Effects of Heat Adaptation on Physiology, Perception, and Exercise Performance in the Heat: An Updated Meta-Analysis

Abstract

Purpose

Heat adaptation (HA) is a popular strategy to combat the negative effects of thermal stress. The HA literature has expanded since a 2016 meta-analysis, and we provide an updated meta-analysis, incorporating 39 additional studies and advanced analysis.

Methods

Following Pubmed searches, full-text original articles using human participants were reviewed using the four-stage PRISMA process. Data were extracted by at least two of the authors. Hedges’ g effect sizes, 95% confidence intervals, and prediction intervals were calculated. Correlations were run where appropriate.

Results

One hundred and thirty-five total articles (96 previous, 39 new) were reviewed. Medium-term (8–14 days), active, constant work HA regimens remain the most common despite a recent focus on isothermal, passive, and short-term (≤ 7 days) alternatives. HA still improves subsequent exercise performance and capacity in the heat (g = 0.7), reduces resting core temperature (g = − 0.6) and heart rate (g = − 0.5), and increases sweat rate (g = 0.4) but the effect sizes are lower than previously reported. HA has a moderate or larger effect (g > 0.5) on lowering sweat onset temperature, mean heart rate, sweat sodium and chloride concentrations, resting thermal sensation, and thirst sensation, and increasing resting plasma volume. There is considerable heterogeneity within the data for most variables.

Conclusion

HA regimens can reduce physiological and perceptual strain and improve subsequent exercise performance and capacity in the heat. Longer regimens may be more effect than shorter ones, but the data are lacking. Passive HA is a practical, effective alternative to active HA.

Keywords

Acclimation / Acclimatisation / Thermal impulse / Thermal strain / Systematic review

References

[1.] Ashworth E, Cotter J, Kilding A. Post-exercise, passive heat acclimation with sauna or hot-water immersion provide comparable adaptations to performance in the heat in a military context. Ergonomics, 2023, 66: 49-60,

[2.] Barley OR, Chapman DW, Mavropalias G, Abbiss CR. The influence of heat acclimation and hypohydration on post-weight-loss exercise performance. Int J Sports Physiol Perform, 2020, 15: 213-221,

[3.] Barry H, Gendron P, Gagnon C, Bherer L, Gagnon D. Passive heat acclimation does not modulate processing speed and executive functions during cognitive tasks performed at fixed levels of thermal strain. Appl Physiol Nutr Metab Physiol Appl Nutr Metab, 2022, 47: 261-268,

[4.] Bartolomé I, Siquier-Coll J, Pérez-Quintero M, Robles-Gil MC, Grijota FJ, Muñoz D, Maynar-Mariño M. 3-week passive acclimation to extreme environmental heat (100± 3 °C) in dry sauna increases physical and physiological performance among young semi-professional football players. J Therm Biol, 2021, 100,

[5.] Borenstein M, Hedges LV, Higgins JP, Rothstein HR. . Introduction to meta-analysis, 2021 New York Wiley,

[6.] Borg DN, O’Brien JL. Comment on “Effects of heat acclimation and acclimatisation on maximal aerobic capacity compared to exercise alone in both thermoneutral and hot environments: a meta-analysis and meta-regression”. Sports Med Auckl NZ, 2022, 52: 1715-1718,

[7.] Bourque CW. Central mechanisms of osmosensation and systemic osmoregulation. Nat Rev Neurosci, 2008, 9: 519-531,

[8.] Callovini A, Fornasiero A, Savoldelli A, Stella F, Low DA, Pellegrini B, Schena F, Bortolan L. Effects of three-exercise sessions in the heat on endurance cycling performance. J Therm Biol, 2021, 98,

[9.] Campbell HA, Akerman AP, Kissling LS, Prout JR, Gibbons TD, Thomas KN, Cotter JD. Acute physiological and psychophysical responses to different modes of heat stress. Exp Physiol, 2022, 107: 429-440,

[10.] Casa DJ, Becker SM, Ganio MS, Brown CM, Yeargin SW, Roti MW, Siegler J, Blowers JA, Glaviano NR, Huggins RA, Armstrong LE, Maresh CM. Validity of devices that assess body temperature during outdoor exercise in the heat. J Athl Train, 2007, 42: 333-342, pmcid: 1978469,

[11.] Chiolero A, Santschi V, Burnand B, Platt RW, Paradis G. Meta-analyses: with confidence or prediction intervals?. Eur J Epidemiol, 2012, 27: 823-825,

[12.] Cohen J. . Statistical power analysis for the behavioral sciences, 1988 New Jersey Lawrence Erlbaum 543

[13.] Daanen HAM, Racinais S, Periard JD. Heat acclimation decay and re-induction: a systematic review and meta-analysis. Sports Med, 2018, 48: 409-430,

[14.] Duvnjak-Zaknich DM, Wallman KE, Dawson BT, Peeling P. Continuous and intermittent heat acclimation and decay in team sport athletes. Eur J Sport Sci, 2019, 19: 295-304,

[15.] Eichna LW, Park CR, Nelson N, Horvath SM, Palmes ED. Thermal regulation during acclimatization in a hot, dry (desert type) environment. Am J Physiol, 1950, 163: 585-597,

[16.] Fenemor SP, Driller MW, Gill ND, Mills B, Casadio JR, Beaven CM. Practical application of a mixed active and passive heat acclimation protocol in elite male Olympic team sport athletes. Appl Physiol Nutr Metab Physiol Appl Nutr Metab, 2022, 47: 981-991,

[17.] Flouris AD, Poirier MP, Bravi A, Wright-Beatty HE, Herry C, Seely AJ, Kenny GP. Changes in heart rate variability during the induction and decay of heat acclimation. Eur J Appl Physiol, 2014, 114: 2119-2128,

[18.] Gale RM, Etxebarria N, Pumpa KL, Pyne DB. Cycling-based repeat sprint training in the heat enhances running performance in team sport players. Eur J Sport Sci, 2021, 21: 695-704,

[19.] Galloway SDR, Maughan RJ. Effects of ambient temperature on the capacity to perform prolonged exercise in man. MedSciSports Exerc, 1997, 29: 1240-1249

[20.] Ganio MS, Brown CM, Casa DJ, Becker SM, Yeargin SW, McDermott BP, Boots LM, Boyd PW, Armstrong LE, Maresh CM. Validity and reliability of devices that assess body temperature during indoor exercise in the heat. J Athl Train, 2009, 44: 124-135, pmcid: 2657027,

[21.] Garrett AT, Rehrer NJ, Patterson MJ. Induction and decay of short-term heat acclimation in moderately and highly trained athletes. Sports Med, 2011, 41: 757-771,

[22.] Gibson OR, James CA, Mee JA, Willmott AGB, Turner G, Hayes M, Maxwell NS. Heat alleviation strategies for athletic performance: a review and practitioner guidelines. Temp Austin Tex, 2020, 7: 3-36,

[23.] Gollan S, Chalmers S, Alderton S, Norton K. Supplemental intermittent-day heat training and the lactate threshold. J Therm Biol, 2017, 65: 16-20,

[24.] Gonzalez-Alonso J, Calbet JA. Reductions in systemic and skeletal muscle blood flow and oxygen delivery limit maximal aerobic capacity in humans. Circulation, 2003, 107: 824-830,

[25.] Greenfield AM, Pereira FG, Boyer WR, Apkarian MR, Kuennen MR, Gillum TL. Short-term hot water immersion results in substantial thermal strain and partial heat acclimation; comparisons with heat-exercise exposures. J Therm Biol, 2021, 97,

[26.] Griefahn B. Acclimation to three different hot climates with equivalent wet bulb globe temperatures. Ergonomics, 1997, 40: 223-234,

[27.] Guy JH, Deakin GB, Edwards AM, Miller CM, Pyne DB. Adaptation to hot environmental conditions: an exploration of the performance basis, procedures and future directions to optimise opportunities for elite athletes. Sports Med, 2015, 45: 303-311,

[28.] Haroutounian A, Amorim FT, Astorino TA, Khodiguian N, Curtiss KM, Matthews ARD, Estrada MJ, Fennel Z, McKenna Z, Nava R, Sheard AC. Change in exercise performance and markers of acute kidney injury following heat acclimation with permissive dehydration. Nutrients, 2021, 13: 841, pmcid: 8000862,

[29.] Heathcote SL, Hassmén P, Zhou S, Stevens CJ. Passive heating: reviewing practical heat acclimation strategies for endurance athletes. Front Physiol, 2018, 9: 1851, pmcid: 6306444,

[30.] Henderson MJ, Chrismas BCR, Fransen J, Coutts AJ, Taylor L. Responses to a 5-day sport-specific heat acclimatization camp in elite female rugby sevens athletes. Int J Sports Physiol Perform, 2022, 17: 969-978,

[31.] Higgins JPT, Thompson SG, Spiegelhalter DJ. A re-evaluation of random-effects meta-analysis. J R Stat Soc Ser A Stat Soc, 2009, 172: 137-159, pmcid: 2667312,

[32.] Horvath SM, Shelley WB. Acclimatization to extreme heat and its effect on the ability to work in less severe environments. Am J Physiol, 1946, 146: 336-343,

[33.] Hutchins KP, Borg DN, Bach AJE, Bon JJ, Minett GM, Stewart IB. Female (under) representation in exercise thermoregulation research. Sports Med Open, 2021, 7: 43, pmcid: 8219822,

[34.] IntHout J, Ioannidis JPA, Rovers MM, Goeman JJ. Plea for routinely presenting prediction intervals in meta-analysis. BMJ Open, 2016, 6, pmcid: 4947751,

[35.] IUPS Thermal Commission. Glossary of terms for thermal physiology. Second edition. Revised by The Commission for Thermal Physiology of the International Union of Physiological Sciences (IUPS Thermal Commission). Pflugers Arch, 1987, 410: 567-587

[36.] Kissling LS, Akerman AP, Campbell HA, Prout JR, Gibbons TD, Thomas KN, Cotter JD. A crossover control study of three methods of heat acclimation on the magnitude and kinetics of adaptation. Exp Physiol, 2022, 107: 337-349,

[37.] Ko Y, Kang J, Seol S-H, Lee J-Y. Effectiveness of skin-heating using a water-perfused suit as passive and post-exercise heat acclimation strategies. J Therm Biol, 2020, 93,

[38.] Liberati A, Altman DG, Tetzlaff J, Mulrow C, Gotzsche PC, Ioannidis JP, Clarke M, Devereaux PJ, Kleijnen J, Moher D. The PRISMA statement for reporting systematic reviews and meta-analyses of studies that evaluate healthcare interventions: explanation and elaboration. BMJ, 2009, 339, pmcid: 2714672,

[39.] Lind AR, Bass DE. Optimal exposure time for development of acclimatization to heat. Fed Proc, 1963, 22: 704-708,

[40.] McCleave EL, Slattery KM, Duffield R, Saunders PU, Sharma AP, Crowcroft S, Coutts AJ. Impaired heat adaptation from combined heat training and “Live High, Train Low” hypoxia. Int J Sports Physiol Perform, 2019, 14: 635-643,

[41.] McGlynn ML, Collins C, Hailes W, Ruby B, Slivka D. Heat acclimation in females does not limit aerobic exercise training outcomes. Int J Environ Res Public Health, 2022, 19: 5554, pmcid: 9103535,

[42.] McIntyre RD, Zurawlew MJ, Mee JA, Walsh NP, Oliver SJ. A comparison of medium-term heat acclimation by post-exercise hot water immersion or exercise in the heat: adaptations, overreaching, and thyroid hormones. Am J Physiol Regul Integr Comp Physiol, 2022, 323: R601-R615, pmcid: 9602919,

[43.] McIntyre RD, Zurawlew MJ, Oliver SJ, Cox AT, Mee JA, Walsh NP. A comparison of heat acclimation by post-exercise hot water immersion and exercise in the heat. J Sci Med Sport, 2021, 24: 729-734,

[44.] Mee JA, Gibson OR, Doust J, Maxwell NS. A comparison of males and females’ temporal patterning to short- and long-term heat acclimation. Scand J Med Sci Sports, 2015, 25(Suppl 1): 250-258,

[45.] Mitchell D, Senay LC, Wyndham CH, van Rensburg AJ, Rogers GG, Strydom NB. Acclimatization in a hot, humid environment: energy exchange, body temperature, and sweating. J Appl Physiol, 1976, 40: 768-778,

[46.] Murray KO, Clanton TL, Horowitz M. Epigenetic responses to heat: from adaptation to maladaptation. Exp Physiol, 2022, 107: 1144-1158,

[47.] Naito T, Haramura M, Muraishi K, Yamazaki M, Takahashi H. Cooling during short-term heat acclimation enhances aerobic capacity but not sweat capacity. Eur J Sport Sci, 2022, 22: 579-588,

[48.] Nybo L, Nielsen B. Hyperthermia and central fatigue during prolonged exercise in humans. J Appl Physiol, 2001, 1985(91): 1055-1060,

[49.] Ormerod JK, Elliott TA, Scheett TP, Vanheest JL, Armstrong LE, Maresh CM. Drinking behavior and perception of thirst in untrained women during 6 weeks of heat acclimation and outdoor training. Int J Sport Nutr Exerc Metab, 2003, 13: 15-28,

[50.] Pandolf KB, Burse RL, Goldman RF. Role of physical fitness in heat acclimatisation, decay and reinduction. Ergonomics, 1977, 20: 399-408,

[51.] Patterson MJ, Stocks JM, Taylor NA. Humid heat acclimation does not elicit a preferential sweat redistribution toward the limbs. Am J Physiol Regul Integr Comp Physiol, 2004, 286: R512-R518,

[52.] Patterson MJ, Stocks JM, Taylor NA. Sustained and generalized extracellular fluid expansion following heat acclimation. J Physiol, 2004, 559: 327-334, pmcid: 1665061,

[53.] Perez-Quintero M, Siquier-Coll J, Bartolomé I, Robles-Gil MC, Muñoz D, Maynar-Mariño M. Three weeks of passive and intervallic heat at high temperatures (100 ± 2 °C) in a sauna improve acclimation to external heat (42 ± 2 °C) in untrained males. J Therm Biol, 2021, 96,

[54.] Periard JD, Cramer MN, Chapman PG, Caillaud C, Thompson MW. Cardiovascular strain impairs prolonged self-paced exercise in the heat. Exp Physiol, 2011, 96: 134-144,

[55.] Periard JD, Racinais S, Sawka MN. Adaptations and mechanisms of human heat acclimation: applications for competitive athletes and sports. Scand J Med Sci Sports, 2015, 25(Suppl 1): 20-38,

[56.] Periard JD, Racinais S, Timpka T, Dahlstrom O, Spreco A, Jacobsson J, Bargoria V, Halje K, Alonso JM. Strategies and factors associated with preparing for competing in the heat: a cohort study at the 2015 IAAF World Athletics Championships. Br J Sports Med, 2017, 51: 264-270,

[57.] Philp CP, Pitchford NW, Visentin DC, Kitic CM, Fell JW, Buchheit M, Minson CT, Gregory JR, Watson G. Can ten days of heat acclimation training improve temperate-condition rowing performance in national-level rowers?. PLoS ONE, 2022, 17, pmcid: 9436058,

[58.] Poirier MP, Gagnon D, Friesen BJ, Hardcastle SG, Kenny GP. Whole-body heat exchange during heat acclimation and its decay. Med Sci Sports Exerc, 2015, 47: 390-400,

[59.] Pryor JL, Pryor RR, Vandermark LW, Adams EL, VanScoy RM, Casa DJ, Armstrong LE, Lee EC, DiStefano LJ, Anderson JM, Maresh CM. Intermittent exercise-heat exposures and intense physical activity sustain heat acclimation adaptations. J Sci Med Sport, 2019, 22: 117-122,

[60.] Pryor RR, Pryor JL, Vandermark LW, Adams EL, Brodeur RM, Armstrong LE, Lee EC, Maresh CM, Casa DJ. Short term heat acclimation reduces heat strain during a first, but not second, consecutive exercise-heat exposure. J Sci Med Sport, 2021, 24: 768-773,

[61.] Racinais S, Alonso JM, Coutts AJ, Flouris AD, Girard O, Gonzalez-Alonso J, Hausswirth C, Jay O, Lee JK, Mitchell N, Nassis GP, Nybo L, Pluim BM, Roelands B, Sawka MN, Wingo J, Periard JD. Consensus recommendations on training and competing in the heat. Sports Med, 2015, 45: 925-938, pmcid: 4473280,

[62.] Racinais S, Periard JD, Karlsen A, Nybo L. Effect of heat and heat acclimatization on cycling time trial performance and pacing. Med Sci Sports Exerc, 2015, 47: 601-606, pmcid: 4342312,

[63.] Reeve T, Gordon R, Laursen PB, Lee JKW, Tyler CJ. Impairment of cycling capacity in the heat in well-trained endurance athletes after high-intensity short-term heat acclimation. Int J Sports Physiol Perform, 2019, 14: 1058-1065,

[64.] Rivas E, Crandall CG, Suman OE, Moustaid-Moussa N, Ben-Ezra V. Exercise heat acclimation causes post-exercise hypotension and favorable improvements in lipid and immune profiles: a crossover randomized controlled trial. J Therm Biol, 2019, 84: 266-273,

[65.] Robinson S, Turrell ES, Belding HS, Horvath SM. Rapid acclimatization to work in hot climates. Am J Physiol, 1943, 140: 168-176,

[66.] Roloff V, Higgins JPT, Sutton AJ. Planning future studies based on the conditional power of a meta-analysis. Stat Med, 2013, 32: 11-24,

[67.] Roussey G, Bernard T, Fontanari P, Louis J. Heat acclimation training with intermittent and self-regulated intensity may be used as an alternative to traditional steady state and power-regulated intensity in endurance cyclists. J Therm Biol, 2021, 98,

[68.] Saillant MM, Charkoudian N, Salgado RM. Individual variability in achievement of short-term heat acclimation during a fixed intensity protocol. J Therm Biol, 2022, 110,

[69.] Sawka MN, Leon LR, Montain SJ, Sonna LA. Integrated physiological mechanisms of exercise performance, adaptation, and maladaptation to heat stress. Compr Physiol, 2011, 1: 1883-1928,

[70.] Schlader ZJ, Simmons SE, Stannard SR, Mundel T. The independent roles of temperature and thermal perception in the control of human thermoregulatory behavior. Physiol Behav, 2011,

[71.] Schmit C, Duffield R, Hausswirth C, Brisswalter J, Le MY. Optimizing heat acclimation for endurance athletes: high- vs low-intensity training. Int J Sports Physiol Perform, 2017,

[72.] Shaw J, Walkington C, Cole E, Gleadall-Siddall DO, Burke R, Bray J, Simpson AJ, Vince RV, Garrett AT. Effectiveness of short-term isothermic-heat acclimation (4 days) on physical performance in moderately trained males. PLoS ONE, 2022, 17, pmcid: 9671432,

[73.] Slivka D, Shute R, Hailes W, Marshall K, Opichka M, Schnitzler H, Ruby B. Exercise in the heat blunts improvements in aerobic power. Eur J Appl Physiol, 2021, 121: 1715-1723,

[74.] Stone BL, Ashley JD, Skinner RM, Polanco JP, Walters MT, Schilling BK, Kellawan JM. Effects of a short-term heat acclimation protocol in elite amateur boxers. J Strength Cond Res, 2022, 36: 1966-1971,

[75.] Sumi D, Nagatsuka H, Matsuo K, Okazaki K, Goto K. Heat acclimation does not attenuate hepcidin elevation after a single session of endurance exercise under hot condition. Eur J Appl Physiol, 2022, 122: 1965-1974, pmcid: 9174628,

[76.] Sunderland C, Morris JG, Nevill ME. A heat acclimation protocol for team sports. Br J Sports Med, 2008, 42: 327-333,

[77.] Tan SCC, Ang WH, Lim LSX, Low ICC, Lee JKW. Efficacy of isothermic conditioning over military-based heat acclimatization and interval training in tropical native males. Med Sci Sports Exerc, 2022, 54: 1925-1935, pmcid: 9632943,

[78.] Taylor HL, Henschel A, Mickelsen O, Keys A. The effect of the sodium chloride intake on the work performance of man during exposure to dry heat and experimental heat exhaustion. Am J Physiol, 1943, 140: 439-451,

[79.] Taylor NA. Human heat adaptation. Compr Physiol, 2014, 4: 325-365,

[80.] Timpmann S, Hackney AC, Tamm M, Kreegipuu K, Unt E, Ööpik V. Influence of Rhodiola rosea on the heat acclimation process in young healthy men. Appl Physiol Nutr Metab Physiol Appl Nutr Metab, 2018, 43: 63-70,

[81.] Travers G, Nichols D, Riding N, González-Alonso J, Périard JD. Heat acclimation with controlled heart rate: influence of hydration status. Med Sci Sports Exerc, 2020, 52: 1815-1824,

[82.] Tyler C, Sunderland C. The effect of ambient temperature on the reliability of a preloaded treadmill time-trial. Int J Sports Med, 2008, 29: 812-816,

[83.] Tyler CJ, Reeve T, Hodges GJ, Cheung SS. The effects of heat adaptation on physiology, perception and exercise performance in the heat: a meta-analysis. Sports Med, 2016, 46: 1699-1724,

[84.] Tyler CJ, Sunderland C, Cheung SS. The effect of cooling prior to and during exercise on exercise performance and capacity in the heat: a meta-analysis. Br J Sports Med, 2015, 49: 7-13,

[85.] Waldron M, Jeffries O, Tallent J, Patterson S, Nevola V. The time course of adaptations in thermoneutral maximal oxygen consumption following heat acclimation. Eur J Appl Physiol, 2019, 119: 2391-2399, pmcid: 6763528,

[86.] Waldron M, Papavasileiou G, Jeffries O, Nevola V, Heffernan SM, Kilduff L, Tallent J. Concurrent adaptations in maximal aerobic capacity, heat tolerance, microvascular blood flow and oxygen extraction following heat acclimation and ischemic preconditioning. J Therm Biol, 2020, 93,

[87.] Weller RS, Buono MJ. The effect of heat acclimation on the sweat lactate concentration vs sweat rate relationship. J Therm Biol, 2022, 109,

[88.] Wickham KA, Wallace PJ, Cheung SS. Sex differences in the physiological adaptations to heat acclimation: a state-of-the-art review. Eur J Appl Physiol, 2021, 121: 353-367,

[89.] Zurawlew MJ, Walsh NP, Fortes MB, Potter C. Post-exercise hot water immersion induces heat acclimation and improves endurance exercise performance in the heat. Scand J Med Sci Sports, 2016, 26: 745-754,