How Long Can I Fast Before I Start Losing Muscle?

Fasting is the process of forgoing food for a period of time to either detox the body or lose weight. Most fasts include drinking water, but some include going without water for a period of time. In the process of fasting, the body turns to muscle at some point to get needed glucose, which results in loss of muscle mass. How fast this happens depends on how much glucose your body is receiving. Fasts can be detrimental to your health, so talk with your doctor before starting one.

A man is doing pullups. (Image: Hlib Shabashnyi/iStock/Getty Images)

How It Works

If you are on a water-only fast, several key changes will happen. The body needs certain basic components, normally provided by diet, to function, the most important of which is glucose. Once the body takes all it can from the last meal you had, it turns to the liver, which stores glucose in the form of glycogen. It takes about 24 hours before the liver is relieved of its glycogen stores.

Muscle Loss

Once the glycogen stores of the liver run out after the first 24 hours, the body turns to fatty acids for fuel, breaking down the fatty acids in the reserves of fat in the body and around the organs. But other parts of the body, primarily the brain and red blood cells, can only function using glucose. The body takes the glucose from glycerol in the fat tissues and the amino acids in muscle, thus beginning the process of muscle deterioration.

Protein Sparing

At some point, usually between 48 to 72 hours after the start of the fast, the body realizes that taking glucose from the muscles is too wasteful. The body then goes through a process known as ketosis, where the liver produces ketone bodies from fatty acids that the brain can use as fuel; only the red blood cells will continue using the muscles for fuel. This reduces the amount the muscles are used as fuel significantly and is referred to as "protein sparing." After 72 hours, ketosis continues as long as there is enough fat in the body. Prolonged fasting can lead to a state marked by harmful levels of ketone bodies and high levels of acidity in the blood. Ketosis usually starts after 48 hours for women and 72 hours for men. Note that ketosis is different than ketoacidosis. Ketosis is usually a controlled state, set about by low-carbohydrate intake. Ketoacidosis releases far more ketone bodies and is often a result of uncontrolled diabetes, according to the Ketogenic Diet Resource website.

Considerations

Most people start to lose muscle mass after 24 to 48 hours. To stop the process, or at least reduce it, consuming a liquid with glucose, usually in the form of sugar can help. Juices and sports drinks with glucose are a popular choice. Protein shakes can also help. Doctors at Jefferson Medical College use a form of liquid fasting for obese patients in order to jump start their weight loss. Consult with a doctor before beginning a fasting regimen.