Dehydration’s Effects on Cerebral Blood Flow  and  Oxygen Metabolism

A subject on an exercise cycle ergometer has his blood flow measured.

An exercise physiology study finds that dehydration lowers cerebral blood flow during maximal exercise with no consequence for oxygen consumption. 

An interesting study published last month looked at how dehydration affects cerebral blood flow and oxygen metabolism (Dehydration affects cerebral blood flow but not its metabolic rate for oxygen during maximal exercise in trained humans, 2014. Trangmar SJ, et al.). Cerebral blood flow is the rate of blood flow through the brain and can be measured in a specific area of the brain with fMRI or using one of the arteries that feeds the brain. In this study, the right common carotid artery was used. Oxygen consumption is a direct measure of metabolism in the brain since all energy storing compounds are metabolized through the reduction of oxygen.

In the aforementioned study, 10 trained male subjects were placed on a cycle ergometer in hydrated, dehydrated and rehydrated states.  As expected, cerebral blood flow was found to increase at maximal exercise capacity from rest. Cerebral blood flow decreased from the initial rise in perfusion as the exercise bout wore on. This increase in cerebral blood flow was diminished in the dehydrated subjects due to a faster rate of perfusion decline.

When the body is dehydrated, blood volume is reduced simply because there is less water content in the blood. The study found that the decline in cerebral blood flow with dehydration correlated with falling arterial carbon dioxide tension. However, extra-cranial perfusion increased in the dehydrated state and with body temperature.  Although cerebral blood flow was diminished in the dehydrated trials, oxygen metabolism was not compromised. This is presumably because oxygen extraction increased in the dehydrated state.  

Thus, this exercise physiology study shows that although dehydration decreases cerebral blood flow, the brain’s metabolism is not diminished.

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