Seven Days of Fasting: How Your Body Transforms Inside and Out

A recent study highlights that significant health benefits and molecular adaptations from fasting are detectable after three days.

Recent findings show that prolonged fasting triggers significant and systematic changes across multiple organs in the body. These results highlight potential health benefits that extend beyond weight loss, but they also reveal that these impactful changes only begin to occur after three full days without food.

Health Benefits of Fasting Unveiled

A recent study published in Nature Metabolism sheds light on how the body responds to extended periods without food, offering valuable insights into the processes occurring during prolonged fasting.

Researchers from Queen Mary University of London’s Precision Healthcare University Research Institute (PHURI) and the Norwegian School of Sports Sciences explored the potential health benefits of fasting, focusing on its underlying molecular mechanisms. Their findings lay the groundwork for future research that could lead to therapeutic treatments, particularly for individuals who may benefit from fasting but are unable to participate in extended fasting or follow fasting-mimicking diets like ketogenic plans.

Historical Context and Modern Techniques

For thousands of years, humans have adapted to survive without food for extended durations. Today, fasting is widely practiced for both medical and cultural reasons, with goals ranging from improved health to weight loss. Historically, fasting has also been used to manage conditions such as epilepsy and rheumatoid arthritis, demonstrating its diverse applications across time and societies.

During fasting, the body changes its source and type of energy, switching from consumed calories to using its own fat stores. However, beyond this change in fuel sources, little is known about how the body responds to prolonged periods without food and any health impacts – beneficial or adverse – this may have. New techniques allowing researchers to measure thousands of proteins circulating in our blood provide the opportunity to systematically study molecular adaptions to fasting in humans in great detail.

Detailed Observations from a Controlled Study

Researchers followed 12 healthy volunteers taking part in a seven-day water-only fast. The volunteers were monitored closely on a daily basis to record changes in the levels of around 3,000 proteins in their blood before, during, and after the fast. By identifying which proteins are involved in the body’s response, the researchers could then predict potential health outcomes of prolonged fasting by integrating genetic information from large-scale studies.

As expected, the researchers observed the body switching energy sources – from glucose to fat stored in the body – within the first two or three days of fasting. The volunteers lost an average of 5.7 kg of both fat mass and lean mass. After three days of eating after fasting, the weight stayed off – the loss of lean was almost completely reversed, but the fat mass stayed off.

For the first time, the researchers observed the body undergoing distinct changes in protein levels after about three days of fasting – indicating a whole-body response to complete calorie restriction. Overall, one in three of the proteins measured changed significantly during fasting across all major organs. These changes were consistent across the volunteers, but there were signatures distinctive to fasting that went beyond weight loss, such as changes in proteins that make up the supportive structure for neurons in the brain.

Expert Insights on Fasting’s Effects

Claudia Langenberg, Director of Queen Mary’s Precision Health University Research Institute (PHURI), said:

“For the first time, we’re able to see what’s happening on a molecular level across the body when we fast. 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.”

Maik Pietzner, Health Data Chair of PHURI and co-lead of the Computational Medicine Group at Berlin Institute of Health at Charité, said:

“Our findings have provided a basis for some age-old knowledge as to why fasting is used for certain conditions. 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.”

Reference: “Systemic proteome adaptions to 7-day complete caloric restriction in humans” by Maik Pietzner, Burulça Uluvar, Kristoffer J. Kolnes, Per B. Jeppesen, S. Victoria Frivold, Øyvind Skattebo, Egil I. Johansen, Bjørn S. Skålhegg, Jørgen F. P. Wojtaszewski, Anders J. Kolnes, Giles S. H. Yeo, Stephen O’Rahilly, Jørgen Jensen and Claudia Langenberg, 30 February 2024, Nature Metabolism.
DOI: 10.1038/s42255-024-01008-9

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