Fasting has had a place in human culture for thousands of years. Some cultures have pursued it as a supposed road to spiritual enlightenment, others have decreed that it must be undertaken at certain times of year for religious reasons.
But to the human body, fasting is one and the same with starvation. Intellectually, the two can be separated: fasting is deliberate, starvation is involuntary. But the effects on the body are identical.
Some people fast regularly, in the belief that the practice helps sweep the body of toxins. There is no medical evidence for this. Nor is there evidence that fasting is an effective way of losing weight, and it is easy to see why.
Once the body realizes that there is no food coming in, it goes into starvation mode. The metabolism shifts into low gear and the body begins to convert glycogen supplies into fuel for living and to keep blood glucose levels normal (fat cannot be converted into glucose for this purpose).
As the glycogen is used up, the water it is stored with goes too, giving the impression that weight is being lost. The first glycogen to be used up is that stored in the liver. After those supplies have been exhausted, the body starts using the glycogen from the muscles. Even a fast lasting a couple of days will therefore result in the loss of muscle tissue.
— Zigverve.com (@zigverve) September 23, 2016
And if no carbohydrate is coming into the body, fat reserves cannot be efficiently burned, leading to the buildup of toxic products called ketones, in the blood. This can cause headaches and fatigue. The result, therefore, of even a short fast, will be the loss of body fluids and the loss of muscle tissue, along with some loss of fat. Fasting can also deplete body stores of phosphate, magnesium, calcium, potassium and sodium.
If a fast is maintained for several days, the appetite will eventually be suppressed — again a potentially dangerous thing.
Fasting is not a good method of dieting, because as soon as old habits are resumed, the weight will come back. Even people who embark on a long-term program of fasting and strict dieting mostly return to their old weight within two years of resuming their regular eating habits.
Very low calorie diets have many of the same effects on the body that fasting has. Diets which provide less than 1000 calories a day cause metabolism to drop, slowing the rate at which energy is burned.
There is evidence that body fat stores are burned more quickly on a well-balanced diet which provides between 1200 and 1500 calories a day, than on diets which provide half this number of calories.
The harsh fact which must be faced by anyone attempting to lose weight is that the process will take time. If a diet is causing you to lose more than about a kilo a week, chances are that most of the extra loss is in the form of muscle and fluid. If they are honest, most people will admit that the extra weight they are carrying was accumulated over a long period. It must be lost in the same way.