The  Influence  of  Hydration  on  Core  Temperature  

Written  By:  William  M  Adams,  MS,  ATC  and  Douglas  J.  Casa,  PhD,  ATC  

Summary  of  Findings  • Exercise  causes  a  rise  in  core  body  temperature  that  is  further  exacerbated  with  dehydration  and  exercise  

in  the  heat.1  • A  review  of  the  existing  literature  (indicated  in  the  graph  below)  comparing  the  rise  in  core  temperature  

to  changes  in  body  mass  loss  during  exercise  in  the  heat  shows  that  the  increase  in  core  temperature  for  every  additional  1%  change  in  body  mass  loss  is  0.22°C  (.4°F).2  An  athlete  who  is  2%  more  dehydrated  than  their  competitor  will  be  competing  at  roughly  1°F  higher  which  can  have  decrements  in  performance  the  higher  the  core  temperature.  1-­‐9  

• With  evidence  indicating  that  dehydration  results  in  an  increased  core  temperature  at  a  given  exercise  intensity,  staying  hydrated  could  help  reduce  the  risk  of  exertional  heat  stroke.10  

• In  the  field  setting,  the  rise  in  core  temperature  is  more  pronounced  in  fixed  intensity  exercise  in  the  heat  compared  to  race  paced  trials,  however,  performance  was  adversely  affected  in  both  conditions  in  those  that  were  dehydrated.6  

• Wearing  protective  equipment  such  as  a  football  uniform  further  exacerbates  heat  strain  by  limiting  the  body’s  ability  to  cool  effectively  via  evaporation.  This  can  have  adverse  effects  when  exercising  in  the  heat  and/or  when  athletes  are  unacclimatized  to  exercising  in  the  heat.7  

• Body  mass  losses  of  3-­‐4%  can  occur  in  exercise  lasting  60-­‐90  minutes,  especially  if  an  athlete  begins  practice  dehydrated  or  there  are  additional  external  factors  affecting  sweat  rate  (environmental  temperature,  protective  equipment/clothing,  etc.).  

 

-0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 Mean ºC per 1% body mass loss

Avg.  ALL  Studies  Armstrong  '97  

Buono  '00  Casa  '10  

Del  Coso  '09  

Del  Coso'08  Edwards  '07  

Gonzalez  '95  Gonzalez  '97  

Gonzalez  '98  Hamilton  '91  

Kay  '03  

Kay  '03  Kenefick  '07  

Kenefick  '07  Kenefick  '07  

Maxwell  '09  

Maxwell  '09  McConnell  '97  

McConnell  '97  

McConnell  '98  McConnell  '98  

Montain  '92  Montain  '92  

Montain  '92  

Montain  '92  Mora-­‐Rodriguez  '07  

Nassis  '02  Sawka'85  

Sawka  '85  

Sawka  '88  

 *  

Changes  in  core  body  temperature  per  every  1  additional  change  in  body  mass  loss.2

 

References  1.  Buono  MJ,  Wall  AJ.  Effect  of  hypohydration  on  core  temperature  during  exercise  in  temperate  and  hot  environments.  Pflügers  Arch.  Eur.  J.  Physiol.  2000;440(3):476–480.  2.  Huggins  RA,  Martschinske  JL,  Applegate  K,  Armstrong  LE,  Casa  DJ.  Influence  of  dehydration  on  internal  body  temperature  changes  during  exercise  in  the  heat:  a  meta-­‐analysis.  Med  Sci  Sports  Exerc.  2012;  44(5S):    524.  3.  Montain  SJ,  Coyle  EF.  Influence  of  graded  dehydration  on  hyperthermia  and  cardiovascular  drift  during  exercise.  J.  Appl.  Physiol.  Bethesda  Md  1985.  1992;73(4):1340–1350.  4.  Montain  SJ,  Sawka  MN,  Latzka  WA,  Valeri  CR.  Thermal  and  cardiovascular  strain  from  hypohydration:  influence  of  exercise  intensity.  Int.  J.  Sports  Med.  1998;19(2):87–91.  5.  Sawka  MN,  Young  AJ,  Francesconi  RP,  Muza  SR,  Pandolf  KB.  Thermoregulatory  and  blood  responses  during  exercise  at  graded  hypohydration  levels.  J.  Appl.  Physiol.  Bethesda  Md  1985.  1985;59(5):1394–1401.  6.  Mora-­‐Rodriguez  R,  Hamouti  N,  Del  Coso  J,  Ortega  JF.  Fluid  ingestion  is  more  effective  in  preventing  hyperthermia  in  aerobically  trained  than  untrained  individuals  during  exercise  in  the  heat.  Appl.  Physiol.  Nutr.  Metab.  Physiol.  Appliquée  Nutr.  Métabolisme.  2013;38(1):73–80.  7.  Casa  DJ,  Stearns  RL,  Lopez  RM,  et  al.  Influence  of  hydration  on  physiological  function  and  performance  during  trail  running  in  the  heat.  J.  Athl.  Train.  2010;45(2):147–156.  8.  Armstrong  LE,  Johnson  EC,  Casa  DJ,  et  al.  The  American  football  uniform:  uncompensable  heat  stress  and  hyperthermic  exhaustion.  J.  Athl.  Train.  2010;45(2):117–127.  9.  Lopez  RM,  Casa  DJ,  Jensen  KA,  et  al.  Examining  the  influence  of  hydration  status  on  physiological  responses  and  running  speed  during  trail  running  in  the  heat  with  controlled  exercise  intensity.  J.  Strength  Cond.  Res.  Natl.  Strength  Cond.  Assoc.  2011;25(11):2944–2954.  10.  Casa  DJ,  Armstrong  LE,  Kenny  GP,  O'Connor  FG,  Huggins  RA.  Exertional  Heat  Stroke:  New  concepts  regarding  cause  and  care.  Curr.  Sports  Med.  Rep.  2012;11(3):  115-­‐23.  

Practical  Applications  • Prior  to  competition,  practice,  or  conditioning  sessions,  ensure  proper  hydration  to  

attenuate  the  rise  in  core  temperature,  especially  when  exercising  in  the  heat.  • Proper  hydration  during  activity  will  also  assist  in  attenuating  a  rise  in  core  temperature  

during  physical  activity.    • Sports  in  which  protective  equipment  is  worn  (eg,  football)  should  ensure  that  frequent  

hydration  breaks  are  implemented  into  a  practice  session  since  equipment  has  been  shown  to  affect  heat  dissipation  from  the  body  during  exercise.  

• Athletes  participating  in  sports  such  as  football,  ice  hockey,  wrestling,  and  rugby  should  maintain  adequate  hydration  to  attenuate  anaerobic  endurance  performance  decrements  during  activity.  

Looking  Ahead  • Future  research  looking  at  whether  there  are  cooling  devices  available  that  can  be  used  

during  exercise  or  a  practice  session  that  will  attenuate  rises  in  core  temperature  and  if  these  devices  decrease  the  increased  influence  of  dehydration  on  core  body  temperature.  

• Examining  the  rise  in  core  temperature  during  exercise  should  be  further  examined  by  evaluating  the  influence  of  hydration  at  the  beginning  of  exercise  (hypohydration,  euhydrated,  or  hyperhydrated)  on  the  rise  in  core  temp  during  an  exercise  session.  

• Research  done  in  a  field  setting  can  sometimes  be  difficult  due  to  the  inability  to  control  for  some  factors.  With  that  said,  performing  additional  field-­‐studies  and  controlling  for  as  many  factors  as  possible  may  help  further  support  the  conclusions  made  in  existing  literature  looking  at  the  effects  of  hydration  on  core  temperature.