When your fuel intake dips below what you need to support the activity of your muscles, bones, and organs, your cells simply can’t function properly. First, your body will make the shift into conservation mode, causing you to burn fewer calories. Undereating will also affect your energy level and even your looks. To reduce your intake without underdoing it, try this step formula: Multiply your weight goal by 10 That’s the minimum number of calories you need per day without physical activity. For example, if you weight 160 pounds but your weight goals is 120, aim for no less than 1,200 calories. Factor in daily activity. If you have a job that keeps you on your feet or you incorporate movement into your daily lifestyle, add another 3 calories per pound of your ideal body weight. Tack on EXTRA for exercise: on the days you work out, add half the calories you burn to your allotted intake. This will give you the extra energy you need to sail through a sweat session, but won’t undo all your hard work.