Scientists reveal the response of human organs to 7-day fasts

New evidence suggests that significant, systematic changes occur in many organs in the body during prolonged fasting. The findings suggest health benefits not only in terms of weight loss, but also that any potentially important health changes only occur after three days without food.

A study published in the journal Nature Metabolism expands the understanding of what happens in the body after a prolonged lack of food.

By identifying the potential health benefits of fasting and their underlying molecular basis, scientists from the Precision Health Research Institute at Queen Mary University of London and the Norwegian School of Sport Sciences have developed a roadmap for future research that could lead to therapeutic interventions, including for people who may benefit from fasting but should not be subjected to prolonged fasting or diets that mimic fasting, such as ketogenic diets.

Researchers observed 12 healthy volunteers who participated in a seven-day fast. The subjects consumed only water during the study period. The volunteers were closely monitored daily to record changes in the levels of about 3,000 proteins in their blood before, during and after the fast. By determining which proteins are involved in the body's response, the researchers could then predict the potential health effects of prolonged fasting by combining genetic information from large-scale studies.

As expected, the researchers observed how the body switches energy sources - from glucose to stored body fat - during the first two to three days of fasting. The volunteers lost an average of 5.7kg of both fat and muscle mass. After three days of eating after fasting, the weight remained the same - the loss of muscle mass was almost completely regained, but fat mass remained at the same level.

For the first time, the researchers observed distinct changes in protein levels in the body after about three days of fasting, indicating a whole-body response to full calorie restriction. Overall, one in three of the measured proteins changed significantly during fasting in all major organs. These changes appeared to be consistent in all volunteers, but there were also signs characteristic of fasting beyond weight loss, such as changes in proteins that make up the support structure for neurons in the brain.

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