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Seven days without food changed body weight, muscle metabolism and exercise capacity in small studies of healthy volunteers. In a 2025 study, participants lost an average of 5.8 kg, including lean mass, and their peak oxygen uptake fell. A separate 2024 study found broad changes in circulating proteins, especially after about three days. These findings describe short-term responses under research conditions—not proven health benefits or evidence that a week-long fast is safe for everyone.
What the seven-day fasting studies actually examined
The headline refers to water-only fasting: participants consumed no food for seven days. That is different from an overnight fast or a typical intermittent-fasting schedule.
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Two small studies examined different outcomes. Kolnes and colleagues’ 2025 Nature Communications study focused on muscle adaptations and physical performance. A separate 2024 Nature Metabolism study tracked changes in proteins circulating in the blood. Neither was a trial showing that fasting treats or prevents disease.
Muscle and exercise performance
The 2025 study included 13 healthy adults—seven men and six women—who completed the fast. Fifteen people had initially been included; two discontinued and were excluded from the analysis. Participants were young, had no significant medical history, did not use prescription drugs or tobacco, and received daily professional care, monitoring for side effects and clinical evaluation. The study was designed to investigate physiological responses, not to prescribe a fasting regimen. Read the study in Nature Communications.
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Changes in circulating proteins
The 2024 study followed 12 healthy volunteers through seven days of water-only fasting and measured thousands of plasma proteins over time. The researchers reported broad changes involving multiple organ systems, with systemic changes emerging after roughly three days. These molecular shifts can suggest avenues for further research, but they do not establish clinical benefits or show that fasting prevents disease. Read the study in Nature Metabolism.
How body weight and composition changed
In the 2025 study, participants’ body weight fell by an average of 5.8 kg, or 7.5%, over seven days. The reported average loss included 4.6 kg of lean mass and 1.4 kg of fat mass. The scale change therefore should not be described as 5.8 kg of fat loss. These are short-term study averages in a small, selected group, not a forecast for any individual.
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What happened to muscle and exercise capacity
Muscle glycogen—stored carbohydrate used as fuel—was approximately halved. The study authors reported that “Maximal isometric and isokinetic strength remained unchanged, while peak oxygen uptake decreased by 13%.” Peak oxygen uptake fell 13% in absolute terms and 7% when normalized to body weight.
That contrast matters: unchanged maximal strength did not mean exercise performance was unaffected. The researchers also found a 13-fold increase in skeletal-muscle PDK4 protein expression. They linked increased PDK4 and inhibitory phosphorylation of pyruvate dehydrogenase to reduced carbohydrate oxidation and lower capacity for high-intensity endurance exercise. The study measured these physiological changes; it does not show how they would affect every person or activity.
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What the protein findings do—and do not—mean
The 2024 study’s blood-protein measurements showed that a seven-day fast was associated with a broad molecular response, with major systemic changes appearing only after roughly three days. A change in a protein level is not itself proof of a health improvement. The study did not establish that these changes prevent disease, produce lasting benefits, or make prolonged fasting a treatment.
Is a seven-day fast safe?
These studies cannot establish that an unsupervised seven-day fast is safe. The exercise study involved only 13 analyzed participants, all a selected group of healthy volunteers receiving daily professional care; it was not designed to assess safety across the public. The absence of serious adverse events in a small monitored study would not prove that the practice is safe for people with different health conditions, medications or circumstances.
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The U.S. National Center for Complementary and Integrative Health warns that not eating for days while drinking large amounts of water or herbal tea can lead to dangerous electrolyte imbalances. NCCIH: “Detoxes” and “Cleanses”.
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Why restarting food can also require care
In its nutrition-support guidance, NICE says nutrition support should be considered for people who have eaten little or nothing for more than five days. It also recommends that people at high risk of refeeding problems receive care from appropriately skilled professionals, with nutrition introduced gradually and monitoring. Risk depends on factors including body mass index, recent weight loss, duration of low intake, electrolyte levels, relevant history and medications. This is clinical guidance, not an individualized refeeding prescription; it does not support a one-size-fits-all plan. NICE nutrition-support recommendations.
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