Fasting has been practiced for thousands of years in many cultures and religions around the world. Some people fast for spiritual reasons, while others try fasting to lose weight or improve their health.
In recent years, fasting diets and intermittent fasting have become extremely popular, with many people believing they may help with metabolism, weight control, inflammation, and healthy aging.
However, scientists have still been trying to understand exactly what happens inside the body during longer periods without food. Now, a new study published in the journal Nature Metabolism offers one of the clearest pictures yet of how prolonged fasting changes the human body at the molecular level.
The research was carried out by scientists from Queen Mary University of London’s Precision Healthcare University Research Institute and the Norwegian School of Sports Sciences. The findings suggest that going without food for several days does much more than simply burn fat.
Researchers discovered that the body enters a very different biological state after about three days of fasting. During this period, widespread changes begin affecting many organs and systems throughout the body, including the brain, immune system, metabolism, and tissue repair processes.
To conduct the study, researchers monitored 12 healthy volunteers who completed a seven-day water-only fast. During the fasting period, participants consumed no calories and drank only water. Scientists collected blood samples every day before, during, and after the fast.
The researchers then used advanced proteomics technology to study about 3,000 proteins circulating in the bloodstream. Proteins are extremely important molecules inside the body. They help carry out nearly all biological functions, including energy production, immune responses, tissue repair, hormone signaling, and brain activity.
By tracking changes in these proteins, scientists could observe how the body responded over time during prolonged fasting.
At the beginning of the fast, the body behaved largely as expected. During the first two to three days without food, the body switched from using glucose from meals as its main energy source to burning stored fat instead.
This process is known as ketosis. During ketosis, the liver produces molecules called ketones, which become an alternative fuel source for the brain and body.
Participants lost an average of about 5.7 kilograms, or roughly 12.5 pounds, during the week-long fast. Some of the weight loss came from fat, while some came from lean tissue such as muscle.
Interestingly, after participants resumed eating for three days, most of the lost lean tissue returned while much of the fat loss remained.
But the most surprising findings appeared after the third day of fasting.
Scientists discovered that many of the body’s largest molecular changes did not happen immediately. Instead, major shifts in protein activity became much more noticeable only after several days without calories.
More than one-third of all measured proteins changed significantly during the prolonged fast. Many of the strongest changes involved proteins connected to the extracellular matrix, a complex support structure that helps maintain tissues and organs throughout the body, including the brain.
Researchers also observed changes involving proteins linked to inflammation, metabolism, and cellular repair.
One important finding was that the protein changes were remarkably similar between participants. This suggests the body may follow a highly organized biological program during prolonged fasting.
Lead researcher Claudia Langenberg from Queen Mary University explained that the study allowed scientists to observe, for the first time, how fasting affects the body on a system-wide molecular level.
The findings may help explain why some earlier studies have linked fasting to benefits beyond simple weight loss.
Scientists believe prolonged fasting may influence biological pathways connected to inflammation, aging, metabolism, cardiovascular health, and even brain function.
Researchers used genetic data from larger human studies to further examine how the protein changes observed during fasting might affect disease risk.
Some of the altered proteins appeared linked to pathways involved in chronic diseases and inflammatory processes. Other changes involved proteins important for supporting neurons and brain structures.
These findings have increased scientific interest in whether fasting could someday contribute to treatments for conditions involving metabolic disease, aging, neurological disorders, or chronic inflammation.
Interest in fasting research has grown rapidly in recent years. Other studies have suggested that intermittent fasting may improve insulin sensitivity, cholesterol levels, blood sugar control, and cardiovascular health.
Some scientists also believe fasting may activate cellular repair systems such as autophagy, a process where cells break down and recycle damaged components.
However, researchers strongly caution that prolonged fasting is not risk-free.
A later study involving extended water-only fasting found signs of increased inflammation, platelet activation, and changes related to blood clotting. Scientists believe some of these effects may represent temporary stress responses inside the body.
Experts also warn that long fasts can increase the risk of dehydration, dizziness, electrolyte imbalance, muscle loss, weakness, low blood sugar, and complications for people with underlying medical conditions.
Because of these risks, prolonged fasting should never be attempted without medical supervision, especially for people with diabetes, heart disease, eating disorders, kidney disease, or other chronic illnesses.
One of the most important discoveries from the study was the timing of the body’s response. Many popular fasting methods involve much shorter fasting windows, but this research suggests some of the body’s most dramatic biological changes may require several days of complete calorie restriction.
Researchers hope the findings may eventually help scientists develop therapies that mimic some of fasting’s beneficial effects without requiring people to stop eating for long periods.
The study findings are important because they provide one of the clearest molecular pictures yet of how prolonged fasting affects the human body. One major strength of the research is the detailed analysis of around 3,000 proteins across multiple body systems during a carefully monitored seven-day fast.
The study also revealed that many important biological changes only appeared after three days without food, which may help explain why prolonged fasting affects the body differently from shorter fasting methods.
However, the study involved only 12 healthy volunteers, so larger studies are still needed. Scientists also caution that prolonged fasting carries risks and may not be safe for everyone. Future research may help identify safer ways to reproduce some fasting-related benefits through targeted medical treatments.
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Source: Queen Mary University of London.
