Does Fasting Speed Up Metabolism?

Metabolism refers to the chemical processes that take place inside your body that allow normal functioning. Your metabolism helps you use energy converted from food to perform basic functions such as respiration and digestion. The higher your rate of metabolism, the more quickly you process energy in food. Fasting may help you lose weight, but it will slow rather than speed your metabolism. This makes weight regain likely.


The energy the body uses has three main factors: your basal metabolic rate, the energy used in physical activity and the thermic, or body heating, effect of food. Because your weight depends on the amount of energy you consume and the level to which you expend it, those with a high BMR, or resting metabolism, typically need to exercise less than those who do not in order to maintain the balance between energy consumed and expended. You take in very few calories when fasting, which can lead to weight loss, but fasting also signals your metabolism to slow down.

Starvation Mode

When you deprive the body of food, it goes into starvation mode. This means your body burns fewer calories so it can conserve energy for requirement at a later time. This is a defense mechanism the body adopts to combat famine. Your metabolism slows to protect your body's fat reserves. Once you've put your metabolism into starvation mode, it works to protect itself from future food shortages. Even when you resume normal eating habits, your metabolism remains sluggish and your body holds onto as much extra fat as it can. You not only may regain the weight you lost during a fast, you may gain additional pounds.

Other Factors

Some other factors that affect metabolic rate include body size, age, gender and genetic predisposition. Large bodies contain more metabolizing tissues, so the bigger you are, the higher your metabolism. Your metabolism slows as you age, and men enjoy higher metabolic rates than women. Scientists have identified a metabolism gene -- sometimes called the fat gene -- so your family history plays a role in the speed of your metabolism. Body fat lowers your metabolism, and muscle tissues raise it. Dietary deficiencies also play a role: Low iodine reduces thyroid performance and can lower metabolism. Illness boosts your metabolism as your body works harder to fight infections. Physical activity speeds your metabolism.


Fasting, in addition to slowing your metabolism, may make you hungrier than you need to be. When your leptin levels fall, your appetite increases. This is one reason nutritionists recommend that you don’t skip meals -- you may overeat later in the day. If you fast to lose weight, your efforts may backfire. If you follow a fast for religious reasons, you can protect yourself against weight gain by gradually increasing your food intake at the end of a fast. This gives your metabolism a chance to adjust.