Long-Term Fasting Doesn’t Harm Muscles—Here’s Why
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Introduction
Fasting is gaining popularity as a way to improve health, lose weight, and fight aging. But one major concern—especially for older adults—is whether it causes muscle loss. A new study published in the Journal of Cachexia, Sarcopenia and Muscle explored how 12 days of long-term fasting affects muscle structure, strength, and energy use. The results may surprise you.
Muscle Size Shrinks Slightly, But Strength Stays
Participants fasted for 12 days, eating only 250 calories per day, mostly from liquids. Unsurprisingly, they lost about 5.9 kg of weight. Muscle volume also went down slightly—about 5% in the thighs and 4% in the calves. But this was mostly due to the loss of glycogen (stored carbohydrate) and water—not actual muscle tissue. One month after fasting, much of the muscle volume had already returned.
Even better: muscle strength didn’t drop. Tests showed no change in how much force participants could produce using their thigh and calf muscles. This suggests that fasting didn’t damage the muscle’s ability to perform.
Fat Moves, Muscles Adapt
While fasting, the body switches from using carbs to burning fat for energy. The study found that fat inside the muscles (called intramyocellular lipids) increased, while fat outside the muscle cells decreased. This shift helps muscles access energy more efficiently.
What’s more, MRI scans showed that fat droplets inside the muscle got smaller—making them easier to burn. This adaptation supports better fat use during exercise, especially when food is limited.
Mitochondria Stay Strong
Mitochondria are the “powerhouses” of our cells, turning nutrients into energy. The study found that mitochondrial function remained strong throughout fasting. In women, it even improved by the end of the fast and stayed higher a month later. This means the muscles remained efficient at using fat for fuel, even under low-calorie conditions.
No Signs of Inflammation or Damage
A key concern with extreme diets is inflammation or structural damage to muscles. But imaging showed no signs of swelling or injury in the muscle tissue. Taurine, a protective molecule found in muscle, remained stable. This shows that fasting didn’t cause harmful stress in the muscle cells.
Conclusion
This study offers encouraging news for those considering fasting as part of a healthy lifestyle. Although there is a small temporary drop in muscle volume, strength and function are preserved—even in older adults. The body adapts by using fat more efficiently and protecting muscle tissue from damage. These findings suggest that long-term fasting, when done safely and with some physical activity, is not harmful to your muscles—and may even make them more resilient.
As always, anyone considering a long fast should consult a healthcare professional, especially if they have underlying health conditions. But with proper guidance, fasting could be a powerful tool for boosting health without sacrificing muscle strength.
Long-Term Fasting Doesn’t Harm Muscles—Here’s Why
Introduction
Fasting is gaining popularity as a way to improve health, lose weight, and fight aging. But one major concern—especially for older adults—is whether it causes muscle loss. A new study published in the Journal of Cachexia, Sarcopenia and Muscle explored how 12 days of long-term fasting affects muscle structure, strength, and energy use. The results may surprise you.
Muscle Size Shrinks Slightly, But Strength Stays
Participants fasted for 12 days, eating only 250 calories per day, mostly from liquids. Unsurprisingly, they lost about 5.9 kg of weight. Muscle volume also went down slightly—about 5% in the thighs and 4% in the calves. But this was mostly due to the loss of glycogen (stored carbohydrate) and water—not actual muscle tissue. One month after fasting, much of the muscle volume had already returned.
Even better: muscle strength didn’t drop. Tests showed no change in how much force participants could produce using their thigh and calf muscles. This suggests that fasting didn’t damage the muscle’s ability to perform.
Fat Moves, Muscles Adapt
While fasting, the body switches from using carbs to burning fat for energy. The study found that fat inside the muscles (called intramyocellular lipids) increased, while fat outside the muscle cells decreased. This shift helps muscles access energy more efficiently.
What’s more, MRI scans showed that fat droplets inside the muscle got smaller—making them easier to burn. This adaptation supports better fat use during exercise, especially when food is limited.
Mitochondria Stay Strong
Mitochondria are the “powerhouses” of our cells, turning nutrients into energy. The study found that mitochondrial function remained strong throughout fasting. In women, it even improved by the end of the fast and stayed higher a month later. This means the muscles remained efficient at using fat for fuel, even under low-calorie conditions.
No Signs of Inflammation or Damage
A key concern with extreme diets is inflammation or structural damage to muscles. But imaging showed no signs of swelling or injury in the muscle tissue. Taurine, a protective molecule found in muscle, remained stable. This shows that fasting didn’t cause harmful stress in the muscle cells.
Conclusion
This study offers encouraging news for those considering fasting as part of a healthy lifestyle. Although there is a small temporary drop in muscle volume, strength and function are preserved—even in older adults. The body adapts by using fat more efficiently and protecting muscle tissue from damage. These findings suggest that long-term fasting, when done safely and with some physical activity, is not harmful to your muscles—and may even make them more resilient.
As always, anyone considering a long fast should consult a healthcare professional, especially if they have underlying health conditions. But with proper guidance, fasting could be a powerful tool for boosting health without sacrificing muscle strength.