By way of Gordon Byrn I found a very interesting article in The New York Times. It is basically the history of the science of food and a look at the politics of the American recommended diet. Because politics distort the science, and the science is obviously incomplete. Read it.
Then read “The Paleo Diet”, and if you're just a little athletic or just interested in knowing more, read “The Paleo Diet for Athletes”, they're sensible books built on common sense and what seems to me to be good, non-reductionist science, and can be boiled down to this: eat lean meat, fruit and vegetables. Avoid legumes, starchy foods (rice, pasta, potatoes), dairy and salt.
And if you're an athlete, starch and sugars are beneficial just before, during and shortly after training and racing.
That's the “how”, the “why” is somewhat more blurry.