What happens when you go 7 days without food?

LONDON — Can fasting for seven days be healthy for the human body? In a pioneering study that sheds light on the age-old practice, European researchers have discovered that the human body undergoes extensive and systematic changes across multiple organs after just three days of abstaining from food. This research not only confirms the health benefits of fasting beyond mere weight loss but also maps out a molecular foundation for these effects, potentially guiding future therapeutic interventions.

Conducted by a collaborative team from Queen Mary University of London’s Precision Healthcare University Research Institute (PHURI) and the Norwegian School of Sports Sciences, the study ventured into largely uncharted territory. It explored in detail how prolonged periods without food affect the body's functioning and health.

Fasting has been practiced for centuries for various reasons, including its supposed health benefits, the study authors note. Despite its widespread popularity, the precise biological impacts of extended fasting remained largely a mystery until now. Thanks to advances in technology, researchers can now measure thousands of proteins in our blood, offering unprecedented insights into how our bodies adapt to fasting.

The study involved 12 healthy volunteers who underwent a seven-day water-only fast, with researchers closely monitoring daily changes in approximately 3,000 blood proteins. This comprehensive analysis revealed a significant shift in the body’s energy sources — from glucose to stored fats — within the first few days. Notably, participants experienced an average weight loss of 12.5 pounds, with fat loss being permanent and lean mass loss almost completely reversible after resuming eating.

More remarkably, after three days of fasting, researchers observed profound changes in protein levels throughout the body, signaling a whole-body adaptation to the absence of caloric intake.

“For the first time, we’re able to see what’s happening on a molecular level across the body when we fast,” says study author Claudia Langenberg, director at Queen Mary University of London's PHURI, in a university release. “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.”

These molecular changes, consistent across volunteers, indicate that fasting affects more than just weight. For instance, proteins that support brain neurons also underwent changes, suggesting potential benefits beyond the physical aspects of weight and fat loss.

“Our findings have provided a basis for some age-old knowledge as to why fasting is used for certain conditions,” notes study author Maik Pietzner, health data chair of PHURI and co-lead of the Computational Medicine Group at Berlin Institute of Health at Charité.

“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.”

This research opens up new avenues for understanding the complex biological responses to fasting and lays the groundwork for developing therapies that could help those who cannot fast due to health reasons.

The study is published in the journal Nature Metabolism.