There are a few other ways to look at it. Take animals, for instance.
Animals, both wild and domestic, that eat only plant matter, are round and sometimes fat. Think of what cattle look like. But animals which only eat meat tend to be lean. This analogy does not extend to fish or whales, of course. Just don't read too much into that. Cooking doesn't make any difference on this point.
Humans, on the other hand, have evolved to eat almost anything that other animals eat and at the same time, many foods have been domesticated (evolved, you might say) to be suitable for humans. It's a big world, too, and the variations are almost endless. There are still many things that humans cannot digest, even if you're surrounded by it (like grass) nor can most other animals. Culture also plays a part. Also, few, if any, animals that we eat are normally meat-eaters, curiously. But it's also worth mentioning that we do not all have the same nutritional requirements. Few people do manual labor like they used to, so we generally do not require high calorie diets like some people used to. And of course, some people do no work whatsoever.
However, nutrition-wise, the basic problem is simply getting enough to eat in the first place. That part is easily assumed away but it remains a problem for people in some places. Getting enough of the right vitamins is an issue on a higher level, beyond getting enough calories. That has sometimes been a problem in this country, too, which we forget. Personally, I don't think there's anything wrong with fast food, but I also worked my way through college working in both a sub shop and a hamburger place. I know where the food comes from and there's nothing wrong with it.
In responding to a post made as I was writing this one, let me say something about processed food. Some very traditional foods are processed more than you might think. And some very basic food stuffs did not occur naturally. They're part of what has been developed in the last five or ten thousand years, during which time humans have scarcely changed. It is true, however, that great advances have been made in New World foods since Europeans got here, foods that had not been consumed in the rest of the world. Even more recently, some cultural prejudices had to be overcome before certain foods were considered suitable for humans. In parts of Europe, for example, corn (maize, that is, 'Indian corn') was considered fit only for cattle.
