If healthy young people don't eat for a week but stay active, they might lose muscle mass but still keep their leg strength — meaning their muscles might protect the parts that actually make them strong.

What we've found so far suggests that during a 7-day fast, leg muscle strength may be preserved even if some lean muscle mass is lost. Our analysis of the available evidence shows that the body might prioritize keeping the functional parts of muscles, especially in active individuals. We reviewed one key assertion from the data, which indicates that in healthy young people who fast for a week but remain physically active, muscle strength in the legs tends to stay intact despite a reduction in overall lean mass [1]. This could mean the body protects the critical components responsible for force production, even when it’s breaking down some muscle tissue for energy [1]. The evidence we’ve reviewed leans toward the idea that strength and muscle size may not decline at the same rate during short-term fasting [1]. It’s important to note that this conclusion is based on limited data—only one assertion was analyzed, supported by 38.0 studies, with no studies refuting it [1]. However, we don’t yet know how this might differ across age groups, fitness levels, or activity patterns. Also, while strength appears maintained, the loss of lean mass itself suggests some physiological change is occurring that we don’t fully understand from this evidence alone. Our current analysis does not confirm why this happens or how long the effect lasts. We also can’t say whether longer fasting periods would have the same outcome. What we can say is that short-term fasting, combined with activity, may not impair leg strength even if the body is losing muscle tissue. Practical takeaway: If you're considering a short fast, you might not lose leg strength even if you lose some muscle mass—but staying active could be a key part of that.