Research results, Transfer of Science or Research

Dr. Glanz, how does the human body respond to heat?

Our body is designed to maintain an internal operating temperature of around 37 degrees Celsius. Human metabolism functions optimally at this temperature, which is why the body strives to stay within this range and, in particular, to prevent overheating. To do so, it needs to dissipate heat. This happens mainly through sweating and by directing more blood to the outer parts of the body.

What effect does that have?

When sweat evaporates from the skin, it removes heat from the body, thereby cooling it. At the same time, the circulatory system can transport warm blood from the body’s core to more peripheral areas, allowing heat to be released into the environment. The increased blood flow causes the blood vessels to dilate. When these vessels lie close to the surface of the skin, the effects are usually easy to see. That is why people often get a flushed face in hot weather.

What does this mean for our brain?

These are perfectly normal responses, and initially our body can cope with them. Problems arise, however, when the ambient temperature gets too high or the heat persists for too long. As blood flow shifts towards the body’s periphery, less blood reaches the internal organs. This also applies, in particular, to the brain.

What consequences can this have?

Reduced blood flow to the brain means that it receives less oxygen and fewer nutrients. The dilation of blood vessels can also cause blood pressure to drop. Fluid loss through sweating can have the same effect. One possible consequence is dizziness.

Sweating also causes the body to lose minerals. These include charged particles such as sodium, magnesium and calcium ions. For our metabolism to function properly, however, their concentrations in the blood, cerebrospinal fluid and other body fluids need to remain within certain normal ranges.

Is that why we should drink mineral water in hot weather?

Exactly. The same applies when exercising, because you sweat. Mineral water or isotonic drinks not only help compensate for fluid loss but also counteract electrolyte deficiency, in other words, a shortage of dissolved minerals.

Coming back to the brain: Its function is based on the activity of nerve cells that exchange electrical signals within networks. This requires a finely balanced mix of different electrolytes. If that balance is disrupted, concentration can suffer and thinking becomes more difficult. A study involving firefighters showed that heat stress impaired concentration and attention.

What about headaches?

Headaches can also be caused by heat and are a typical symptom of sunstroke. In this situation, the meninges can become irritated and swell. These layers of tissue surround the brain and are extremely sensitive to pain – unlike the brain itself.

Another structure that is important for homeostasis – that is, maintaining a stable internal environment in the brain – is the blood-brain barrier, which can also be impaired by overheating. It controls the exchange of substances between the blood and brain tissue and also helps keep harmful substances out. Particularly in cases of severe hyperthermia or heatstroke, experimental studies suggest that this barrier becomes more permeable, which may promote inflammation and fluid accumulation in the brain.

The brain’s neurotransmitter systems also respond to heat. One of these is the serotonergic system. This is the network of nerve cells that uses the neurotransmitter serotonin to regulate various functions of the body and brain. There is evidence of increased serotonin release in the brain during overheating, which is attributed to heightened activity of the brain’s serotonin system. Serotonin is produced both inside and outside the brain, with most of the body’s serotonin being produced in the gut. However, these two systems are largely separate, as serotonin from the bloodstream does not normally enter the brain in significant amounts. There is currently insufficient evidence to establish whether heat-induced damage to the blood-brain barrier allows peripheral serotonin to enter the brain in relevant amounts.

What role does serotonin play in the brain?

Serotonin influences various aspects of our metabolism, including the sleep-wake cycle and impulse control. As with electrolytes, the right concentration of serotonin is important. However, the ways in which heat alters these functions via the serotonergic system are complex and not yet fully understood. An imbalance in serotonin is found, among other things, in mental health conditions such as depression, obsessive-compulsive disorder, and anxiety disorders. A balanced serotonin level is therefore important for emotional well-being. Ultimately, however, the symptoms of overheating result from numerous interconnected processes in which other neurotransmitters also play a role.

Speaking of the sleep-wake cycle, what happens when nights are unusually warm?

Sleep quality suffers. People may wake up intermittently, and phases of deep sleep can change. Disrupted sleep is harmful to the brain, partly because it interferes with natural cleaning and repair processes.

Heat can therefore put the brain under stress in a variety of ways – from fluid and electrolyte loss to sleep deprivation. There is no single mechanism. Rather, several factors interact.

If even a healthy brain can struggle with heat, what happens when someone has a medical condition?

A weakened brain generally finds it more difficult to cope with heat. It is less able to compensate for the strain because its cerebral reserve capacity – its ability to compensate for additional stress – is already reduced. This can affect, for example, the hypothalamus, a region of the brain that plays an important role in regulating body temperature.

Epidemiological studies have shown an association between high ambient temperatures and behavioral changes, for example, increased violent crime and higher suicide rates. Studies also suggest that symptoms of psychiatric and neurological disorders can worsen during periods of heat. Among people with these conditions, increases in hospital admissions and mortality – the frequency of deaths – have also been observed. There is evidence of such effects in conditions including schizophrenia, Parkinson’s disease, and dementia.

Are there any other effects?

Yes. In hot weather, people with dementia face an increased risk of delirium, a state of severe confusion. And if they stay at home for extended periods during hot spells because it is cooler there, they may have less contact with other people. This social isolation can also have a negative impact on mental well-being. Heat can also disrupt familiar daily schedules and routines.

At high temperatures, therefore, it is not only people with cardiovascular disease who are particularly vulnerable, even though they are often perceived as the main risk group. Heat can also pose serious problems for people with psychiatric or neurological conditions.

What about medication?

That is an important consideration. Regardless of the particular medical condition, it is important to bear in mind that many medications are sensitive to heat and need to be stored in a cool place. Some medications can also have different effects than they do at normal temperatures because metabolism changes in hot conditions.

Some active ingredients affect sweat production or the sensation of thirst. There are also medications administered via patches, with the active ingredient being absorbed through the skin. This option is available, for example, for a commonly used dementia medication. It can be taken as a tablet, but if the tablet is not well tolerated, a patch can be a useful alternative. However, heavy sweating can cause the patch to come loose or reduce the absorption of the active ingredient.

The safe use of medications in hot weather is therefore a broad issue. Specific recommendations are available in the form of the CALOR list. Its name derives from the Latin word for heat.

What are your tips for coping with hot weather?

The familiar advice is also the most effective: Drink enough fluids, avoid strenuous physical activity, and seek out a shady place whenever possible.

Interview: Dr. Marcus Neitzert

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DZNE is among the world’s leading research centers dedicated to neurodegenerative diseases, including Alzheimer’s, Parkinson’s, and ALS. These conditions are linked to dementia, movement disorders, and other severe health impairments. They place an enormous burden on patients and their families, as well as on society and the health care system. DZNE plays a key role in developing novel strategies for prevention, diagnosis, care, and treatment – and in bringing them into practice. With ten sites across Germany, it collaborates closely with universities, university hospitals, and other research institutions both in Germany and internationally. DZNE is publicly funded and is a member of the Helmholtz Association and the German Centers for Health Research. https://www.dzne.de/en

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Weitere Informationen:
https://www.dzne.de/im-fokus/meldungen/2026/graue-zellen-im-hitzestress/ German version of this press release