Our digestive system and anatomy are similar to those of our ancestors, the primates. Eating food that is not meant for our anatomy can cause disease. If you were to compare the anatomy of human beings and those of other classes of animals, you would find that we are anatomically herbivorous. For example, our teeth, mostly molars and premolars, are useful for eating plant-based foods, while carnivores have large canines for the purpose of tearing flesh. We also do not have claws or teeth to kill the prey with.
If we eat the food nature has designed for us to eat, it is easy to steer clear of diseases. This means that whole, unrefined, natural, plant-based foods will help us keep the doctor away throughout our lifespan.