The Weight of Evidence
An interesting research design and fairly nicely controlled overall, especially with having a control condition where no activity was performed. What were the main results?
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- As expected and desired, the hydration status during the dehydrated trials were lower, based on both body mass and urine specific gravity. There were also no differences in the energy costs between the hydrated and dehydrated exercise trials. As expected, the energy costs in the exercise trials were much higher than the control conditions where subjects rested for 45 minutes.
- Somewhat surprisingly, no differences were present in voluntary food intake across the three experimental conditions, each totaling approximately 5,000 kJ. The surprising aspect was that food intake remained similarly high even after no exercise. Therefore, when calculated relative to energy costs of those 45 minutes, relative food intake was MUCH higher in the control compared to exercise conditions.
- Breaking down the food intake further, there were no differences in the intake of carbohydrates, fats, or proteins. The only significant difference came in water intake, with the dehydrated condition eliciting more water intake than either the control or hydrated conditions.
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To The Table and The Road
This study provides some interesting insight into the effects of exercise and hydration on appetite. Namely, the primary conclusions were that neither the presence/absence of exercise, nor your hydration status, altered your conscious eating behavior.
First off, there is a major confounder. The need for scientific control, namely fasting from 9 p.m. onwards, meant that the subjects likely would have been equally hungry by the completion of the experiment at approximately 9 a.m. Therefore, they may all have been predisposed towards eating, minimizing the chances of seeing differences across conditions. This is compounded by the lack of data presented on the subjects' normal breakfast caloric intake, making it difficult to determine whether the exercise suppressed appetite down to non-exercising levels, or whether the subjects ate more than normal in the control conditions.
A second consideration is that, again for scientific control of prior nutrition, the study was done in the morning. Therefore, it may be difficult to extrapolate to the rest of the day, especially as many of us tend to consume most of our calories during lunches, snacks, and dinner compared to breakfast.
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Those considerations aside, there are important ideas generated here. The first is that we may need to be careful about our caloric intake during periods of lower training, as it appears that we may not subconsciously down-regulate our appetite properly to match the reduced energy needs.
A second finding is that our subjective thirst sensation seems to be a good match with our reduced hydration status. This adds to the idea that drinking to satisfy thirst may be sufficient as a gauge to maintaining hydration.
Ride fast, eat well, and have fun!
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References
1. Kelly PJ, Guelfi KJ, Wallman KE, and Fairchild TJ. Mild dehydration does not reduce postexercise appetite or energy intake. Med Sci Sports Exerc 44: 516-524, 2012.
Stephen Cheung is an Associate Professor of Kinesiology and a Canada Research Chair in Environmental Ergonomics at Brock University, with a research specialization in the effects of thermal stress on human physiology and performance. He can be reached for comments at thermal.doc@gmail.com.
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