The typical diet limits two things–carbohydrates and total calories. The result is short-term weight loss of a few pounds, and then rapid weight gain, often to a level a pound or two above where you started.
It’s not hard to understand why. It takes about 3 grams of water to store one gram of glycogen, the stored form of carbohydrate. Low-carbohydrate diets eliminate glycogen–and the water it takes to store it. Water is heavy. Losing glycogen reduces weight by eliminating some of the body’s water, not its fat.
Meanwhile, diets that limit calories to less than about 1200 a day fool the body into thinking it’s starving. The body reacts by slowing its basal metabolic rate (BMR), the rate at which calories are burned while resting. The longer you diet, the slower your BMR, and as BMR decreases, the pounds come off more slowly.
When the diet ends and calorie consumption returns to normal, renewed glycogen storage quickly adds lost water weight. In addition, BMR remains low for a while, so you burn calories more slowly and regain weight more quickly, especially if you eat fats.