Going without food for several days causes much more than hunger and weight loss.
According to new research published in Nature Metabolism, prolonged fasting triggers widespread biological changes throughout the body, affecting the brain, metabolism, immune system, and many other organs.
Scientists say the findings provide one of the most detailed views yet of what happens inside the human body during extended fasting.
The study was conducted by researchers from Queen Mary University of London and the Norwegian School of Sports Sciences. Their work suggests the body enters a completely different biological state after around three days without food.
Fasting is not a new idea. Humans have practiced fasting for thousands of years for religious, cultural, and medical reasons. In recent years, fasting diets have become extremely popular because some studies suggest they may help with weight loss, blood sugar control, inflammation, and healthy aging.
Still, scientists have not fully understood how the body changes during longer fasting periods.
To investigate this question, researchers recruited 12 healthy volunteers for a seven-day water-only fast. During the fast, participants consumed no calories and drank only water.
The scientists collected blood samples every day before, during, and after fasting. They then used advanced laboratory techniques called proteomics to examine around 3,000 proteins circulating in the bloodstream.
Proteins are essential molecules that help cells perform nearly every function in the body. Changes in protein activity can reveal how organs and tissues respond to stress, energy shortages, and environmental changes.
During the first few days of fasting, the body shifted from using glucose from food to burning stored fat for energy. This is a normal survival response that humans evolved over thousands of years to survive periods without food.
As fat breakdown increased, the body entered a metabolic state called ketosis. In ketosis, the liver produces ketones, which become an alternative fuel source for the brain and other tissues.
Participants lost an average of about 5.7 kilograms, or approximately 12.5 pounds, during the week-long fast. The weight loss included both fat and lean tissue such as muscle.
However, after participants resumed eating for three days, most of the lost lean tissue returned while much of the fat loss remained.
The researchers were especially surprised by what happened after day three.
Instead of changing immediately, many of the body’s major molecular responses appeared only after several days without calories.
More than one-third of the proteins measured in the bloodstream changed significantly during prolonged fasting. Many of the strongest changes involved proteins related to the extracellular matrix, a structural support system that helps maintain tissues and organs throughout the body.
These proteins are important for tissue strength, repair, and communication between cells. Researchers also identified changes involving proteins linked to inflammation, metabolism, and brain support structures.
Scientists found that the protein changes were highly consistent among participants, suggesting the body may follow a coordinated biological program during extended fasting.
Lead researcher Claudia Langenberg explained that the study provided the first detailed molecular picture of what fasting does across the entire body.
Researchers believe these findings may help explain why fasting has been linked to potential health benefits beyond weight loss.
Using genetic data from larger studies, scientists examined how the protein changes observed during fasting might influence long-term health risks.
Some of the protein changes appeared connected to pathways involved in chronic disease, inflammation, metabolism, and brain function.
Researchers say the findings may eventually help scientists develop treatments that imitate some fasting-related biological effects without requiring people to stop eating for long periods.
Interest in fasting research has grown rapidly over the last decade. Other studies have suggested that intermittent fasting may improve insulin sensitivity, cholesterol levels, blood pressure, and cardiovascular health.
Scientists have also studied cellular repair processes such as autophagy, where cells break down and recycle damaged components during periods of nutrient shortage.
However, experts strongly warn that prolonged fasting is not completely safe.
Later studies examining water-only fasting found evidence of increased inflammation, platelet activation, and changes involving blood clotting pathways. Researchers believe some of these effects may represent temporary stress responses, but they also highlight the need for caution.
Extended fasting may increase the risk of dehydration, dizziness, weakness, muscle loss, electrolyte imbalance, and complications for people with underlying health conditions.
Doctors warn that people with diabetes, eating disorders, cardiovascular disease, kidney disease, or other chronic illnesses should never attempt prolonged fasting without medical supervision.
One of the most important discoveries from the study was that many of the body’s strongest biological responses occurred only after several days without food. This suggests prolonged fasting may affect the body differently than shorter fasting methods that have become popular in recent years.
Scientists hope future research will help determine which biological changes are beneficial and which may carry risks.
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Source: Queen Mary University of London.
