The brain has a remarkable ability to cope with dehydration during exercise

Brains hard at work. Martin Rickett/PA Wire

When it comes to physical exercise, we don’t tend to take into account how important our brain is for keeping our whole body going. But our ability to control our muscles – to keep them contracting and relaxing – and move our bodies precisely how we want them to, is ultimately determined by our brain.

This vital organ is in command of generating the precious electrical impulses that repeatedly spark muscle contraction and keep our bodies moving how we want them to. The exact way the human brain copes with extreme conditions of environmental stress and exercise is not yet fully understood. But new research I worked on at Brunel University’s Centre for Sports Medicine and Human Performance shows how the brain reacts to being dehydrated during intense exercise.

It is general knowledge that when people work or compete in hot environments and become markedly dehydrated from losing significant amounts of body fluids from sweating, they feel tired sooner and their performance deteriorates drastically. But working out the role of the brain in this fatigue is a major challenge.

It’s long been established that dehydration from exercising in the heat detrimentally affects blood flow to the body’s muscles – which is fundamental to transporting oxygen and producing the energy needed to keep muscles working effectively. With this background, we asked whether the brain might also experience an energy deficit, which could explain why athletes feel tired sooner and slow down or stop exercising prematurely when they are noticeably dehydrated.

We set out to better understand the impact of dehydration on the supply of blood and oxygen to the brain and the consequences that reductions in this might have on the metabolic processes that generate the energy required for optimal brain functioning. The combination of precise measures of blood flow and blood samples to and from the brain allowed us to gain new insight into the human brain at work in exercise.

The ‘smart’ mechanism

As well as establishing how important it is for the brain to stay hydrated, we found that the human brain possesses a “smart” mechanism to cope with the challenge of reduced blood and oxygen supply. We collected data on blood flow to the brain using new techniques to measure the blood velocity and diameter of the internal carotid artery, the main vessel supplying blood to the brain. We also measured oxygen levels in the blood supplying the brain and the internal jugular vein, which drains blood directed from the brain.

These measures allow us to determine the difference between the amount of oxygen going into the brain and how much is extracted from the circulation for metabolic use. We then calculated the brain aerobic metabolism during different exercise stages and conditions. Measurements were obtained in ten trained males during incremental cycling to volitional exhaustion in a hot environment, and compared under control, dehydrated and rehydrated states.

Published in The Journal of Physiology, the data we collected showed that when humans exercise to the point of exhaustion, dehydration causes an early reduction in brain blood flow. But, to compensate for this, there is an increase in oxygen extraction from the blood circulating in the brain, which protects the brain’s ability to process oxygen and function.

This data showed that the brain copes better than the body’s muscles with the stress of dehydration and exhaustive exercise. From an evolutionary perspective, this makes perfect sense – brain function lies on top in the hierarchy of the human body systems, as small impairments in its function could be deadly.

Dehydration reduced body mass, increased internal body temperature, lowered cerebral blood flow and impaired exercise capacity. Conversely, a regular uptake of fluids prevented the body mass and temperature changes and restored normal exercise capacity and brain blood flow dynamics.

These findings advance our understanding of how the human brain responds to strenuous exercise. It is now clear that conditions invoking extreme stress on the body reduces blood flow to many parts of the body including the brain. But this vital organ is able to preserve oxygen consumption which is of utmost importance for sustaining its function.

The findings also clearly substantiate the recommendation that people drink fluids during exercise, as this helps optimise performance.