The purpose of our study was to determine whether CHO in fluid ingested during 1 hour of steady-state exercise in the heat would accelerate the rise in core temperature or elicit greater heat production, resulting in an elevated core temperature. This concern was raised by Montain and Coyle,1
who examined graded dehydration by feeding participants with fluids containing CHO but did not test for a CHO effect because they did not include a CHO-free control beverage. Subsequently, several groups3–,6
have reported elevations in core temperature during exercise performance testing when CHO was ingested. These studies involved various durations of exercise sessions (60–130 minutes) and included performance testing at varied intensities4–,6
; hence, the design of these studies did not allow for a clear conclusion regarding a CHO effect. The potential for CHO intake to elevate core temperature during exercise warranted further investigation because of implicated effects on overheating, fatigue, and the risk of heat illness in athletes.
The protocol for our study consisted of consuming fluid 30 minutes before and every 15 minutes during steady-state exercise, a pattern that was based on recommendations to prevent dehydration during exercise25,28,29
and similar to that used in earlier research in which core temperature elevation was reported.3–,6
Our participants ingested a total of 93.7 g of CHO immediately before 30 minutes of rest and during the first 45 minutes of the 1-hour bout of exercise. Carbohydrate concentration of the fluid was similar to that in prior studies, and CHO was administered to our volunteers at a comparable rate (g/kg body weight).3,6
Our participants received 0.016 g CHO/kg body weight·min−1
during the observation, which was within the range of the 0.012 to 0.023 g/kg·min−1
used previously by authors3–,6
reporting the CHO effect on core temperature. Exercise increased core temperature as expected, but we did not detect a difference in core temperature when comparing the 2 beverage treatments, which differed only by the presence of CHO. The effect size for the CHO influence on core temperature ranged from 0.3 to 0.5, with variability (SD) exceeding the mean difference in the rate of change of core temperature. In addition, we did not observe an increase in mean o2
at rest or during exercise as a result of the CHO feeding.
Aside from varying exercise intensity and metabolic rate during exercise, other factors exist to explain the rise in core temperature observed previously in association with CHO feedings. In prior research,3–,6
body mass was not maintained during exercise. The acute reduction in body mass ranged from 0.7% to 4.4%, suggesting that dehydration might have an effect and promote an elevation in core temperature. The findings of Fritzsche et al3
support an interaction with hydration. Within the trials that produced similar power output, ingesting CHO without fluids during 2 hours of exercise resulted in elevated core temperature compared with the temperature when ingesting nothing. Both trials resulted in similar dehydration amounts of 4%.3
When the same dose of CHO was ingested with fluid to minimize dehydration (1% reduction in body mass), core temperature was approximately 0.75°C lower than when CHO was ingested without fluids. Interestingly, during the CHO trial with fluids, volunteers generated more metabolic work (greater power output). We designed our investigation to control for dehydration and minimize the change in body mass during exercise to remove this potential confounding factor.
The gradient between body temperature and the environment may also be a factor in establishing whether CHO ingested during exercise raises core temperature. In one of the earlier studies19
on steady-state exercise in which core temperature was similar with and without CHO, participants severely underconsumed beverage and experienced dehydration of approximately 5%. However the environment was much milder than in more recent studies (approximately 25°C). Similar to previous authors,3–,6
we reported the CHO-induced core temperature elevation using environmental conditions that were more extreme. As they did, we controlled the relative intensity, and volunteers sustained efforts at 65% of peak o2
. Subsequent researchers should investigate the interactive effects of more extreme environmental conditions, exercise duration and intensity, CHO dose, and compromised hydration status to clarify the feeding effect of CHO during exercise on core temperature response.
A final, but unlikely, possibility for an exaggerated elevation in core temperature associated with CHO intake in prior studies is because of the thermogenic effect of feeding.30–,32
With glucose ingestion, metabolic rate increases by 5% to 6% of the energy content of the feeding (75 g of glucose ingested in 5 minutes) and 5% to 10% over basal metabolic rate.30,33,34
Fructose can also increase resting metabolism by as much as 10%.33,34
In our study, the CHO sources were sucrose, glucose, and fructose in amounts that contributed nearly a 50:50 mixture of glucose and fructose. This blend is rapidly absorbed20–,22
and oxidized during exercise at higher rates than a single form of carbohydrate.23,24
Given that heat production during exercise is dramatically greater than at rest,35
either any thermogenic effect of feeding is likely within the error of our measurements or volunteers were capable of absorbing or dissipating this small amount of heat without raising core temperature. Unlike rest conditions, CHO would not be expected to elicit an elevation in metabolic rate during exercise of more than 1% of the energy production in our study.