Fasting denies your body the nutrients it needs to function properly. Though short-term fasts may have some benefit, longer-term fasting can affect your muscles and overall health. Many "detox-style" fasts last one to five days and include drinking nothing but tea, water or broth. This can make your muscles feel weak or fatigued. You're certainly unlikely to build muscle mass by including too many fasting periods in your diet plan.
Energy
Your muscles need energy to function. Cells in your muscles use glucose as an energy source by converting it to adenosine triphosphate, or ATP. If the muscles can't use glucose from the blood, they draw on reserves of an energy source called glycogen stored in fatty adipose tissue. However, when muscles draw on these resources, they're effectively depleting the body of its energy reserves. That means that, eventually, the energy will run out.
Fatigue
Some people experience muscle fatigue when fasting for a day or more. Because your cells have less energy from food to draw on, energy conversion takes longer and is less efficient. This can make your muscles feel weaker than before. This particularly applies if you exercise while fasting. Your muscles won't have much available energy to work properly when running or playing sports. The longer your fasting period, the more pronounced this effect will become.
Protein
Your muscles contain large amounts of protein. Cutting out all protein from your diet means that the body doesn't have the amino acids and proteins it needs to work. After up to 16 hours without food intake, your body starts to break down proteins stored in the muscles to build glucose. This can weaken your muscles and reduce their size. This is why bodybuilders eat high-protein diets -- to build muscle mass. The inverse also applies -- if you remove protein through fasting, you lose muscle mass.
Considerations
The effect of fasting on your muscles depends partly on the duration of your fasting period. However, it also depends on your weight and body-fat percentage before fasting. For example, in a 1982 study published in "The Journal of Nutrition," scientists measured the effects of fasting on lean and obese rats. They found that over a period of two months, the fatter rats lost almost twice as much body fat as lean rats. However, they only lost 1/5 of the body protein of lean rats. It seems that the leaner you are before fasting, the greater the effect it will have on your muscles.
References
- American Cancer Society; Fasting; Nov. 1, 2008
- Boston Globe Online; If You Feel Urge to Fast, Keep It Short; Judy Foreman; May 18, 1998
- "Journal of Nutrition"; Effects of Fasting on Muscle Protein Turnover, the Composition of Weight Loss, and Energy Balance of Obese and Nonobese Zucker Rats; M.A. Dunn; October 1982
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