I'm not in love with the food reward hypothesis, but since the PHD was forwarded by Mark and is on my radar here we go....they do postulate that this is the reason we gravitate toward a 50% diet of carbs (not recommended, but a reason):
"Those three evolutionary arguments were present in capsule form in the first edition, but the next one is entirely new to this edition: a discussion of the food reward system of the brain. Food reward evolved to motivate us to go seek out healthy, nourishing foods. If it evolved in the Paleolithic environment, then it tells us what foods were hard to get in the Paleolithic, but still necessary for health – these were the foods that Paleolithic foragers had to be motivated to work for.
But in our modern world, all foods are easy to obtain; they are on the shelves of supermarkets. So those foods that were hard to get in the Paleolithic are going to be precisely the ones that we overeat today.
The presence of an innate food reward system explains why peoples around the world eat very similar diets – almost always 50% carb, 15% protein, 35% fat, or close to it. If this food reward system evolved for the Paleolithic food environment, which was radically different from the modern agricultural and industrial global food production system, then we can’t trust our unconscious food buying impulses. If we just go to the supermarket and put whatever seems desirable into our cart, we will overeat all the things that our Paleolithic ancestors tended to undereat.
Worse, we will be tempted to eat junk foods designed to appeal to the tastes that signal healthfulness to the food reward system, without supplying the nutrients that actually deliver healthfulness.
On the other hand, this evolutionary argument is encouraging. It tells us that, if we eat Paleolithic foods and if we educate ourselves to select healthy foods in the optimal proportions, then we can be confident that our brains will find our meals to be delicious and satisfying.
A healthy diet is also a delicious and satisfying diet! There is no need to suffer to be healthy. There is no need to suffer to lose weight. If you are suffering on a diet, you are doing something wrong."