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Seven days without food can change body weight, body composition, muscle metabolism and exercise capacity. In small studies of healthy volunteers undergoing water-only fasts, participants lost both lean mass and fat, while maximal leg strength was maintained but peak oxygen uptake fell. These findings describe closely monitored research participants—not a safe or beneficial fasting plan for everyone.
What the seven-day fasting studies actually examined
The headline reflects two separate studies of seven-day water-only fasting, not ordinary overnight fasting or a typical intermittent-fasting schedule. One study examined muscle and exercise performance; another tracked proteins circulating in the blood. Neither was a test of fasting as a treatment.
Muscle and exercise performance
In a 2025 Nature Communications study, 13 healthy adults (seven men and six women) completed a seven-day water-only fast. The researchers initially included 15 participants; two discontinued and were excluded from the analysis. Participants were young, had no significant medical history, and did not use prescription drugs or tobacco. They received daily professional care, side-effect monitoring and clinical evaluation. The study was designed to examine muscle adaptations and exercise performance, not to establish a fasting recommendation. Read the study.
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Researchers reported an average body-weight decrease of 5.8 kg, or 7.5%, over the seven days. On average, 4.6 kg of the decrease was lean mass and 1.4 kg was fat mass. These short-term changes should not be described as 5.8 kg of fat loss.
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- Author: Joel Fuhrman.
- Publisher: St. Martin's Griffin
- Pages: 255
- Publication Date: 1995
- Edition: a
Muscle glycogen—the stored form of carbohydrate—was approximately halved. Maximal isometric and isokinetic leg strength remained unchanged, but absolute peak oxygen uptake decreased by 13%; the decrease was 7% when peak oxygen uptake was expressed relative to body weight. The authors also measured a 13-fold increase in skeletal-muscle PDK4 protein expression. They link this change, alongside inhibitory phosphorylation of pyruvate dehydrogenase, to reduced carbohydrate oxidation and lower capacity for high-intensity endurance exercise. The practical distinction is that preserved maximal strength did not mean all aspects of exercise performance were preserved.
Changes in circulating proteins
A separate 2024 Nature Metabolism study followed 12 healthy volunteers through a seven-day water-only fast and measured thousands of plasma proteins over time. The researchers found broad changes in circulating proteins, with major systemic shifts emerging after roughly three days. These molecular results can suggest questions for future studies; they do not establish clinical benefits, disease prevention or a treatment effect. Read the study.
What the findings mean—and what they do not
- Scale weight is not the same as fat loss. In the 2025 study, the average weight decrease included substantial lean-mass loss as well as fat-mass loss.
- Strength and endurance are different outcomes. Maximal leg strength was maintained in that study, while peak oxygen uptake fell and high-intensity endurance capacity was reduced.
- Protein changes are not proof of health benefits. The 2024 study measured circulating proteins, not whether participants experienced better health or avoided disease.
- The participants were not representative of everyone. The studies were small and involved healthy volunteers; the exercise study also included daily professional monitoring.
Is a seven-day water fast safe?
These studies cannot establish that a seven-day fast is safe for the general public or for people with medical conditions or medications. The fact that no serious adverse events were reported in a small, professionally monitored study would not prove safety for an unsupervised fast. The National Center for Complementary and Integrative Health cautions that going without food for days while drinking large quantities of water or herbal tea could lead to dangerous electrolyte imbalances. NCCIH: “Detoxes” and “Cleanses”.
Clinical nutrition guidance also matters after a period of very low intake. NICE says nutrition support should be considered for people who have eaten little or nothing for more than five days. For people at high risk of refeeding problems, it recommends care from appropriately skilled professionals and gradual introduction of nutrition with monitoring. Risk depends on factors including body mass index, weight loss, duration of low intake, electrolyte levels, medical history and medications; the guidance is not an individualized refeeding plan. NICE nutrition-support recommendations.
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Bottom line on the evidence
Seven-day water-only fasting produced measurable changes in small groups of healthy research volunteers: weight loss included lean mass and fat, muscle glycogen fell, and peak oxygen uptake declined even though maximal leg strength remained unchanged. A separate study found broad changes in circulating proteins. These results describe what happened under research conditions; they do not show that a week-long fast is a proven health treatment or a safe do-it-yourself practice.
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