Scientists reveal what seven days of fasting does to the human body

Scientists have uncovered the far-reaching impacts of a seven-day fast on the human body, beyond merely reducing weight. Research published in 2024 in Nature Metabolism reveals that prolonged fasting triggers a range of biological changes across multiple organs, with noticeable effects emerging after approximately three days without food.

Researchers from Queen Mary University of London and the Norwegian School of Sports Sciences conducted the study, aiming to uncover the molecular changes associated with extended fasting to potentially develop treatments that mimic these effects. This could be particularly valuable for individuals who may benefit from fasting's biological effects but cannot safely engage in prolonged fasting or adopt fasting-mimicking diets like ketogenic diets.

Going without food for several days forces the body to burn stored fat. Initially, the body derives most of its energy from glucose, which is derived from food. Once this energy source is depleted, the body begins to rely more on stored fat for fuel. However, the specific biological changes that occur throughout the rest of the body during fasting have been less clear.

Using advanced protein analysis techniques, scientists followed 12 healthy volunteers who underwent a seven-day water-only fast. They measured changes in about 3,000 proteins in their blood before, during, and after the fast.

The study found that the body shifts its primary energy source from glucose to stored fat during the first two or three days of fasting. Participants lost an average of 5.7 kilograms, with the loss coming from both fat mass and lean mass, which includes tissues such as muscle and other water-rich tissues. After three days of resuming eating, most of the lost lean mass had returned, while fat mass reduction persisted.

The most striking finding was that, around three days into fasting, researchers detected distinct changes in the levels of proteins throughout the body. These changes suggested a coordinated whole-body response to calorie restriction rather than merely altering energy acquisition.

Approximately one-third of all the proteins measured changed significantly during fasting, affecting major organs. Many of these changes were consistent across participants, indicating that fasting induces widespread biological responses beyond weight loss. Additionally, researchers observed biological signatures that could not be explained by weight loss alone.

For instance, changes were detected in proteins associated with neuron structure and function in the brain, suggesting that prolonged fasting may influence a broader range of biological processes than previously thought.

Claudia Langenberg, Director of Queen Mary's Precision Health University Research Institute, highlighted that the study provides a molecular-level view of what happens to the body during fasting. While safe weight loss through dietary methods like intermittent fasting is known, the research offers evidence for health benefits beyond weight loss, but only after three days of total caloric restriction.

This timing is crucial, as intermittent fasting often involves shorter periods without food, and the study does not imply that shorter fasting schedules would produce the same effects. Instead, it helps identify when significant molecular responses to prolonged fasting begin to appear. Understanding these mechanisms could eventually lead to developing treatments that harness fasting's beneficial biological effects, benefiting individuals who cannot safely fast or follow fasting-mimicking diets.