It's probably genetic. Some people can eat a high carb diet and feel fine, and not suffer the metabolic damage that others would. But that's definitely a minority of the world population. The Okinawans seem to eat a lot of a certain type of purple sweet potato which contains a good balance of vitamins and minerals, but they also have a strong social element - elders are highly respected and given responsibilities that keep them focused - and naturally they exercise a lot due to the terrain, so it's not just the food.