Body for Life (BFL) is a 12-week nutrition and exercise program, and also an annual physique transformation competition. The program utilizes a low-fat high-protein diet. It was created by Bill Phillips, a former competitive bodybuilder and previous owner of EAS, a manufacturer of nutritional supplements. It has been popularized by a bestselling book of the same name. Medical experts have described Body for Life as being effective, if difficult to follow. It has sometimes been described as a fad diet, but this criticism is not universal. Exercise The human body adapts itself to changes in nutritional intake. If the calorie intake is reduced, the body responds by slowing down its metabolism, and by burning muscle in preference to fat. This reduces the metabolism long-term. When the diet comes to an end and normal calorie intake is restored, the individual starts to gain weight even faster than before. This is known as yo-yo dieting. Diets that focus exclusively on calorie reduction often fail in this way. With these concerns in mind, Body for Life addresses energy expenditure (i.e. exercise) in addition to energy input. For best results, Body for Life holds that this exercise should include weight training to build skeletal muscle and increase the metabolism over the long term. This also helps to maximise the energy expenditure and fat loss from aerobic exercise. Body for Life's exercise program is more complicated than its diet program. It suggests exercising six days a week, normally Monday to Saturday, and alternating between weight training and aerobic exercise. The seventh day, usually Sunday, is a rest day (referred to as the "free day", during which no exercise is done and unhealthy, normally fatty foods may be eaten). Weight training sessions alternate between exercises for the upper body and exercises for the lower body. This allows the exercised muscles enough time to recover fully before the next training session. Each fortnight follows the same pattern: Intensity index Body for Life uses Gunnar Borg's Rating of Perceived Exertion (known as the Borg scale) for assessing the intensity of exercise based on how hard you feel you are working. It uses the variant developed by the American College of Sports Medicine, which uses a scale of 0 to 10: 0 is no exertion at all. 2 corresponds to very light exercise. For a healthy person, this is like walking slowly at their own pace for several minutes.