't think it has much to do with the diet itself. It's a 'restricted' diet and it's more psychological than anything else. Hence, why I'll never attend PETA meetings! There are concentrated amounts of veg posers.