Fueling the Flame: How Your Body Adapts to 7 Days Without Food 

without food for several days

Study identifies the response of every organ in the human body to a prolonged fast of seven days without food

It is unlikely that our cave ancestors ate every day. The relevance of fasting in the human species goes beyond mere dietary restriction, encompassing spiritual, religious, health and cultural identity dimensions.

New findings reveal that the body experiences significant and systematic changes in multiple organs during prolonged periods of fasting. The results demonstrate health benefits beyond weight loss, but also show that any potentially harmful health changes appear to occur only after three days without eating food. The study, published in Nature Metabolism, represents an advance in the understanding of what happens in the body after prolonged periods without eating food.

Fasting, More than 3 days to be effective

By identifying the potential health benefits of fasting and their underlying molecular basis, researchers at the Precision Healthcare University Research Institute (PHURI) at Queen Mary University of London and the Norwegian School of Sports Sciences offer a roadmap for Future research could lead to therapeutic interventions, including for people who may benefit from fasting but cannot undergo prolonged fasting or diets that mimic fasting, such as ketogenic diets.

Any potentially harmful changes to health appear to occur only after three days without eating food.

Over millennia, humans have developed the ability to survive without food for long periods. Fasting is practiced by millions of people around the world for various medical and cultural purposes, including health benefits and weight loss. Since ancient times, it has been used to treat diseases such as epilepsy and rheumatoid arthritis.

During fasting, the body changes its source and type of energy, going from consumed calories to using its own fat reserves. However, beyond this change in fuel sources, little is known about how the body responds to prolonged periods without eating food and the possible repercussions – beneficial or adverse – that this may have on health. New techniques that allow researchers to measure thousands of proteins circulating in the blood provide the opportunity to systematically and in great detail study molecular adaptations to fasting in humans.

Water only fasting

The researchers followed 12 healthy volunteers who participated in a seven-day water-only fast. The volunteers were closely monitored daily to record changes in the levels of about 3,000 blood proteins before, during and after fasting. By identifying which proteins are involved in the body’s response, researchers were able to predict the possible health effects of prolonged fasting by integrating genetic information from large-scale studies.

The volunteers lost an average of 5.7 kg of both fat and lean mass

As expected, the researchers observed that the body switched energy sources – from glucose to fat stored in the body – in the first two or three days of fasting. The volunteers lost an average of 5.7 kg of both fat and lean mass. After ending the fast, with only three days of eating, the volunteers recovered their lean mass almost completely, but they did not recover their fat mass.

For the first time, researchers observed that the body experienced changes in protein levels after three days of fasting, indicating a whole-body response to total caloric restriction. Overall, one in three proteins measured changed significantly during fasting in all major organs. These changes were constant in all volunteers, but there were hallmarks of fasting that went beyond weight loss, such as changes in the proteins that form the support structure of neurons in the brain.

Intermittent Fasting, Not just for weight loss

Claudia Langenberg, Director of Queen Mary’s Precision Health University Research Institute (PHURI), said: ‘For the first time we can see what happens at a molecular level in 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 that go beyond weight loss. But the study results show that these benefits are only seen after three days of total calorie restriction, later than previously thought.

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

“Our findings have provided a basis for some ancient knowledge about why fasting is used for certain conditions. Although fasting can be beneficial in treating some conditions, it is often not an option for patients suffering from illnesses. “We hope these findings can provide insight into why fasting is beneficial in certain cases, which can then be used to develop treatments that patients can take.”

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