Going without food for several days does far more than force the body to burn fat. Research published in Nature Metabolism revealed that extended fasting sets off widespread biological changes throughout the body, including shifts linked to the brain, metabolism, and immune system.
Scientists found that many of the most significant effects did not appear right away. Instead, the body seemed to enter a very different biological state after about three days without food.
The findings offer one of the clearest pictures yet of what prolonged fasting does inside the human body at a molecular level. Researchers say the work could eventually help scientists develop treatments that mimic some of fasting’s effects without requiring people to stop eating for days.
What Happens to the Body During Prolonged Fasting?
Humans evolved to survive periods of food scarcity. When food is unavailable, the body switches from using glucose from meals to relying on stored fat for energy.
Fasting has been practiced for thousands of years for religious, cultural, and medical reasons. It has also attracted growing attention in recent years because of studies linking fasting and intermittent fasting to weight loss, improved metabolic health, and cellular repair processes.
Still, scientists have had only a limited understanding of exactly how the body responds during longer fasts.
To investigate, researchers from Queen Mary University of London’s Precision Healthcare University Research Institute (PHURI) and the Norwegian School of Sports Sciences monitored 12 healthy volunteers during a seven day water-only fast. Blood samples were collected daily before, during, and after fasting.
Using advanced proteomics technology, the team tracked roughly 3,000 proteins circulating in the bloodstream. These proteins can provide clues about what is happening across organs and tissues throughout the body.
The Biggest Changes Started After Day Three
As expected, the body quickly shifted from burning glucose to burning fat within the first two to three days of fasting.
Participants lost an average of 5.7 kilograms (about 12.5 pounds), including both fat and lean tissue. After participants resumed eating for three days, most of the lean tissue loss returned while much of the fat loss remained.
But researchers discovered something more surprising.
Large scale changes in protein activity throughout the body did not begin immediately. Instead, major molecular changes became much more noticeable after around three days without calories.
More than one third of the proteins measured changed significantly during fasting. Some of the strongest shifts involved proteins linked to the extracellular matrix, which helps provide structural support for tissues and organs, including neurons in the brain.
The protein changes were remarkably consistent among volunteers, suggesting the body may follow a highly coordinated response to prolonged fasting.
“For the first time, we’re able to see what’s happening on a molecular level across the body when we fast,” said Claudia Langenberg, Director of Queen Mary’s Precision Health University Research Institute (PHURI).
“Fasting, when done safely, is an effective weight loss intervention. Popular diets that incorporate fasting, such as intermittent fasting, claim to have health benefits beyond weight loss. Our results provide evidence for the health benefits of fasting beyond weight loss, but these were only visible after three days of total caloric restriction — later than we previously thought.”
Potential Benefits Beyond Weight Loss
Researchers used genetic data from large human studies to examine how the protein changes observed during fasting might influence long term health.
The results suggested possible links to improvements in several biological pathways associated with disease risk and inflammation. Scientists also identified changes connected to proteins involved in brain support structures.
The findings have fueled interest in whether fasting could someday help researchers develop therapies for conditions involving metabolism, aging, inflammation, or neurological health.
Interest in fasting research has continued growing since the 2024 study was published. More recent reviews have linked intermittent fasting to improved insulin sensitivity, healthier lipid levels, and possible benefits for brain function and cardiovascular health.
Other recent studies on prolonged fasting have also found that the body enters a deep ketogenic state during multi day fasts, dramatically changing how cells use fuel.
