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One of the points Lesley makes is that the idea of ‘nature’ is actually a cultural construct. What do we mean when we say something is ‘natural’? I think that, in general, we mean that it hasn’t been altered or intervened with in anyway. Which is completely impossible. Everything we do changes our body in some way. Not doing something changes our body in some other way. Everything you eat becomes a part of you. And if you don’t eat, well, that has other implications. Breathing air, drinking water, wearing clothes, walking, driving, sitting, standing, sleeping, all of these things alter the body in some way. The body is always in flux, and we can’t live without taking in things from our environment, things which change us. An unaltered body is, by definition, not alive. (This is highly influenced by a presentation I recently attended by Rachael Kendrick on metabolism, and while I’m sure I’m this is an obscene misappropriation of her argument, I found it very interesting. Kendrick isn’t always entirely fat-positive, but she does an excellent critique of medial science and obesity epidemic discourse.)

The ideal ‘natural’ body is also frequently invoked in anti-fat rhetoric, particularly in the figure of the ‘caveman’. In fact, some people call for a return to this way of eating (if not this way of living). The idea is that the human body is ideally suited to a palaeolithic lifestyle and that our digestive systems work best if we eat only foods that were around 2 million years ago, and avoid all that new-fangled stuff like ‘grains’ and ‘beans’. This idea basically harnesses the discourse of evolution in the service of what amounts to a creationist argument. It posits that the ideal human design was arrived at somewhere in the deep and distant past, and has remained constant ever since. It denies evolution as an ongoing process, and most importantly, ignores the fact that the caveman body was as much a product of its environment as the modern human body is.

I think "natural" can work as a hypothesis for making things better, but it's just a hypothesis, not a source of reliable truth.