Carbohydrate Metabolism Alters the Heat Acclimation Responses in Sedentary Individuals

It is well established that heat acclimation improves cardiovascular and thermal strain, which subsequently improves heat tolerance. However, these adaptations are often observed under a fixed external work‐rate heat acclimation (HA) protocol that leads to physiological habituation. Additionally, exercising under acute heat stress also alters metabolism (ie., increases muscle glycogen utilization) compared to cool conditions that can modulate internal heat production and oxygen uptake. This study tested the hypothesis that HA adaptations in core body temperature, heart rate and tolerance to thermal stress, utilizing a clamped hyperthermia model, are improved by alterations in carbohydrate metabolism and exercise economy. Twelve healthy sedentary subjects (7M/5F, 28±6y, 77.9±17kg), completed a 10‐day(90min/day exercise bout) clamp controlled (internal work‐rate)hyperthermia HA (42°C, 28% RH) and control (CON: 23°C, 42% RH) protocolsin a counter balanced design separated by at least 2 months. Internal work‐rate was clamp‐controlled on days 1 through 9, by maintaining an increase in core body temperature of 1.5 °C above baseline for the last 60 min of exercise. This was achieved by exercising the initial (day 1) 30min at 118±29 watts (external metabolic work rate) which then progressively increasing each day to a peak work rate of 140±39 watts by day 9. The last 60min of exercise were reduced to 11±4 watts each day. For days 1 and 10 subjects exercised at identical external work rates. A similar internal heat production was achieved on days 1 and 9 ( P >.05), but this value was attenuated by ~ 20% ( P <.01) on day 10 despite the workload being identical to day 1.HA attenuated the increased internal core temperature by ~38% on Day 10 compared to days 1 and 9 (Interaction effect, time x day, P <.0001) see figure 1A. Similarly, HA ( P <.01) attenuated the increase in final heart rate on day 10(Δ67±18 bpm), relative to day 1 and 9 (Δ89±16 vs 81±15 bpm, P >.05). The physiological strain index was reduced on day 10 HA ( P <.001,4.8±0.6 AU) in comparison to day 1 and 9 (7.5±1.3 vs 6.5±0.8 AU, P >0.05). Neither, HA nor CON regimens improved peak aerobic capacity ( P >0.05; preHA:37.1±6.9, postHA: 37.9±7.2, preCON: 36.7±6.3, postCON: 37.0±6.4 ml·kg·min −1 ).During HA, carbohydrate (CHO) oxidation was not different between day 1 and 9(2.1±0.9 vs. 1.9±0.9 g·min −1 P >.05,), but was attenuated by ~45% on day 10 (1.1±0.4 g·min −1 ; P < .001). Exercise economy on day 1 and 9 were similar (101.9±19 vs 91.4±23 mL·O2·km −1 , P >.05), but was reduced by ~19% on day 10 ( P <.001, 78.5±17 mL·O2·km −1 ).Pearson's Correlations for CHO and heat production resulted in a very strong positive correlation (r=.89, P <.0001) accounting for 80% variance over the HA days (see figure 1B). These data indicate that HA utilizing the controlled hyperthermia model improves thermal and cardiovascular strain when comparing similar external work‐rates(day 1 vs 10), however when controlling for internal heat production and CHO oxidation (days 1 vs 9), the attenuated core temperature, heart rate and physiological strain on day 10 are influenced by the reduced carbohydrate oxidation and improved exercise economy. Support or Funding Information Support funded partly by the American Heart Association Predoctoral Fellowship, the National Swimming Pool Foundation, and the Carl V. Gisolfi Memorial Research Fund from the American College of Sports Medicine Foundation. 1Change in rectal core temperature (Tre) during 90 min of exercise during the heat acclimation (HA) and control (CON) protocols (A) and the correlation coefficient between carbohydrate oxidation and internal work‐rate (Wkg −1 ) during HA (B) under similar external work‐rate for days 1 and 10 (118±29 vs 117±28 W, P >.05) and similar internal work‐rates for day 1 and 9 (7.1 ±2 vs 6.2±2 Wkg −1 , P >.05). Results are expressed as means ± SD. †, Statistically significant main interaction (time × day), P <.0001: a, Day 10 statistically different than days 1 and 9 HA ( P <. 0001); b, c, d, Each CON day (1, 9, 10) statistically different than days 1 and 9 (P < 0001). Strong correlations with a reduced leftward shift in CHO oxidation/Internal heat production on day 10 HA compared to day 1 HA with a similar external work‐rate.