Going without food for several days affects the body in ways that go beyond just burning fat. Research published in Nature Metabolism shows that extended fasting triggers a range of biological changes across the body, including alterations in metabolism, brain function, and the immune system.
Interestingly, many of the most significant effects didn’t show up immediately. It seemed that after about three days without food, the body transitioned into a different biological state altogether.
This study offers one of the clearest glimpses into the molecular changes linked to prolonged fasting within the human body. Researchers believe this could lead to treatments that replicate some benefits of fasting without the need for people to refrain from eating for extended periods.
What Happens to the Body During Prolonged Fasting?
Humans have biologically adapted to handle food shortages. When food is scarce, rather than using glucose from meals, the body starts burning stored fat for energy.
People have practiced fasting for religious, cultural, and medical reasons for millennia. Lately, it has drawn considerable interest due to research associating fasting and intermittent fasting with weight loss, enhanced metabolic health, and cellular repair.
However, scientists have previously had quite a limited grasp of how the body responds during longer fasting periods.
To explore this, researchers from Queen Mary University of London and the Norwegian School of Sports Sciences studied 12 healthy participants undergoing a seven-day water-only fast. Blood samples were taken daily, before, during, and after the fasting period.
Utilizing advanced proteomics technology, the team monitored approximately 3,000 proteins present in the bloodstream. These proteins could shed light on activities across various organs and tissues in the body.
The Biggest Changes Started After Day Three
As anticipated, there was a swift transition from burning glucose to utilizing fat within the first two to three days of fasting.
On average, participants lost around 5.7 kilograms (approximately 12.5 pounds), which included both fat and lean tissue. After they resumed eating for three days, most of the lean tissue loss returned, while a significant portion of the fat remained lost.
But researchers stumbled upon something more unexpected.
Significant changes in protein activity throughout the body didn’t start right away. Notably, observable molecular changes became more pronounced after about three days without food.
More than a third of the proteins evaluated showed substantial changes during the fasting period. Some of the most significant alterations were seen in proteins associated with the extracellular matrix, which plays a crucial role in providing structural support for tissues and organs, including neurons in the brain.
The changes in proteins were remarkably consistent among the participants, suggesting a well-coordinated response by the body during prolonged fasting.
“For the first time, we’re able to see what’s happening on a molecular level across the body when we fast,” noted Claudia Langenberg, Director of Queen Mary’s Precision Health University Research Institute.
“Fasting, when conducted safely, serves as an effective weight loss method. Popular diets that incorporate fasting, such as intermittent fasting, claim benefits beyond weight loss. Our findings provide evidence for health benefits of fasting that extend beyond simply reducing weight, but these effects only appeared after three days of complete caloric restriction—later than we previously assumed.”
Potential Benefits Beyond Weight Loss
Researchers utilized genetic data from large human studies to investigate how the observed protein changes during fasting could impact long-term health.
The results indicated possible connections to improved pathways related to disease risk and inflammation. Additionally, the team identified changes associated with proteins involved in supporting brain structures.
These findings have sparked interest in whether fasting could help develop therapies for issues related to metabolism, aging, inflammation, or neurological health.
Interest in fasting research has surged since the 2024 study was published. More recent reviews have connected intermittent fasting to better insulin sensitivity, healthier lipid levels, and potential benefits for brain function and cardiovascular health.
Other recent studies exploring prolonged fasting have also identified that the body enters a deep ketogenic state during multi-day fasts, fundamentally altering cellular fuel usage.
Scientists Also Found Possible Risks
While fasting may lead to beneficial biological changes, researchers warn that extended fasting carries risks.
A subsequent proteomics study looking at prolonged water-only fasting found signs of increased inflammation, platelet activation, and alterations in blood clotting-related pathways. Researchers suggested these could represent temporary stress responses, but underscored the necessity for more investigation into the long-term health implications of extreme fasting.
Experts also caution that prolonged fasting can raise the chances of dehydration, electrolyte imbalances, dizziness, muscle loss, and complications for individuals with existing health issues.
Due to these risks, extended fasting shouldn’t be done without medical supervision, particularly for those with diabetes, eating disorders, cardiovascular issues, or other chronic conditions.
Why Scientists Are Excited About the Findings
Researchers highlighted the timing of the body’s response as one of the most critical discoveries.
Many popular fasting methods emphasize shorter windows of fasting, but this study suggests that some of the body’s notable molecular changes may require several days of total calorie restriction to occur.
“Our findings validate long-held beliefs about why fasting is applied in certain situations,” explained Maik Pietzner, Health Data Chair of PHURI and co-leader of the Computational Medicine Group at Berlin Institute of Health at Charité.
“Although fasting might be helpful in treating some conditions, it often isn’t feasible for patients with health issues. We hope these insights can help clarify why fasting can be beneficial in specific contexts, leading to the development of treatments that patients could actually follow.”
