Burning more calories than you eat is the goal of almost every diet and weight loss plan. Calories are the body's source of energy, and if you don't eat enough calories to sustain your body, your body will begin burning fat instead. If you eat far too few calories, you could develop serious health problems.
When you exercise, your body first relies on its stores of glycogen, a byproduct of glucose that can rapidly provide energy. After you burn through all your glycogen, your body begins burning fat to maintain your movement. This is why exercise can help you lose weight. A pound of fat is equal to 3,500 calories, so to lose one pound, you'll need to burn 3,500 calories through exercise.
If you experience a chronic caloric deficit -- which occurs when you don't get enough calories to sustain your activity level -- your body will begin burning through muscle. While you might lose weight, you could actually end up looking flabbier as your muscle mass diminishes. Muscle requires more energy to maintain, and therefore burns more calories than fat. Consequently, if you burn muscle, your body might actually end up burning fewer calories.
If you continue burning more calories than you take in, your body will enter starvation mode. As your body burns through fat and muscle, it will begin burning through tissues and organs, slowly breaking down bodily functions. You may have trouble maintaining your energy, experience heart palpitations and fainting spells, break bones or develop anemia and low blood pressure. Insufficient food intake can kill you, which is why trying to lose weight too quickly is so dangerous. The Mayo Clinic recommends losing no more than a pound or two per week.
Proper Caloric Intake
The number of calories you need each day is determined by your activity level, weight goals, age, sex and overall health. While the "average" person needs 2,000 calories per day, you might need many more if you're highly active. Consistently getting too few calories can interfere with your fitness goals and weaken your body. Talk to your doctor to determine the number of calories you need each day.