There are countless variations of the three-day diet. Any weight-loss diet that has a "quick start" phase, whether it's Weight Watchers, Atkins, South Beach, Dukan, or ITG, takes advantage of the fact that we all lose a lot of water weight before we start losing fat.
Everyone (except a few people who have a rare genetic disease) carries around a few pounds (3-4 pounds, 1-2 kilos) of a stored form of energy called glycogen. It's the main form of stored energy in the liver. It's also the main form of energy stores in the muscles, and the chemical that gives muscles their bulk.
The body makes glycogen from glucose and water. It uses four molecules of water for each molecule of glucose sugar that it stores. When it is time to use the energy stored in glycogen, enzymes in the liver or the muscles "unzip" the glucose from the water so it can be burned (or used anaerobically) to fuel the body. Glucose released in muscles is used only in those muscles, while glucose released from the liver can be used anywhere in the body.
When does the body use this stored energy? If you restrict your consumption of food for a day or two, the body will use glycogen before it burns fat. The kidneys flush the extra water out with urine, so the net effect is losing a lot of weight while depriving yourself of a relatively few calories. However, once glycogen is depleted, the fast weight loss is over. Your diet hits its first plateau as you have to start burning fat, which (mostly) doesn't involve the release of water.
There are ways to niggle the amount of weight loss to be just a little longer and just a little more. Some diets severely restrict sodium. This may cause the kidneys to excrete more water as they keep sodium concentrations in a narrow range.
Some diets emphasize foods that stimulate bowel movement, such as the cabbage soup diet. These diets take off a few pounds (maybe a kilo or so) of weight that had been carried around in the form of fecal matter.
It only takes about 1200 calories of deprivation to deplete the body's glycogen supply, and it only takes about 1200 excess calories (about the calorie value of a fast-food sandwich) to put it right back on. Three-day diets are fine if you need to lose just a little weight to fit into a wedding dress or a tux or for a photo shoot, but they are of little or no long-term value.
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