Written By: Muskan Gupta | Published : May 18, 2026 8:07 PM IST
Fasting (Image AI Generated)
Fasting has been used for centuries, both for religious and cultural reasons, as well as for health. However, scientists are now starting to discover what goes on within the body during a prolonged fast. According to a recent study published in the journal Nature Metabolism, after the 3rd day without eating, fasting for 7 days can result in profound changes in the human body.
The Queen Mary University of London and Norwegian School of Sports Sciences scientists conducted a seven-day water-only diet on 12 healthy individuals. Scientists took blood samples every day throughout the experiment and monitored nearly 3,000 proteins in the body, to get a better understanding of how fasting impacts various organs and systems.
The study identified that within 2-3 days the body starts to use a different energy source. The body is typically getting its energy from glucose in food. However, when someone fasts it goes onto burning the body fat. The result of this process is called ketosis. The average weight loss for the participants was approximately 5.7 kg over the week-long fast, with some of this being due to water loss and lean tissue loss.
The most remarkable aspect to the scientists was that the most significant biological changes were not immediate. It will seem like the body was in a different "mode" only after about three days of fasting. Over 33% of protein measurments had significant changes. These proteins included some associated with brain function, immune system and metabolism, and tissue repair.
Scientists also observed changes in proteins linked to inflammation and the protein's surroundings around brain cells. This indicates that fasting could be having other effects other than just weight loss. This study may help address some of the puzzling links between fasting and improved metabolic health and decreased disease risk.
Fasting may affect inflammation and brain health
A further study in Nature Communications revealed that after seven days of fasting, the participants' muscle strength remained largely unchanged. But, it reduced the ability to perform high intensity exercise and endurance because lower carbohydrate stores in the body.
But these findings are interesting, experts say, and they also caution that it's not an ideal diet for everyone. Long fasts cause dehydration, dizziness, electrolyte imbalance, muscle loss, fatigue and other health issues for individuals with diabetes, heart disease, eating disorders, and chronic diseases.
The study does not recommend that anybody fast for seven days. In fact, the researchers say that more research is needed because only 12 people participated in the experiment. Despite that, the study provides scientists with an unprecedented glimpse into the molecular mechanisms of the human body during long fasts.
