Scientists reveal how 7 days of fasting transforms the body

Going without food for seven days does a lot more than make your stomach angry. A 2024 study published in Nature Metabolism found that prolonged fasting triggers a coordinated biological response across the body, affecting proteins linked to multiple organs and biological systems. Surprisingly, many of the biggest changes did not show up until participants had gone about three days without calories.

Researchers from Queen Mary University of London's Precision Healthcare University Research Institute (PHURI) and the Norwegian School of Sports Sciences wanted to see what was actually happening inside the body during an extended fast beyond people losing weight. The result was essentially a molecular play-by-play of the body adapting to having no incoming food.

The body switches fuel sources first

Humans are pretty good at dealing with periods without food. Evolution had to make sure we could keep functioning when the next meal wasn't guaranteed. Normally, the body gets much of its readily available energy from glucose derived from food. Once that supply disappears, it gradually starts relying more heavily on stored fat.

That part isn't exactly breaking news. What scientists haven't understood as well is what happens to the rest of the body during prolonged fasting. Does the brain respond differently? Do organs change how they function? Are there biological effects that have nothing to do with simply losing weight?

To investigate, researchers tracked 12 healthy volunteers through a seven-day water-only fast. Every day, they collected blood samples and measured roughly 3,000 proteins before, during and after the fast. Because proteins perform thousands of jobs throughout the body, changes in their levels can reveal what's happening across different biological systems.

Day three appears to be a big deal

During the first two or three days, participants' bodies increasingly shifted from using glucose toward using stored fat for energy. But around day three, researchers started seeing something much more.

Protein levels began changing across the body in patterns that suggested fasting wasn't simply changing the body's fuel supply. It was triggering a much wider, coordinated response. About one-third of the proteins measured changed significantly during the fasting period, with effects associated with all major organs.

Some of these changes were consistent across the participants and appeared to represent biological responses that couldn't be explained by weight loss alone.

One particularly interesting finding involved proteins associated with the structural support of neurons, the specialized cells responsible for transmitting information through the nervous system.

The participants lost weight but that's not all

The 12 participants lost an average of 5.7 kilograms during the seven-day fast. Some of the weight came from lean mass, which includes muscle and other tissues that aren't body fat.

But something interesting happened after they started eating again. Three days after the fast ended, most of the lost lean mass had returned, while the reduction in fat mass remained. That distinction helped researchers separate some of the body's temporary responses to fasting from changes associated with losing weight.

A seven-day fast is not your next wellness hack

The study involved complete calorie restriction for seven days, not a typical intermittent-fasting schedule. So if you occasionally skip breakfast or eat within an eight-hour window, you should not assume your body is experiencing the same molecular changes seen in this experiment.

The researchers specifically found that many of the broader biological effects became visible only after roughly three days without calories. According to Claudia Langenberg, director of PHURI, the findings provide evidence of potential health effects beyond weight loss. She emphasized that these effects appeared later than previously thought.

Scientists want to copy the biology

If researchers can figure out exactly which molecular pathways are responsible for potentially beneficial effects of fasting, they may eventually be able to develop treatments that activate some of those pathways without requiring someone to go several days without food.

That could mean a lot for people who might benefit from certain biological effects of fasting but cannot safely undergo prolonged fasting because of their health. According to Maik Pietzner, Health Data Chair of PHURI and co-lead of the Computational Medicine Group at Berlin Institute of Health at Charité, the findings could help explain why fasting has historically been used in connection with certain conditions and potentially point toward treatments that patients can actually use.

Source: ScienceDaily