How exactly does hunger affect metabolism?

This is how the body reacts to hunger

Evolution prepared humans for phases in which there was no adequate food supply. A person can survive for up to two or even three months without food - provided they have enough water and are healthy. "We are all selected starvation artists," explains Joachim Gardemann, professor of human biology and humanitarian aid at the Münster University of Applied Sciences. "Hunger is not a disease, but a competence of the human body." This may sound cynical when it comes to starving people in Africa, but from the medical point of view it is a vital strategy of the human organism.

An unethical hunger experiment

What scientists know about hunger is based to a large extent on an experiment that would no longer be conceivable these days: any ethics committee would stop it immediately. In the mid-1940s, the American scientist Ancel Keys investigated what hunger does to people. For three months, 36 test subjects consumed only half the calories they actually needed. Keys' goal was for each subject to lose a quarter of their weight during this time. In the following three months he rebuilt the test subjects with different menus.

Above all, the psychological effects of constant hunger became clear. Many men withdrew and became apathetic. Hunger covered everything, they were only interested in things to do with food. Some even dreamed of cannibalism. At the same time, their senses were sharpened to the limit: the test subjects were able to smell and hear much better than before the start of their studies.

The brain knows the tricks

The hunger center in the brain becomes active

The hunger center in the hypothalamus plays a central role in what happens in the human body when there is hunger. The metabolic center in the brain becomes active as soon as the blood sugar level drops. As a first measure, this part of the brain ensures that the adrenal gland releases the stress hormone adrenaline - so that people can mobilize all their energy to successfully search for food. If there is no food, the brain uses plan B.

In order to function, the brain needs glucose, which is glucose. Although the brain makes up only two percent of a person's body mass, it makes up about half of the body's consumption of glucose. So the brain uses a trick to secure its entire glucose supply. And it works like this: Without insulin, glucose cannot get into the muscles. So the brain gives the signal to stop producing insulin. Result: the muscles go empty-handed. "The brain controls the metabolism in such a way that it survives itself," says Joachim Gardemann. "Every organ shrinks during severe hunger to about half of its original weight until death occurs. Not so the brain: it decreases by a maximum of two to four percent." No wonder if the brain exclusively secures the glucose reserves ...

If the food deprivation continues, the body falls back on protein for energy production. This measure is also at the expense of the muscles, which largely consist of protein. The body can produce dextrose from chopped proteins, the amino acids. So the body goes over to converting the amino acids in the muscles into glucose. "We don't need as much muscle mass as we have in a well-nourished state," explains the scientist. "Muscles are nothing more than protein stores." First of all, humans can get over the muscle loss.

Why you can smell hunger

After eight to ten days, the body switches its metabolism to a kind of energy-saving program: essential activities are shut down, run on the back burner: heart rate, blood pressure and body temperature drop - similar to an animal in hibernation. "That is the best thing you can do when there is less food available," says Joachim Gardemann.

In addition, the body starts to tap into its fat deposits. To do this, he converts fatty acids into so-called ketone bodies. These ketone bodies are an extremely important source of energy and make survival in times of hunger possible in the first place, because they are the only compounds that the brain can use besides glucose. The fact that the metabolism of a starving person falls back on the fat deposits can sometimes even be smelled. Because one of the ketone bodies that are excreted via the kidneys and the air we breathe is acetone with its characteristic nail polish smell.

With prolonged starvation, more and more negative consequences occur: The skin's barrier function decreases, the immune system becomes weaker, and inflammation spreads. The most serious thing is that the body gradually converts the heart muscle into brain food - and so do all other vital organs, because they too consist to a large extent of proteins. After a while the person is only skin and bones, and children look like old people. Man dies when his organs fail. "Often it is the heart that gives up first," says Gardemann, "You have a starving child in your arms, it looks at you - and from one second to the next it is dead."

Children often have nothing to add in times of hunger

Prerequisite for the conversion to the starvation metabolism

A person can only starve for a maximum of three months if the metabolism has reprogrammed itself as described: The brain partially switches to ketone bodies as an energy source. This means that less glucose is needed and the protein reserves in the vital organs are spared. The prerequisite for this is that the body has given the necessary hunger signal, i.e. the stop of insulin release. But that's exactly what doesn't always work, says Joachim Gardemann. "If someone suffers from malaria, AIDS or other diseases, they have so much inflammation in their blood that the pancreas continues to release insulin." And that means that the hunger metabolism does not get going. The result: the body breaks down all of the protein within a very short time, there are no ketone bodies as a source of energy, and the fat reserves remain untouched.

Very quickly, all the proteins that keep people alive have fallen victim to the brain's hunger for glucose. A child who does not manage to change its metabolism, for example because it has an infection, "is already dead a few weeks later," says doctor Gardemann.

Author: Brigitte Osterath / Birgit Görtz
Editor: Matthias von Hein