Fast weight loss is unhealthy for a number of reasons:
Not only is fast weight loss unhealthy, but your body is actually designed to prevent itself from losing weight. If you try to lose more than the recommended one to two pounds of fat per week, your body will turn to and burn its own muscle for energy. The loss of lean muscle mass negates any weight loss you might be trying to accomplish, as muscle is what makes your body healthy, toned and in motion. Having more muscle makes your resting metabolism work harder, which means you burn more calories while you're at rest. You must consume at least 1,200 calories a day to prevent the loss of muscle.
When a body consumes too few calories it will actually slip into 'famine mode,' where it purposefully slows down its resting metabolism, or basal metabolic rate (the amount of calories your body burns when at rest), in order to avoid expending energy. Your body will strain to save calories, preventing weight loss and even harming your metabolism's natural functions.
Striving for fast weight loss usually means entering a nasty pattern; dramatically lowering your caloric intake only starts a cycle of yo-yo dieting and bingeing, all of which leads you further in the wrong direction. The subsequent feelings of guilt are enough to bring anyone more mental grief than they deserve.
Some of the more popular diets that promote fast weight loss are in fact nutritiously deficient, as they rely on liquid shakes or diets built around restricted food to cut calories and shed the pounds. These are not only unhealthy, as a healthy body requires a well-balanced diet incorporating all the food groups, but losing weight the right way means exercising regularly in order to keep your muscles and blood moving and to maintain an active metabolism, all of which contribute to losing the weight for good.