@aviona
Yes one can get by with very close substitutes but the fact of the matter is that we are omnivores and designed to eat a variety of different substances. Humans have canines and molars designed for both tearing flesh and masticating vegetable matter. Human canines are not as pronounced as other mammals but if we look to our closest relatives the bonobo chimps one can see a template closest to our species. They will catch and eat small mammals, mice, birds, insects but unlike the common chimp rarely cannibalize each other for meat. Chimps also eat vegetable matter all day. Certainly the cannibalism lobby is not one we need to support in Washington, and if we were to eat the carbohydrates that chimps consume we would have garbage cans full of fruits and vegetables for our daily fill. Still, the best sources of protein are animal sources-mainly fish, eggs and lean meats. Also the level of protein humans consume may be extreme, but for the health of the body it’s best to consume some and certainly more than our closest relatives. And if one is to be a vegetarian it is vital to eat a variety of different proteins which is rarely done.
Animal sources of protein tend to be complete. Other protein sources lack one or more “essential” amino acids—that is, amino acids that the body can’t make from scratch or create by modifying another amino acid. Called incomplete proteins, these usually come from fruits, vegetables, grains, and nuts. (Harvard Scool of Public Health)
Furthermore there is growing evidence of the nature of high protein, low carbohydrate diets to lose weight. I can definitely testify to that I find high protein, low carb diets personally work best Certainly the types of proteins we consume is also significant.