The term diet has been spun into something that it is not. More often than not if one says 'Diet' it is seen as a temporary or fad like thing in order to lose weight or augment your lifestyle for temporary gain. Diet in its definition means the continual eating habits of a creature. So, we know cows mostly eat grass and that is their diet. Predators are carnivores and subsist mostly on meats. Somehow a diet has become misconstrued over the years as something else. It is now a marketable term and not true to its meaning. So, with trepidation, I often say I am on the Paleo diet and it is misunderstood as something akin to the Atkins diet for meat eaters or that I only consume what cavemen ate which is wholly untrue and often leads to crooked eyebrows or being told I don't need to be on a diet, that eating red meats will kill me, or some other myth perpetuated by misguided information. This mental programming of the word diet along with the 'diets' in question is part of the problem with health and nutrition. Diet is part of your lifestyle and what you can handle ingesting on a continual basis and what your body should be fed habitually.
We as humans don't really have a solidified diet per se according to nature. There are things which we should and should not eat but we have a very robust system that allows us both meats and vegetables and some in-between items. There are a lot of things that we cannot adapt to though and should be removed from the body altogether in order to achieve optimal energy levels and ease digestion. We can eat and pass pennies but that doesn't mean it is good for our body and you may or may not have adverse affects from doing so. The same can be said for gluten, legumes, dairy, sugar etc. all of which can have adverse affects, hidden or visible.