Meat-based diets were crucial to human brain development. Vegan and vegetarian diets can lead to underdevelopment of the brain and reduced intelligence. An overly restrictive diet can do more harm than good. The nutritional basis for human brain development for at least 3 million years has been the meat of hunted and, for about 10,000 thousand years, raised animals. Its irreplaceable nutrients provided our ancestors with energy and building blocks for the development of the brain of rational man (Homo sapiens). The results of modern research indicate that restrictive vegan and vegetarian diets are responsible for underdevelopment of the brain, reduced intelligence and poor mental health. Exercise extreme caution especially in feeding children is also recommended by medical experts.