Objective: The purpose of this study was to compare glucose and insulin responses during an oral glucose tolerance test (OGTT) in cold (C), neutral (N), and hot (H) environments. Methods: Eleven males completed three 4-hour climate-controlled OGTT trials (C, 7.2°C; N, 22°C; and H, 43°C). Participants remained semireclined for 60 minutes before ingesting a 1.8 g/kg glucose beverage. Skin and rectal core temperatures were continuously monitored. Blood was collected just before glucose ingestion (time 0) and at 15, 30, 60, 90, 120, and 180 minutes, and analyzed for serum glucose, insulin, hematocrit, and hemoglobin. Expired gases were collected upon entering the chamber (−60 minutes), before glucose ingestion (0 minutes), and at 60, 120, and 180 minutes to determine Vo₂ and respiratory exchange ratio. Results: Rectal core temperature was greater in the H condition compared with both C and N ( P < .001). Rectal core temperature was not different between C and N, whereas skin temperature was different across all trials (H greater than N greater than C). The Vo₂ was greater in C than in both H and N during all time points. Carbohydrate oxidation was greater in C compared with H and N ( P < 0.001). Glucose was higher during H compared with C and N ( P ≤ 0.002). Glucose was elevated in C compared with N. Insulin was higher in H compared with C ( P = 0.009). Area under the curve for serum glucose was greater in H compared with C and N ( P ≤ 0.001); however, there was no significant difference in area under the curve for insulin. Conclusions: These data indicate that after an OGTT, glucose and insulin are elevated in a hot environment.

Methods Subjects were randomly assigned to a 6-wk plyometric treatment (P; n = 11) or control group (C; n = 11). Preintervention and postintervention outcomes included body composition, vertical jump, sit-and-reach, maximal oxygen consumption (V˙O2max), speed at onset of blood lactate, 3-km time trial performance, ECR, and a vastus lateralis muscle biopsy for protein analysis. Results Plyometric intervention resulted in improved time trial (P, 2.6% faster, P = 0.04; C, 1.6%, P = 0.17). V˙O2max improved in the P group (5.2%, P = 0.03), whereas the C group increased by 3.1% (P = 0.20). The ECR decreased in the P group as the result of 6 wk of plyometric training (P = 0.02 for stage 3), whereas it increased in the C group (P = 0.02 for stage 3). The ECR correlated strongly with performance at stages 2, 3, and 4 (r > 0.8, P < 0.001) independent of group. There was no significant main effect of group, time, or interaction on any of the protein isoforms analyzed. A negative correlation was found between the ECR at stage 7 and MHC IIa (r = −0.96, P < 0.001), and the ECR at stage 6 with titin isoform 1 (T1)/T2 ratio (r = −0.69, P = 0.007) independent of group. Conclusion Six weeks of plyometric training improved running performance and the ECR despite no measurable changes in MHC and titin isoforms. However, higher MHC IIa and lower T1/T2 isoform ratios correlated to lower ECR....

It is unclear whether the capabilities of recreationally active modern humans are unique due to present training practices or linked to the selection resulting from migration, escape, scavenging, and hunting and/or endurance running in early Homo. The purpose of this study was to determine upper values for total energy expenditure (TEE) and water turnover during sustained work for periods of 12-24 and 12-48 h, respectively and compare them with other species and proposed activities of early Homo. Stable isotopic water (²H₂ ¹⁸O) was used during competitions in hot environments to establish energy expenditure rates of approaching 10 times resting metabolism (RM) for 12.7 and 26.8 h, respectively. These events demonstrate pronounced hydration demands, with water output rates ranging from 25–95% of initial total body water for events lasting 12–48 h, respectively. These results provide new evidence for a high, sustained work (0.5–2 days) output and hydration demand in humans compared to other species in hot environments. Although the span for sustained metabolic activity in humans is large, it does not require elite level training status so long as adequate exogenous fuel and water are accessible. Because these values far exceed reported expectations/needs for foraging and persistence hunting in early Homo, it remains unclear if the phenomenon of the metabolic range is a modern human characteristic. While modern recreational endurance participants can demonstrate a TEE approaching 10 times RM, the rationale and need for such a high human metabolic ceiling is unclear when considering the energy demands of early Homo.

This study compared 2 commercially available beverages, an oral rehydration solution (ORS; 60.9 mM Na+; 3.4% carbohydrate) and a sports drink (SDS; 18.4 mM Na+; 5.9% carbohydrate), on hydration and metabolism during submaximal exercise in the heat. Ten male subjects completed two 90-min exercise trials (39ºC, 30%) of walking at 50% VO2max followed by a 30-min rest period in the heat while wearing wildland firefighter personal protective clothing. After 45 min of exercise, fluid delivery by either ORS or SDS replaced 150% of sweat loss. Subjects continued the exercise for 45 additional minutes followed by a 30-min rest period. Blood samples were collected pre-exercise (0 min), post-exercise (90 min), and post-trial (120 min) to measure plasma volume (%) and blood glucose (mg·dL-1). Expired gases were collected twice for 3 min for substrate oxidation. The sweat rate and percent dehydration did not differ between the groups (P=0.86 and P=0.79, respectively). Changes in plasma volume did not differ (P=0.55). Hemoglobin levels significantly increased in both groups post-trial (P=0.009). Blood glucose was significantly greater post-trial in SDS versus ORS (116±19 vs 103±13 mg·dL-1, respectively; P=0.01). Fat oxidation was lower post-exercise in SDS vs ORS (0.38±0.1 vs 0.47±0.2 g·min-1, respectively; P=0.049). These data indicate no difference in fluid retention between ORS or SDS when supplemented during exercise in the heat. This implies that fluid volume, and not drink contents, may be more important when ingested during exercise in a hot environment.

Objective: To document the effects of wildland firefighter (WLFF) critical training (CT) on physiologic markers of muscle damage and acute overreaching. Methods: Eighteen male and three female WLFFs were studied during an 11-day CT. Upper-body (US), lower-body (LS) muscle soreness and body weight (BW) were collected daily. Blood was collected on days 1, 4, 8, and 11 to measure creatine kinase (CK), lactate dehydrogenase (LDH), cortisol, and testosterone. Skinfolds were taken on days 1 and 11 to estimate body fat (BF) and lean body weight (LBW). Results: CT resulted in a significant depression in BF and elevation in LBW, while main effects of time were seen in US, LS, CK, LDH, cortisol, and testosterone/cortisol ratio (P < 0.05). Conclusion: These data suggest WLFFs undergo significant physiological stressors resulting in muscle soreness, damage, and possible overreaching during CT.