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In this column, I wanted to highlight two recent publications that I thought would be of interest given the hot days of summer. Both of these are completely out of my area of expertise, so I present a brief synopsis without interpretation. I will also provide the reference for interested readers.
“The effect of dehydration on muscle metabolism and time trial performance during prolonged cycling in males”
Rationale: There are a number of studies that have examined the physiological consequences of dehydration during exercise (and cycling specific) performance. However, maintaining hydration during a prolonged event is often difficult. Although it is known that fluid loss can contribute to a decrease in exercise performance, it is not known how much changes in energy metabolism contribute to the decrease in performance. Since it is known that glycogen depletion correlates with fatigue, it is possible that changes in substrate use (carbohydrate versus fat) could contribute to the onset of fatigue with dehydration. This study examined the effects of progressive dehydration on time trial performance and several physiological parameters by studying subjects after an overnight fluid restriction followed by further dehydration during the exercise bout.
Methods: Nine trained (but not elite) males completed the protocol. During the dehydrated trial, subjects did not drink fluids after 6 pm on the night prior to testing and abstained from fluids throughout the exercise trial. For the control trial, subjects remained hydrated. During the exercise bout, subjects cycled at ~65% VO2peak for 90 min followed by a time trial to complete a given amount of work in as fast of time as possible. Blood and muscle were sampled for analysis.
Results: With a 2-3% body weight dehydration, subjects had increased carbohydrate use, greater muscle glycogen use, a 13% decrement in performance (time to complete the time trial), increased heart rate, increased core temperature, and increased perceived exertion.
Conclusions: According to the authors, the results may indicate that athletes training in a dehydrated state induce a greater cellular and whole body stress, which may enhance adaptations when training, but will hinder performance during competition.
The effect of dehydration on muscle metabolism and time trial performance during prolonged cycling in males. Logan-Sprenger HM, Heigenhauser GJ, Jones GL, Spriet LL. Physiol Rep. 2015 Aug;3(8). PMID: 26296770.
“Does wearing clothing made of a synthetic “cooling” fabric improve indoor cycle exercise endurance in trained athletes?”
Rationale: In humans, the evaporation of sweat from the skin surface accounts for ~80% of heat loss during physical activity. Failure to adequately thermoregulate during exercise can inhibit exercise performance. Since clothing presents a barrier to evaporative heat loss, clothing that minimizes resistance to evaporation could enhance exercise performance compared to restrictive clothing. The purpose of this study was to test the hypothesis that wearing clothing made of a synthetic fabric promoted by the manufacturer as having superior cooling properties would improve exercise performance of endurance trained athletes under ambient laboratory conditions.
Methods: Twenty competitive male and female amateur athletes completed the study. The study was a controlled, randomized, double blind, crossover study with cycling exercise performed with synthetic clothing control or synthetic clothing advertised as “cooling”. Exercise testing was performed as an incremental cycling test followed by exercise to fatigue at a given workload. A variety of physiological parameters were monitored during the exercise bout.
Results: There was no difference in exercise time to exhaustion between the fabrics. Further, there were no differences in cardiorespiratory parameters, or core or surface body temperature. Finally, there was no difference between perceived comfort, cooling, or performance between the two garments.
Conclusions: According to the authors, wearing a garment composed of a synthetic “cooling” fabric does not improve exercise performance of trained athletes, nor does it enhance thermoregulatory, cardiometabolic, ventilatory, and perceptual responses to exercise.
Does wearing clothing made of a synthetic "cooling" fabric improve indoor cycle exercise endurance in trained athletes? Abdallah SJ, Krug R, Jensen D. Physiol Rep. 2015 Aug;3(8). PMID: 26290527