As we age, our basil metaalbalic rate tends to decline. If you follow a calory restricted diit, you count your calories and cut out 500 calories a day, then pretty soon your body will burn 500 at last. That's why your weight plateaus. The science behind increasing your metabolic rate is much harder because there are no studies that do it. But one of the things that help maintain it, and you get this from studies of fasting, is fasting.