Obesity in Western countries is on the rise. Modern, affluent lifesyles have reduced chances for exercise and increased the availability of rich, appetizing food. The problem is that our bodies still respond to calories in the same way as those of our prehistoric ancestors. For early humans, hunting and gathering meant an uncertain food supply. Their bodies were programmed to eat well when food was available, and excess energy was stored as fat. Later, during the inevitable periods of shortage, their bodies slowed the rate at which energy was used and drew on the fat until times were good again.
  Modern weight-loss diet plans have typically promoted a sharp decrease in food consumption. However, frustrated dieters often find that the weight lost is quickly regained as soon as normal eating habits are resumed. The ancient process is stillat work: when dieters' bodies receive the message that food is in abundance, the body stores the excess calories as fat.
  For those seeking to take off weight and keep it off, scientists have introduced the glycemic index―a ranking of certain foods based on their immediate effect on blood-sugar levels. Carbohydrate-rich foods such us bread and rice, which break down quickly during digestion and often contribute to weight gain, have the highest rating. Those that break down slowly, releasing glucose gradually into the bloodstream, are ranked lower. The index focuses on foods that are high in carbohydrates, since foods high in fat or protein do not significantly raise blood-sugar levels.
  Before the development of the glycemic index, scientists assumed that our bodies absorbed and digested simple sugars more quickly than other foods, producing rapid increases in blood-glucose levels. However, it is now known that potatoes, a carbohydrate-rich food, have an index number higher than that of chocolate. Conventional wisdom aside, the serious dieter would be better off with a chocolate bar than a baked potato.