When I ask this question I am usually referred to Gary Taubes. Also see Wikipedia. I don't have time to evaluate the evidence, but I'm pretty skeptical of nutrition science in general.
Edit: Eliezer has spent a few years doing the only thing you can do: try a bunch of diets on yourself and measure the results.
Michael Rae--Aubrey de Grey's co-author and the most knowledgeable person about matters of nutrition and supplementation that I know--doesn't have a high pinion of Gary Taubes. Most of his remarks are spread over several messages on the Calorie Restriction Society mailing list; here's a representative sample:
...More Calories cause people to gain weight and develop metabolic disruptions. Nothing Taubes presents meaningfully challenges this. And even if loading up on lard really didn't cause one to gain weight and doing the same with green veg (or even Wonde
I recently came out against paleo in the open thread, and realized that I probably haven't yet heard the strongest arguments in favor of a paleo diet. So, what are said arguments?
EDIT: Or more generally, why should I eat less carbohydrates and more protein / fat?