According to a study from Queen Mary University of London and the Norwegian School of Sports Sciences, published in Nature Metabolism, a seven-day water-only fast in 12 healthy volunteers produced widespread changes in the body's proteins. The researchers tracked about 3,000 blood proteins before, during, and after the fast, and found that roughly one-third changed significantly, with effects touching all major organs.
Most of the notable changes did not appear until around day three. Before that, the body mainly shifted from glucose to stored fat as fuel. After three days, the protein patterns suggested a coordinated whole-body response, including changes in proteins linked to structural support around neurons in the brain. The authors say these effects go beyond weight loss and could eventually inform treatments that mimic fasting for people who cannot fast safely.
Participants lost an average of 5.7 kilograms, from both fat and lean mass. Three days after eating resumed, most lean mass had returned, while the reduction in fat mass remained. The single source does not report any conflicting findings; it frames the results as an early step toward understanding fasting's broader biological effects.