Weight gain and weight loss are gradual processes. Most people don't become overweight in a short period of time, and although it's possible to lose weight quickly through crash diets and relentless exercise, the safest and healthiest way to lose weight is slowly, over time. Losing weight too fast can be detrimental to your overall health and damaging to your lean mass, or muscle.
Your body requires a certain amount of calories to perform basic functions, such as digestion, breathing and cellular repair. This basic number of calories is your body's basal metabolic rate, or BMR. Larger bodies have a higher BMR, as do muscular bodies, because muscle needs more energy to maintain itself. Weight loss occurs when you consume or burn fewer calories than your body needs to maintain its current weight, or a combination of both.
Losing weight too quickly affects your body in several ways. If you don't meet your body's energy needs through caloric intake, your body uses it stored energy -- primarily fat -- to meet its functional needs and the demands you place upon it. During initial weight loss, your body sheds some amount of fluid, lean tissue and fat. If your weight loss continues at a fast rate, your body will slow its metabolism and reserve incoming calories for survival. Because muscle requires more calories than fat to sustain itself, your body may begin to metabolize its muscles to preserve energy.
Your body's order of preference for energy is incoming calories, stored fat and muscle. When you lose weight too quickly, your vital organs, such as your brain, heart, kidneys and liver, pull from the most available energy source. When your body can no longer maintain its muscle, your vital organs are first to metabolize muscle so they can continue functioning. If you reach a point where you have very little fat or muscle, your body will metabolize your organs -- which are also made of lean tissue -- to keep your brain functioning. This can lead to heart attack, stroke, and liver and kidney failure.
To lose a pound of fat, you need to burn or cut 3,500 calories. The standard recommendation for healthy weight loss is to lose no more than 2 lbs. per week, which means you should cut or burn between 500 and 1,000 calories each day. Combining exercise with calorie reduction has multiple benefits, because you can meet your body's nutritional needs and increase your strength and endurance while losing unwanted fat.
Fast weight loss is not sustainable, notes Mayo Clinic preventive medicine specialist Donald Hensrud. Changing your lifestyle through proper diet and regular exercise is the key to healthy, sustainable weight loss. An obese person may lose a large amount of weight in a short amount of time while on a very low-calorie diet, Hensrud states, but only if it's medically supervised. For the general population, it's not recommended for women to consume fewer than 1,200 calories per day, while men need, at a minimum, 1,500 daily calories.