Throughout history, for reasons of health, well known personalities have opted not to eat to satiety - to under-eat - with known benefits. Here, for the benefits of under-eating a very simple explanation is offered and discussed in some detail. Our distant ancestors, whose bodies we inherited, ate and weighed a good deal less than we do. They had to run around every day to scour the countryside for their scarce food, which kept them lean, hungry and healthy. We, not so healthy relative over-weights, load up our shopping carts once a week at the corner supermarket and simply eat too much for our ~50000-year-old body design. Under-eating or not to eat one's fill appears to bring us somewhat more in line with this design. Thereby, it causes our way of life to more closely resemble the healthier one of our ancient forbears and enables us to potentially live longer, to 100 and more. The putative underlying physiological mechanisms for the health benefits of under-eating are of great interest and therefore are touched upon here.