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Fasting
By Ellen Coleman, RD, MA, MPH
Fasting is sometimes used by people to achieve rapid weight loss. Natural Hygiene, a Spartan form of naturopathy, recommends fasting for adults or children, sick or healthy, to "rejuvenate" the body and eliminate toxins. Other proponents endorse fasting for weight loss -- claiming that fasting trains the body to selectively burn body fat. From a physiological point of view, fasting fails on both accounts.
After the first day of fasting, liver glycogen is depleted. Since the body tries to maintain serum glucose within a normal range, protein stores from muscle (and eventually even from organs such as the heart and kidneys), are broken down to supply amino acids which can be converted to glucose. After a week-long fast, the loss of muscle tissue may comprise as much as one-third of the total weight loss.
A prolonged fast may lead to anemia, impairment of liver function, kidney stones, postural hypotension, mineral imbalances, and other undesirable side effects. Deaths from prolonged fasting have occurred, usually in people who believed this would "purify" their body or cure them of some disease.
Fasting is also ineffective as a weight loss method because it decreases the body's metabolic rate. Since fewer calories are required, less weight is lost.
Fasting for more than three days will increase fat metabolism, but this occurs at the expense of the individual's health. Prolonged fasting also impairs kidney function. Body fat loss can be accomplished far more effectively and healthfully with exercise and a low-fat diet.
The large initial weight loss (3 to 5 lb) associated with fasting is due to the water loss associated with muscle and liver glycogen (carbohydrate) depletion. When the individual resumes eating, glycogen and water stores are replenished and body weight is regained.
Electrolyte losses also result from this fasting-induced diuresis. The only things that are "cleansed" from the body are the minerals needed for muscle contractions, nerve transmissions, regulation of body fluids, and other physiological functions. Even though the body can rely on its fat, protein, and carbohydrate stores for energy for a period of time, the body needs the vitamins and minerals supplied by food to metabolize these fuel stores and to carry on all the necessary physiological functions.
Fasting and fad diets can be very dangerous for individuals with such chronic conditions as diabetes, coronary heart disease, liver or kidney disease, infection, or pregnancy. Fasting and fad diets are especially unsuitable for individuals who include exercise as part of their weight-loss regimen. Since muscle glycogen is the preferred fuel for most types of exercise, low-carbohydrate diets will impair the ability to exercise by reducing muscle glycogen stores. These diets also make active people more susceptible to low blood sugar due to insufficient dietary carbohydrate.
Individuals on these diets are likely to feel irritable due to inadequate dietary carbohydrate and the subsequent ketosis that results from fat breakdown. Ketosis can also cause nausea and ketone-induced metabolic acidosis can cause central nervous system depression. The accompanying symptoms -- sluggishness, loss of coordination, inability to concentrate -- are incompatible with exercise. Lastly, dehydration and electrolyte losses impair body temperature regulation and increase the risk of heat illnesses.
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