Going several days without eating does much more than force the body to burn stored fat. Research published in 2024 found that prolonged fasting produces widespread and coordinated biological changes in multiple organs, with some of the most notable effects appearing only after about three days without calories.
The findings, published in nature metabolismThey offer an unusually detailed look at how the human body responds during prolonged fasting. The researchers found evidence of biological effects that went beyond weight loss, although many of the potentially health-related changes did not become evident until participants went about three days without eating.
Scientists from the Precision Healthcare University Research Institute (PHURI) at Queen Mary University of London and the Norwegian School of Sports Sciences carried out the work. By identifying the molecular changes associated with fasting, the researchers hope to provide a basis for future studies that could eventually lead to treatments capable of reproducing some of the effects of fasting.
This could be especially important for people who could benefit from certain biological effects of fasting but who cannot safely undergo prolonged fasting or follow approaches that mimic fasting, such as ketogenic diets.
How the body adapts to fasting
Humans have evolved to tolerate long periods without eating, an ability that was essential in times when meals were unpredictable. Today, millions of people fast for cultural, religious, medical reasons or to lose weight.
Fasting also has a long history in medicine. It has been used in various forms to help manage conditions such as epilepsy and rheumatoid arthritis.
One of the clearest changes during fasting has to do with the body’s energy supply. Under normal conditions, much of the energy available in the body comes from glucose derived from food. Once that incoming energy is gone, the body gradually begins to consume more stored fat.
Scientists already understood that basic metabolic change. What has been much less clear is what effect prolonged fasting has on the rest of the body, including whether it produces biological changes that could be beneficial, harmful, or both (beneficial or adverse).
Modern protein analysis has made it possible to investigate these questions in much greater detail. Proteins perform a huge variety of functions in the body, from forming tissues to controlling chemical reactions and transmitting signals between cells. Therefore, measuring thousands of proteins in the bloodstream can provide a broad picture of how different organs and biological systems are responding.
Following a seven-day water-only fast
For the study, researchers followed 12 healthy volunteers who completed a seven-day water-only fast.
Participants were closely monitored every day. The scientists measured changes in around 3,000 proteins in the blood before fasting, during the fasting period and after starting to eat again.
The researchers then combined those protein measurements with genetic information from large population studies. This allowed them to investigate what biological pathways changed during fasting and predict some of the potential health consequences associated with those changes.
As expected, the body began to switch from glucose to stored fat as the main source of energy during the first two to three days without food.
Participants lost an average of 5.7 kilograms during fasting, and the loss came from both fat mass and lean mass. Lean mass includes tissues that are not body fat, such as muscle and other water-rich tissues.
Three days after the participants ate again, their total weight remained below the baseline level. However, most of the lost lean mass had returned, while the reduction in fat mass persisted.
Around the third day an important change appeared
The most surprising finding was what happened after about three days of fasting.
At that time, researchers began to detect distinct changes in protein levels throughout the body. The pattern suggested that complete calorie restriction was producing a coordinated whole-body response rather than simply altering the way the body obtained energy.
Approximately one-third of all proteins measured changed significantly during the fasting period, with effects related to all major organs.
Many of those changes appeared consistently across participants. The researchers also detected biosignatures that could not be explained by weight loss alone.
One example involved changes in proteins associated with the structural support of neurons in the brain. Neurons are the specialized cells that transmit information throughout the nervous system, and the proteins that surround and support them help maintain their structure and function.
The results suggest that prolonged fasting can influence a much broader range of biological processes than simple fat metabolism.
Potential benefits go beyond weight loss
Claudia Langenberg, director of Queen Mary’s Precision Health University Research Institute (PHURI), said:
“For the first time, we can see what happens at a molecular level throughout the body when we fast. 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 full calorie restriction, later than we previously thought.”
The moment is important. Intermittent fasting can involve much shorter periods without food, while this study looked at complete calorie restriction that lasted several days. Therefore, the results do not mean that shorter fasting programs necessarily produce the same effects.
Instead, the study helps identify when some of the more substantial molecular responses to prolonged fasting begin to appear.
Could scientists reproduce the effects of fasting?
Understanding why fasting produces particular biological changes could eventually prove more important than fasting itself.
If researchers can determine which molecular pathways are responsible for potentially beneficial effects, it will be possible to develop treatments that activate those pathways without requiring patients to stop eating for several days.
That possibility could be especially useful for people whose medical conditions make prolonged fasting impractical or unsafe.
Maik Pietzner, chair of health data at PHURI and co-director of the computational medicine group at the Berlin Health Institute at Charité, said:
“Our findings have provided a foundation for some long-standing knowledge about why fasting is used for certain conditions. While fasting can be beneficial in treating some conditions, many times, fasting will not be an option for patients suffering from health problems. We hope that these findings can provide information about why fasting is beneficial in certain cases, which can then be used to develop treatments that patients can pursue.”
The study provides a detailed molecular map of how the human body adapts during prolonged fasting. It also highlights a notable threshold: While the shift toward burning stored fat begins in the first few days, many of the broader biological changes associated with fasting become detectable only after about three calorie-free days.
That distinction could help researchers separate the effects of weight loss from the deeper biological responses triggered by prolonged fasting, while also providing new clues about how those responses might one day be reproduced without requiring people to go long periods without eating.