Research Blog

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.

Is Breakfast Important for Health?

breakfast foods

A study finds 6 weeks of skipping breakfast does not alter resting metabolic rate, but reduces light, spontaneous physical activity. 

“Eat your breakfast.”

Breakfast is often labeled the most important meal of the day, but why? Weight loss programs stress the importance of breakfast because of its ability to lift ones resting metabolism and lower the amount of food eaten later in the day. A study released earlier this month shows this logic may be flawed. Breakfast is good for us, but not for the reasons we may think.

The aforementioned study was published in The American Journal of Clinical Nutrition (The Casual Role of Breakfast in energy balance and health: a randomized controlled trial in lean adults, 2014.  James Betts, et al.). The study randomly divided 33 lean subjects into two experimental groups for six weeks: fasting and breakfast. The subjects who were in the breakfast group were instructed to eat at least 700 Calories before 11 am. The fasting group was instructed to ingest nothing but plain water until after 12 pm. Subjects were monitored for a variety of metabolic indicators.

The results appear to show that breakfast is not a necessary component of a healthy lifestyle. In fact, an article in Science trumpeted  “Eat Breakfast to Lose Weight?  Not So Fast.” The study found that resting metabolic rate was no different between the breakfast eaters and fasters. In the fasters and breakfast eaters, resting metabolic rate was about 1450 Calories per day. However, breakfast eaters did have a small, but measurable, increase in diet induced thermogenesis (that is, energy expenditure from metabolizing food). The breakfast group reported eating significantly more calories per day than their fasting counterparts. Breakfast eaters ate an average of 2730 Calories per day relative to fasters who ate an average of 2191 Calories per day. The breakfast group, it was found, had significantly more daily carbohydrate calories. This debunks the commonly-held belief that one eats more calories after skipping breakfast to overcompensate for lost breakfast calories. Body mass and fat composition did not differ between groups before or after the 6 week experimental condition. Furthermore, there was no difference in absolute or relative body mass response between either of the 6 week experimental treatments.

Additional health indicator measurements showed no difference between groups as well. There was no difference in fasting glucose or insulin levels between groups. Cholesterol levels also showed no significant differences. Lastly, there was no difference in a variety of metabolic regulating hormones.  

Despite these findings, one variable was found to differ between groups: physical activity thermogenesis. Physical activity thermogenesis is a measure of calories burned through all physical activity. The breakfast group was found to expend 1450 daily Calories on physical activity compared to 1007 daily Calories for those who skipped breakfast. When these calories were partitioned, it was found that breakfast eaters did significantly more light physical activity in the morning. The authors noted that light intensity physical activity is generally spontaneous in nature. Thus, even if resting metabolic rate is the same, those who eat breakfast will spend more time above the resting metabolic rate.  Given how important physical activity is to long-term health, elevating physical activity thermogenesis would be a strong health motivator for eating breakfast. The difference in physical activity thermogenesis between the breakfast group and fasting group can be seen in the figure below.

breakfast affects physical activity

The authors argue that their experimental design elicits effects (or lack thereof) not found in other studies from skipping breakfast. They point to their comprehensive list of metabolic factors and randomization of subjects into experimental conditions as strength of the study. They conclude that breakfast may not be as important as previously thought. Other, recent studies have made similar, controversial conclusions. In fact, one study found no difference in health indicators after separating subjects into breakfast and fasting groups for 16 weeks.

However, the fact that breakfast raises physical activity is important. It is likely that 16 weeks, and certainly 6 weeks, is not enough time for differences in health indicators to emerge. Physical activity can prevent a variety of disease that occur far down the road. Plus, the combination of breakfast and exercise contributes to happiness and well being! So, ignore the controversial headlines, but don’t ignore the importance of breakfast.

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