Question:

Why is it that people who want to lose weight are told to exercise and people that want to gain weight are told the same thing? If exercise builds muscle why would people that want to lose weight want to exercise when its building muscle?

Answer:

Basically, exercise is the best means of changing your body, which is why it is recommended to people regardless of goals. If you are trying to gain weight, you should utilize different forms of exercise and nutrition than someone trying to lose weight, however. Though the information may look similar there are differences.

One of the main differences is in the amount of food you eat. In order to build muscle and gain weight, you must eat more calories than it takes to maintain your bodyweight. In order to lose weight, you must eat less. The types of food you eat will be similar, only the amounts will be different. This is why it can be confusing.

A similar concept applies to exercise. You're still using the same basic exercises, you're just targeting your exercises, rep ranges, rest periods and intensity levels to muscle gain rather than fat loss. For example, if you are trying to lose fat, you would use shorter rest periods and add more cardio training into your program. To gain muscle, you use somewhat longer rest periods for moderate reps and reduce the amount of cardio you're doing.

Since maintaining muscle mass is important for weight loss (muscle burns calories even at rest), eating high protein and lifting weights are critical. Basically, the same things that build it will help to keep it, which is why the information looks fairly similar.