Study reveals biological changes in body after three days of fasting

A study published in Nature Metabolism in 2024 has revealed that multi-day fasting triggers extensive biological changes across various organs, particularly after approximately three days of calorie deprivation.

Researchers from the BHR Institute for Precision Health Research at Queen Mary University of London, in collaboration with the Norwegian School of Sport Sciences, monitored 12 healthy volunteers during a seven-day water-only fast. By analyzing approximately 3,000 proteins in the blood, the team observed that while the body initially relies on glucose, it shifts significantly toward using stored fat for energy by the second or third day.

On average, participants lost 5.7 kilograms, comprising both fat and lean mass (such as muscle and water-rich tissues). While lean mass was largely recovered upon resuming food intake, fat mass continued to decrease. Notably, about one-third of the measured proteins showed significant changes, indicating a systemic biological response that extends beyond simple weight loss, including changes in proteins related to cellular structural support.