How we evolved to survive hunger and malnutrition

People are facing famine and starvation on the Gaza Strip, a Palestinian territory, which has been the focus of the Israel-Hama war.

Tedros Adhanom Ghebreyesus, the Director-General of the World Health Organization, has described "severe levels of malnutrition" which have led to the deaths of at least 10 children. The Palestinian Health Ministry, which is run by Hamas, said meanwhile that 15 children had died of malnutrition and dehydration.

In northern Gaza, an estimated 300,000 people are living with food and clean water shortages.

Prolonged hunger is an extreme burden for the body. But our evolution has trained the human body to survive without food for weeks if necessary.

It doesn't work for everyone, and when there are other factors, such as disease, that weaken the immune system even more than the lack of food, a person's chances of survival are poor.

Below, we look at how the body tries to survive hunger and starvation.

The body is programmed to survive

There's a "hunger center" in the brain's hypothalamus, a region that plays a central role when a person starves. It gets active as soon as blood sugar levels drop.

First, the hypothalamus instructs the kidney's adrenal glands to release the stress hormone adrenaline. This gives us strength to search for food.

But if we fail to find food, the brain resorts to Plan B: It scrounges around the rest of the body for glucose.

The brain needs glucose, a form of sugar — also known as blood sugar — to function. Glucose accounts for only 2% of a person's body mass, but the brain consumes about half of that.

In order to obtain this essential sugar, the brain tricks the body: It sends a signal to stop insulin secretion. That stops the body's muscles from getting glucose and allows the brain to get it instead.

During severe hunger, each organ shrinks to about half its original weight until the point that they fail and the person dies. The brain is the only exception: It decreases by a maximum of 4% due to its ability to store glucose.

The rest of the body turns to protein for energy production. This also comes at the expense of the muscles, which consist largely of protein. The body converts amino acids — which are proteins — into glucose.

Can you smell extreme hunger?

After eight to ten days, the body switches its metabolism to an energy-saving program.

Like an animal in hibernation, essential activities, such as heart rate, blood pressure and body temperature drop to run at a minimum. When food is limited, it's the best thing the body can do.

The body starts tapping into its fat reserves by converting fatty acids into so-called ketone bodies.

Ketone bodies are an extremely important source of energy. They are the only compounds the brain can use alongside glucose.

But when converts those fatty acids into ketone bodies, it can emit the distinct smell of nail polish. That's because acetone is among the ketone bodies excreted through the kidneys and breath.

The longer hunger lasts, the worse things become: The skin's barrier function decreases, the immune system weakens and inflammation spreads.

Why organs fail because of starvation

Gradually, the body converts all its vital organs into brain food. After a while, the person becomes nothing but skin and bones, and the organs start to fail. The heart often gives up first.

A person can only survive hunger for an extended period if the metabolism reprograms itself, as described above, to allow the brain to survive on less glucose. That makes it possible to maintain protein reserves in the vital organs.

In order for all of this to function smoothly, the body must give that initial hunger signal — the one that stops insulin secretion. But it doesn't always work.

For example, people with malaria, HIV/AIDS or other diseases have so many inflammatory substances in their blood that the pancreas will continue releasing insulin, blocking the so-called starvation metabolism.

Long-term physical impact of starvation

People do recover from starvation. But some face long-term physical and psychological effects. These can include irreversible organ damage or dysfunction, impaired immune function and a loss of bone density.

Starvation can affect hormones, including insulin, cortisol and thyroid.

People who have experienced starvation are often also more likely to develop gastrointestinal problems.

Starvation weakens the immune system, making the body more susceptible to infectious diseases, such as cholera, measles and malaria.

Starvation passes from mother to child

Malnourished pregnant mothers can pass on the negative effects of starvation to their babies.

In a 2022 study, researchers at Pennsylvania State University in the US examined individuals who had been exposed to the Dutch Hunger Winter, a famine at the end of World War II, to study the long-term effects of starvation on children.

Out of all the age groups studied, researchers found that malnutrition in utero caused the most severe negative long-term health effects.

Babies born under those circumstances had faced an increased risk of diabetes, cardiovascular disease and obesity later in life, as well as muscular-skeletal deficiencies and auditory impairment.

Starvation's psychological effect

In the mid-1940s, researchers set out to understand the physiology of starvation via an experiment would be inconceivable today.

Facilitated by American scientist Ancel Keys, the study tracked 36 study participants fed half the calories required for normal survival for three months.

The psychological effects of constant hunger became particularly clear. Many participants withdrew and became apathetic.

Hunger overshadowed everything. They were only interested in things related to food. Some even dreamed of cannibalism. Meanwhile, their senses sharpened: the subjects could smell and hear much better than before.

This article was originally published in German.