What is the Connection Between Fasting and Weight Loss?

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  • Written By: Dale Marshall
  • Edited By: Kristen Osborne
  • Last Modified Date: 30 January 2019
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People who want to lose weight quickly are sometimes intrigued by the claims of diets that purport to exploit the connection between fasting and weight loss. Of all the confusing and sometimes conflicting claims made about diet and weight management, one fact is irrefutable: calories consumed are used for daily energy requirements, and any excess calories are stored as fat. It stands to reason, then, that when there's an insufficient supply of energy in the bloodstream, the body will turn to its stored reserves of energy to make up the deficit. How this is accomplished illustrates the relationship between fasting and weight loss, and why "fasting diets" so often fail in the long run after showing sometimes extraordinary short-term results.

Strictly speaking, fasting is an extreme diet strategy that deprives the body of any caloric intake at all. Shutting off the calorie supply, however, also closes off the supply of nutrition. Since the body doesn't store nutrients for later use, most diets that characterize themselves as “fasting diets” actually permit some caloric intake, usually juice and vitamins. In other cases, people will fast not primarily for weight management, but for physical or religious purification. The extent to which fasting accomplishes physical purification, or detoxification, is open to debate, but regardless of the reason for the fast, there will be a connection between the fasting and weight loss experienced.


It's well documented that the human body can survive for months with little or no food, as long as there's a supply of water. It does this by resorting to a special starvation strategy when faced with a drastic reduction of caloric intake, possibly developed over centuries of sporadic, “feast or famine” diets characteristic of how our earliest ancestors lived. When faced with the possibility of starvation, the body reduces the metabolism; that is, it uses less energy. It also derives energy from burning the body's own cells, but it doesn't turn first to the energy stored in fat; it primarily uses the energy stored in muscle cells because those cells require more energy to maintain. It's a simple economic strategy; when faced with a shortage of any necessary resource, part of the response is to use less of that resource, and another part is to shut down or eliminate non-essential activities that use the resource. A body in survival mode will consider fat cells, which store energy, as more essential than muscle cells, which require energy.

The math is relatively simple. A pound of fat contains about 3,500 calories, and a pound of muscle contains between 900 and 1,000 calories. Depending on a number of factors, such as gender, weight and height, the average adult body requires between 1,800 and 2,500 calories daily for routine maintenance and activity, with more calories being required by more active people. Thus, if the fasting body burns fat cells, an average person should lose 3.5 – 6 lbs (1.6 – 2.7 kg) in a week, yet reports of 10 – 15 lbs (4.5 – 6.8 kg) and more lost by fasters in a week are not uncommon. The reason for this is that the cells being burned for energy aren't fat cells, but muscle cells. Since a pound of muscle contains far fewer calories than a pound of fat, more pounds of muscle must be consumed to provide the body's caloric needs.

Those who seek to exploit the relationship between fasting and weight loss to accomplish dramatic weight loss in a relatively short period of time, then, may find that while the weight loss goal is achieved, some degree of physical fitness has also been lost because so much muscle has been consumed by the fast. In addition, the overwhelming majority of research, both scientific and anecdotal, suggests that in the vast majority of such cases &emdash; perhaps more than 90% &emdash; all the weight lost, and more, will be regained &emdash;and most of the weight regained will be fat.

Thus, while there's a strong connection between fasting and weight loss, the cumulative effects of a fast undertaken for weight management may prove to be more detrimental to a person's health than any actual weight loss achieved.



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