A groundbreaking study published in Nature Metabolism has revealed how the human body undergoes dramatic molecular changes during extended periods without food. The research shows that the body experiences significant, systematic changes across multiple organs during prolonged fasting, with potentially health-altering effects appearing only after three days without food.
The History of Fasting
Humans have developed the ability to survive without food for long periods over thousands of years. Today, millions of people worldwide practice fasting for medical or cultural reasons. It is often pursued for health benefits and weight loss, and has been used since ancient times to treat conditions such as epilepsy and rheumatoid arthritis.
The Study
Researchers from Queen Mary University of London's Precision Healthcare University Research Institute (PHURI) and the Norwegian School of Sports Sciences tracked 12 healthy volunteers through a seven-day water-only fast to understand the impacts of fasting. The findings could lead to therapeutic interventions for people who may benefit from fasting but cannot undergo prolonged fasting or fasting-mimicking diets, such as ketogenic diets.
During fasting, the body changes its source and type of energy, switching from consumed calories to using its own fat stores. However, little was known about how the body responds to prolonged periods beyond this fuel change. The researchers used new techniques to measure thousands of blood proteins, providing a clear understanding of how the body changes during fasting.
What Happens When You Fast
By identifying the proteins, the researchers were able to predict potential health outcomes. They found that the body switches energy sources from glucose to stored fat within the first two or three days of fasting. Participants lost an average of 5.7 kg of both fat mass and lean mass. After three days of eating again, the weight loss remained; muscle was mostly regained, but fat loss persisted.
The Three-Day Threshold
One of the most significant findings was that the body underwent distinct changes in protein levels after about three days of fasting, representing an induction of 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. Beyond weight loss, there were alterations in proteins that make up the supportive structure for neurons in the brain.
What the Researchers Think
Claudia Langenberg, Director of Queen Mary's Precision Health University Research Institute, stated: "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."
Fasting Is Not for Everyone
While fasting may have many health benefits, the researchers emphasized that it is not suitable for everyone. Maik Pietzner, Health Data Chair of PHURI and co-lead of the Computational Medicine Group at the 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, oftentimes it won't be an option for 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 follow."
