What happens when you starve and work out?

Answer 1

You have to specify what kind of exercise you're talking about.

If you do resistance and you starve your caloric intake, then your body will literally "freak out" and lower your metabolism drastically, thus you will burn very few calories as your body can manage. Your muscles will not develop and possibly break down due to increased stress and low calorie intake. (They will go through hypertrophy if you have a decent caloric and protein intake, though.) And whenever you eat, your body will treat the food as holy-food and save as many calories as possible. That's the problem with crash diets, you will lose weight, but once you start eating again, you'll gain it right back again.

The same happens with aerobic workouts.

The best way to build up muscle and keep a decent weight is to have a caloric intake according to your exercise. If you do bodybuilding, you want to eat around 2500+ a day and 1/2 - 1g per pound of body weight. Results are slower, but once you get into the habit you'll get your results, and best of all... you can keep them, and you can have "cheat" days every so often without affecting your weight.

Crash DIETS = BAD

Easy going DIETS = BEST