True-true. dieting without exercise can cause your body to adapt to a minimal amount of food and slow your metabolic rate, or the rate at which calories are burned. in fact, the fewer calories consumed, the slower metabolism will become. when you diet without exercise, fat and protein stores (muscle) are used to produce energy. when protein is converted to energy, the muscle mass decreases and causes metabolism to slow.the only way to avoid the loss of muscle and drop in metabolism is through a combination of exercise and moderate caloric restriction. it is also the best way to ensure that you will maintain weight loss. the exercise should include some weight lifting, since it is the best form of exercise to maintain muscle mass.a good guideline would be to create a caloric deficit of about 500 calories per day. half should be from diet and half from exercise.