Scientists Had Volunteers Go 7 Days Without Food. They Didn't Expect What Happened After Day 3

Fasting doesn't just cause your stomach to rumble. It can also potentially trigger biological changes that alter how almost every tissue in your body uses energy. In a study published in Nature Metabolism, researchers noted major shifts in proteins affecting almost every organ system about three days after fasting. These changes affected everything from how the body uses sugar to brain neuron survival. 

In the study, the researchers recruited 12 healthy volunteers and had them complete a 7-day water-only fast. The scientists measured levels of about 3,000 blood proteins before, during, and after the fast. As expected, the participants' bodies shifted away from glucose (sugar) as their primary energy source and began burning more fat around the second or third day. The participants lost an average of 12.6 pounds, with the loss coming from both fat and lean tissues (muscle and other water-containing tissues).

About three days after participants started eating again, most lean tissue weight returned, while total body weight and fat mass were lower than at baseline. The findings provide evidence that fasting may be beneficial for managing some health conditions. Future research will likely focus on uncovering why fasting may cause certain protein changes. 

It's long been known that the body adjusts how it uses energy during periods of calorie restriction. This adaptation allows humans to survive when food isn't readily available. During periods of fasting, the body switches its primary energy source from carbohydrates to fats and molecules derived from fat called ketones. Some animals rely on this backup energy system far longer than humans. For example, grizzly bears hibernate for months at a time, relying on body fat during this period. The Nature study provides evidence that the changes to the body's systems during a fast aren't limited to energy processing. 

The researchers noted that about a third of the 3,000 proteins analyzed changed significantly during the fasting period. The proteins affected could potentially affect nearly all major organ systems. For example, they noticed a strong decline in EPHA1, a protein that has been linked to changes in neurological, cancer, and Alzheimer's disease development. They also noted a decrease in brain-derived neurotrophic factor, which supports nerve development and survival. 

Many proponents of fasting make claims about potential benefits beyond weight loss, and this study provides supportive evidence that some of these claims may have something to them. One popular protocol is intermittent fasting, which involves shorter fasting periods of around 16 hours. More research is needed to understand if intermittent fasting leads to similar body changes as the longer fasting period in this study. 

Dietary changes are occasionally used in the treatment of certain health conditions. For example, some types of epilepsy may respond to a low-carbohydrate diet called the ketogenic diet when other treatments aren't effective. Although, disease improvements have been observed in people following a low carbohydrate diet, the exact reason why a carbohydrate-restricted diet may be beneficial isn't entirely known. This study showed fasting caused changes in proteins associated with the structural support of neurons in the brain, which could at least partially explain the connection. 

It may seem like we already know everything there is to know about the human body, but even our biology textbooks may have been getting some body parts wrong for over a century. Every molecule in the human body comes from the food, water, and air so it makes since that restricting food could have affects on many aspects of human health. In theory, understanding these molecular pathways may allow researchers to leverage and recreate them, which could allow medical professionals to deliver some of the fasting benefits to people without the actual need to fast.