The best site I've found that argues for a natural diet is ecologos.org. It's not advertising anything. It's just one person on a mission to tell the world the truth about what diet is natural for humans. It's entertaining if you care about what is true, regardless of having to bend your mental habits and tastes a little to appreciate correctness and correction rather than only sloppy thinking.
Many products and diet-related sites such as mercola.com and various blogs and forums use the word "natural" a lot, but mean food that has a little more to do with what occurs in nature than with what occurs by high technology, not whether it's natural for humans to get that food. Ecologos draws a clear distinction between "natural" and "cultural," putting more on the cultural side than less observant and less logical thinkers do, and has an argument for a natural diet consistent with that.
Some advocates of a meat-eating "Paleodiet" argue that in the Paleolithic, humans ate meat to survive, so that's what humans adapted to eating, by evolution, and what we should eat now. All meat eating by humans is culture, not nature. It involves using tools and cooking, which is how we get the archaeological evidence that it was done. Tools and cooking are culture and artifice, and without them killing and eating wild game is one of the hardest things a human could do, totally against instinct, and quite dangerous. Any meat eater who doesn't recognize that meat eating is artificial and cultural, requiring tools and training, is living in a fantasy world. Meat eaters who are realistic and concerned with preparedness for wilderness survival would agree that tools and training for hunting, cleaning, and cooking meat must be emphasized, or else faced with a live wild beast or a stinking carcass, eating meat in any quantity is not going to happen. Especially not if there's fruit on a tree nearby, and the human hasn't been properly acculturated to think "Fruit is bad for you, it has too much sugar," but instead follows his or her instincts to eat what looks, smells, and tastes good, and is easy to get by hand.
It is a great fallacy to believe that whatever humans have done culturally for a while, we're adapted to doing at a basic biological level. The equivalent fallacy is to believe that whatever we are starting to do, we will soon adapt to doing, by miraculous seeming powers of evolution to cause extremely unlikely, for all we know impossible abilities, for no particular reason. After all: "Evolution is cleverer than you are." (Orgel's Second Rule) So, humans will soon be adapted to breathing in the vacuum of space, because humans are spacewalking on a regular basis. At least humans will soon lose some of our toes, becoming more like horses, because we've been wearing shoes for a few hundred or thousand years. That one was what one of my sisters heard from a biology teacher, who believed it.
Species live well on foods and in conditions that satisfy their biological nutritional needs. If they can live on artificial food or in artificial conditions for a while, food and conditions adjusted to let them survive, that doesn't cause their biological needs to change. No selection pressure means no evolution by selection.
Rational culture should aim to fit human biological needs, not to excuse irrational, unexamined culture with rationalizations of evolutionary adaptation. Dietary culture is rapidly changing, not a stable tradition, because people know we are unhealthy compared with what's possible and we were worse in health before modern conveniences such as refrigeration. Therefore, it's absurd to rationalize cultural diets as something we're adapted to eating, when they certainly evolve many times faster than humans hypothetically could.