Occupational heat stress increases kidney injury (KI) risk due to the development of hyperthermia and dehydration. Workplace recommendations aim to maintain core temperature below 38.0°C and prevent dehydration (<2% body mass loss) in unacclimated workers. We hypothesized that KI risk would be attenuated when adhering to occupation heat stress recommendations compared to a work-matched positive control trial that was non-compliant with the recommendations. Twelve healthy adults (6 women) completed 4 h exposures to wet bulb globe temperatures of 30±2°C (Compliant [C]) and 36±0°C (Non-compliant [NC]). Subjects walked on a treadmill evoking a rate of metabolic heat production of 462±96 W. Consistent with the recommendations, work-rest ratios were prescribed as 15 min of walking and 45 min of rest per hour and subjects were provided 237 mL of sport drink every 15 min and drank ad libitum. Core temperature is presented as an average over the 4 h exposure. Percent change in body mass from pre- and postexposure provided an index of body water loss. Urine samples were collected preexposure, and immediately and 1 h postexposure. KI biomarkers of urinary insulin-like growth factor-binding protein 7 [IGFBP7] and tissue inhibitor of metalloproteinase 2 [TIMP-2] were measured and normalized to urine flow rate. The peak change in the product of IGFBP7 and TIMP-2 (Δ[IGFBP7xTIMP-2]) provided an overall index of KI risk. Data are presented as mean±SD. Average core temperature was lower in the C (37.3±0.3°C) compared to the NC (38.2±0.2°C; p<0.01) trial. Percent changes in body mass did not differ between the C (-0.3±0.6%) and NC (-0.7±1.0%; p=0.19) trials. UFR decreased from pre (C: 5.7±3.1 mL/min; NC: 5.2±3.1 mL/min) to postexposure (C: 2.2±1.8 mL/min; NC: 0.8±0.5 mL/min; p≤0.02) but was not different between trials (p=0.07) and returned to preexposure values 1 h postexposure (p≥0.21). IGFBP7 did not change over time (p=0.27) and was not different between trials (p=0.56). In contrast, TIMP-2 increased from preexposure (C: 0.2±0.2 ng/min; NC: 0.4±0.4 ng/min) to postexposure (C:7.1±17.7 ng/min; NC: 49.3±103.3 ng/min) in both trials (p≤0.01) and was higher in the NC trial both postexposure (p=0.02) and 1 h postexposure (C: 0.2±0.2 ng/min; NC 14.0±44.3 ng/min; p<0.01). Finally, peak Δ[IGFBP7xTIMP-2] was higher in the NC trial (+16.1±39.7 [ng/mL2]/1000) compared to the C trial (+0.8±1.5 [ng/mL2]/1000; p=0.02). Simulated occupational heat stress elevates KI risk, likely due to distal tubule injury as evident by increases in urinary TIMP-2. Nonetheless, violating the recommendations augmented KI risk when work duration was matched. RCT: NCT04767347. Supported By: R01OH011528 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.
INTRODUCTION Federal and local entities issue guidance that aims to maintain core temperature (TC) below 38.0°C in unacclimated workers exposed to heat. These heat stress recommendations prescribe work‐rest ratios as a function of wet bulb globe temperature (WBGT) and metabolic heat production (Hprod). Hydration recommendations are less nuanced and suggest drinking 237 mL of a cool sport drink every 15‐20 min for work in the heat exceeding 2 h. This drinking regimen is based on the premise that ad libitum drinking during heat exposure results in dehydration [defined as >2% body mass loss (BML)] due to inadequate drinking. To our knowledge, this hydration guidance has not been examined under heat stress recommendation compliant scenarios. Thus, it is unknown if ad libitumdrinking results in dehydration during recommendation compliant work in the heat. PURPOSE Test the hypothesis that ad libitum drinking during heat stress recommendation compliant work in the heat will result in >2% BML. METHODS Six, healthy, unacclimated, euhydrated subjects (2 women; 27±5 y) completed four trials consisting of 4 h (half a workday) of exposure to WBGTs of 23.9±0.2°C (A), 26.3±0.9°C (B), 28.4±0.1°C (C), 29.2±0.7°C (D). Subjects walked on a treadmill evoking a Hprod of 481±95 W, which is the average Hprod for outdoor workers in the U.S. NIOSH‐compliant work‐rest ratios were prescribed as a function of WBGT and Hprod [work:rest per hour (in min) ‐ A: 60:0, B: 45:15, C: 30:30, D: 15:45] and subjects were provided 237 mL of a cool (10±4°C) flavor preferred sport drink (Gatorade) every 15 min and drank ad libitum. BML was determined from changes in nude body mass and correction for fluid intake and urine loss provided an estimate of sweat rate. TC (telemetry pill) is reported as an average over the trial and as the percent of time TC exceeded 38.0°C (Time >38). Thirst perception (1 = not thirsty, 9 = very, very thirsty) is reported as an average over the trial. Data are presented as mean±SD (parametric) or median (interquartile range) (nonparametric). RESULTS Average TC was higher in Trial A (37.8±0.4°C; p=0.05) and Trial B (37.6±0.3°C; p=0.03) versus Trial D (37.3±0.4°C) but did not differ between the other trials (p≥0.20). Time >38 did not differ between trials (A: 24 (5,43)%, B: 6 (0,5)%, C: 0 (0,5)%, D: 0 (0,0)%; p=0.17). Sweat rate was higher in Trial A (0.5±0.1 L/h) versus Trial D (0.4±0.1 L/h; p=0.05) but did not differ between any other trials (p≥0.12). Thirst perception (A: 2±1 a.u., B: 2±0 a.u, C: 2±1 a.u, D: 2±0 a.u; p=0.20) and fluid intake (A: 1.6±0.6 L, B: 1.5±0.5 L, C: 1.7±0.5 L, D: 1.6±0.7 L; p=0.88) did not differ between trials. BML did not differ between trials (A: ‐0.9±0.9%, B: ‐0.8±0.6%, C: ‐0.5±0.4%, D: ‐0.3±0.6%; p=0.24). One subject in one trial lost >2% body mass (Trial A: ‐2.7%). CONCLUSION Ad libitumdrinking during recommendation compliant work in the heat rarely resulted in dehydration. Modification of work‐rest ratios across a range of WBGTs while working at a fixed Hprod mostly maintained TC <38.0°C when ad...
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