Going without food for several days does far more than force the body to burn stored fat. Research published in 2024 found that prolonged fasting produces widespread and coordinated biological changes across multiple organs, with some of the most notable effects appearing only after about three days without calories.
The findings, published in Nature Metabolism, offer an unusually detailed look at how the human body responds during an extended fast. Researchers found evidence of biological effects that went beyond weight loss, although many of the potentially health related changes did not become apparent until participants had gone roughly three days without food.
Scientists from Queen Mary University of London’s Precision Healthcare University Research Institute (PHURI) and the Norwegian School of Sports Sciences conducted the work. By identifying the molecular changes associated with fasting, the researchers hope to provide a foundation for future studies that could eventually lead to treatments capable of reproducing some of fasting’s effects.
That could be especially important for people who might benefit from certain biological effects of fasting but cannot safely undergo a prolonged fast or follow fasting mimicking approaches such as ketogenic diets.
How the Body Adapts to Fasting
Humans have evolved to tolerate extended periods without food, an ability that was essential during times when meals were unpredictable. Today, millions of people fast for cultural, religious, medical, or weight loss purposes.
Fasting also has a long history in medicine. It has been used in various forms to help manage conditions including epilepsy and rheumatoid arthritis.
One of the clearest changes during fasting involves the body’s energy supply. Under normal conditions, much of the body’s readily available energy comes from glucose derived from food. Once that incoming energy disappears, the body gradually begins drawing more heavily on stored fat.
Scientists already understood that basic metabolic shift. What has been much less clear is what prolonged fasting does throughout 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 those questions in far greater detail. Proteins carry out an enormous range of jobs in the body, from building tissues to controlling chemical reactions and transmitting signals between cells. Measuring thousands of proteins in the bloodstream can therefore provide a broad picture of how different organs and biological systems are responding.
Tracking 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 each day. The scientists measured changes in about 3,000 proteins in their blood before the fast, throughout the fasting period, and after they began eating again.
The researchers then combined those protein measurements with genetic information from large population studies. This allowed them to investigate which biological pathways were changing during fasting and to predict some of the possible health consequences associated with those changes.
As expected, the body began shifting away from glucose and toward stored fat as a major energy source during the first two or three days without food.
Participants lost an average of 5.7 kilograms during the fast, with the loss coming 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 participants resumed eating, their overall weight remained below where it had started. However, most of the lost lean mass had returned, while the reduction in fat mass remained.
A Major Shift Appeared Around Day Three
The most striking finding involved what happened after approximately three days of fasting.
At that point, researchers began detecting distinct changes in the levels of proteins throughout the body. The pattern suggested that complete calorie restriction was producing a coordinated whole body response rather than simply altering how the body obtained energy.
About one third of all the proteins measured changed significantly during the fasting period, with effects linked to all major organs.
Many of those changes appeared consistently among the participants. The researchers also detected biological signatures 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 surrounding and supporting them help maintain their structure and function.
The results suggest that prolonged fasting may influence a much wider range of biological processes than simply fat metabolism.
Potential Benefits Extend Beyond Weight Loss
Claudia Langenberg, Director of Queen Mary’s Precision Health University Research Institute (PHURI), said:
“For the first time, we’re able to see what’s happening on a molecular level across 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 total caloric restriction – later than we previously thought.”
The timing is important. Intermittent fasting can involve much shorter periods without food, while this study examined complete calorie restriction lasting several days. The results therefore do not mean that shorter fasting schedules 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 Fasting’s Effects?
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 may become 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, Health Data Chair of PHURI and co-lead of the Computational Medicine Group at Berlin Institute of Health at Charité, said:
“Our findings have provided a basis for some age-old knowledge as to why fasting is used for certain conditions. While fasting may be beneficial for treating some conditions, often times, fasting won’t be an option to patients suffering from ill health. 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 are able to do.”
The study provides a detailed molecular map of how the human body adapts during prolonged fasting. It also highlights a notable threshold: while the switch toward burning stored fat begins within the first few days, many of the broader biological changes associated with fasting become detectable only after about three days without calories.
That distinction could help researchers separate the effects of weight loss from the deeper biological responses triggered by extended fasting, while providing new clues about how those responses might someday be reproduced without requiring people to undergo prolonged periods without food.
