What a Seven-Day Fasting Study Found After Day Three

A seven-day fasting study is making the rounds again, but before you rethink breakfast, there’s some context worth knowing.

ScienceDaily ran the story again on October 5, 2026, after publishing coverage on May 17. Both cover research first released in Nature Metabolism on March 1, 2024. The small study followed 12 healthy volunteers through a closely monitored, seven-day water-only fast.

Researchers found that participants started using more stored fat within the first few days. Around day three, they also detected more widespread changes in blood proteins linked to organs throughout the body. Here’s what researchers found during that week, and why the timing matters.

What Did the Seven-Day Fasting Study Measure?

Researchers from Queen Mary University of London’s Precision Healthcare University Research Institute (PHURI) and the Norwegian School of Sports Sciences monitored five women and seven men daily. Participants drank water but ate nothing for seven whole days.

The paper, “Systemic proteome adaptions to 7-day complete caloric restriction in humans,” looked at how the body responded. Unlike intermittent fasting, which usually limits when you eat, this study involved seven days with no food at all.

What Happens During the First Three Days of Fasting?

Within the first two to three days, participants began relying more on stored fat for energy instead of glucose, or sugar. Researchers also measured roughly 3,000 proteins in their blood before, daily during, and after the fast. This approach, called proteomics, let them follow changes that wouldn’t show up on a scale.

“For the first time, we’re able to see what’s happening on a molecular level across the body when we fast,” Claudia Langenberg, director of Queen Mary’s Institute, said in a press release.

Burning stored fat was an early response to going without food. After about three days, researchers also detected changes in blood proteins linked to all major organs.

What Changed After Three Days of Fasting?

About one-third of the proteins measured changed significantly during the fast, with a similar overall pattern across participants. Researchers grouped the changes into nine response patterns based on when protein levels rose or fell.

“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,” Langenberg said in the press release.

The study measured molecular changes and predicted their possible effects. It didn’t show that participants became healthier after three days.

What Did Researchers Find Beyond Weight Loss?

Participants lost an average of about 12.6 pounds, including both fat and lean mass. Three days after they started eating again, most of the lost lean mass had returned. Their body fat was still below its starting level.

Some findings couldn’t be explained by weight loss alone. Researchers saw changes in proteins in the extracellular matrix, the material that surrounds and supports cells. One example was tenascin-R, a brain-specific protein involved in the support structure around nerve cells.

The team used genetic data from large population studies to predict how some of these changes might affect health. The results pointed to both potential benefits and risks, giving researchers specific leads to investigate next.

The longer-term hope is to develop treatments that reproduce some of fasting’s effects without requiring people to stop eating for days. Co-lead researcher Maik Pietzner said this could be especially useful for patients whose poor health makes fasting unsuitable. The team also hopes to find alternatives for people unable to follow fasting-mimicking diets, such as ketogenic diets. These findings have not established an FDA-approved treatment.

For now, the clearest finding is the timing. These 12 closely monitored volunteers began using more stored fat within two to three days. Around 72 hours, blood tests also showed a coordinated response involving major organs throughout the body. The next step is to find out what those changes mean for health and whether any benefits can be achieved without a prolonged fast.