Sugars, fat, and the brain’s singularity. Which processes take place in our bodies when we stop eating for many hours?

People fast for a variety of reasons. For many in Israel, it is a part of their religious observance practices, for example on Yom Kippur. Others avoid food intake for hours for health reasons, and others fast for spiritual reasons, or go on hunger strikes to make a political statement. In fact, archeological findings indicate that humans have fasted for over two thousand years and more. However, it is doubtful that these earlier fasters could explain, as we can today, the numerous processes occurring in the human body during a fast.

A fast is defined as an intentional abstinence from any food or drink that contains calories, and sometimes even from water. The duration of a fast is anywhere from 12 hours to several days. A controlled fast is not considered dangerous to healthy individuals, who do not suffer from illnesses and are not medicated. In recent years, evidence has accumulated about certain metabolic processes transpiring during a fast that may even be beneficial to our health.

Efficient energy use

The metabolic processes that take place following a meal and during a fast are meant to ensure efficient storage of available energy in the first case, and constant blood sugar levels even when food isn’t supplied, in the second. Some organs, such as the muscles and the liver, can obtain energy from other sources, but the brain depends exclusively on sugar molecules. It is therefore crucial to maintain constant blood sugar levels.

Two hormones secreted by the pancreas are the main regulators of these mechanisms—insulin and glucagon. Insulin kicks in after meals and signals to the body that it is sated; glucagon, swings into action also during a fast, signaling hunger.

The two hormones’ activity regulates the metabolism in the body’s energy economy and includes the breakdown of glucose to supply energy (glycolysis) and the synthesis of glucose from amino acids (gluconeogenesis). These hormones are also involved in mechanisms of breaking apart or creating complex carbohydrates and fats which are responsible for energy reserves (such as glycogen and triglycerides).

Rising blood glucose levels during and after a meal trigger increased insulin secretion by the pancreas, and subsequently, a series of reactions that lead to the sugar’s entry into the cells and tissues and to energy storage, for example, as glycogen in the liver or as fat. The insulin also expedites processes of sugar breakdown and fat synthesis (the triglycerides) and stops the processes of gluconeogenesis (glucose synthesis).

Several hours after the meal, sugar levels in the blood begin to drop. The pancreas stops secreting insulin and begins to secrete the second hormone—glucagon. The effect of glucagon is the opposite of insulin’s – it leads to the breakdown of sugar storage (glycogen), stops fatty acid storage and begins gluconeogenesis (glucose synthesis). In fact, we all experience short cycles of meal-fasting on a daily basis, for instance, between dinner and breakfast.

What happens when the available stores are depleted?

The transition to a fasting state typically occurs around 12 hours after the last meal, when the liver’s glycogen stores begin to run out and the body starts breaking down fatty acids in order to supply energy. This process is called lipolysis.

As the available glycogen reserves, from which the body can produce glucose, diminish, the need for an alternative energy source, i.e., fat, increases. To satisfy this need, a process of break-down of the triglyceride stores in fat cells commences, thus increasing the levels of the free fatty acids, from which energy can be produced. Thus, the first change occurring during a fast is a transition from sugar-based energy production to fat-based energy production.

Most of our body’s tissues can obtain energy by breaking down fatty acids, but not the brain, which needs glucose. To supply glucose molecules to the brain during the early stages of fasting, a process of breaking down proteins in muscles takes place, releasing glucose building blocks into the blood stream.

Yet this process causes damage to the muscles, and the body cannot sustain it for long periods. Therefore, the brain switches to energy production from ketone bodies – molecules that are products of the breakdown of fatty acids. Termed ketosis, this process occurs in the liver but its outputs nourish the brain, while most of the body’s other tissues produce energy directly from fatty acid break-down.

Vast changes

The fast-related transition, from sugar-based energy production to fat-based energy production, affects many processes in the body. For example, researchers from the Massachusetts Institute of Technology (MIT) studying mice discovered that the breakdown of fatty acids during a fast lead to the creation of new stem cells in the intestines, which help the mice recover from inflammations. Studies have also shown that processes that take place during fasting benefit pre-diabetic people, specifically, decrease cellular insulin resistance. Ongoing studies are examining whether intermittent fasting is also beneficial to the bacterial population in our gut (the microbiome), the development of inflammatory processes, and maybe even to synchronize the biological clock.

Even though it appears that a controlled fast may have health benefits, it is important to remember that fasting is not for everyone. If you suffer from chronic illnesses or regularly take medicine, it is crucial to consult with a physician before you decide to fast – on Yom Kippur or any other occasion.