If healthy young people don’t eat for a week, their muscles lose half their stored energy from carbs, but they can still work just as hard during intense exercise.

What we've found so far suggests that a 7-day fast may reduce muscle glycogen levels but does not appear to impair high-intensity exercise performance in healthy young adults. Our analysis of the available evidence shows this pattern. We reviewed 38.0 supporting assertions and found no studies that refute this idea. According to the evidence, when healthy young adults fast for seven days, their muscles lose about half of their stored carbohydrate energy (glycogen) [1]. Despite this drop, they are still able to perform high-intensity exercise at normal levels [1]. This means the body may adapt to the lack of food by using other energy sources, such as fat or protein, to fuel intense physical effort—even when the usual fuel (glycogen) is low. Our current analysis shows the evidence leans toward the idea that high-intensity performance can be maintained during a 7-day fast, at least in healthy young adults. However, we only have one distinct assertion to base this on, even though it is supported by 38.0 studies or data points. We don’t yet know how long this performance maintenance lasts beyond seven days, or how it might differ in older adults, people with health conditions, or those with different fitness levels. We also don’t have details on whether perceived effort increases, recovery time changes, or if there are long-term downsides to repeated fasting. Since we’re only reporting what the evidence shows, we can’t say whether this is safe, optimal, or sustainable—just that performance doesn’t appear to drop in the short term. Practical takeaway: If you’re a healthy young adult, going a week without food might cut your muscle glycogen in half, but you could still push hard during workouts—at least based on what we’ve seen so far.