The  Influence  of  Hydration  on  Sweat  Rate  

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

Summary  of  Findings  • Scientific  Definitions:  Hypohydration  is  the  steady-­‐state  condition  of  decreased  total  body  

water.  Dehydration  is  the  process  of  losing  body  water  (eg,  during  exercise).  • Sweat  rates  during  exercise  vary  depending  on  the  exercising  individual,  environmental  

conditions  (temperature  and  relative  humidity),  fitness  level,  exercise  intensity,  duration,  equipment  worn,  and  acclimatization  status  of  the  individual.  During  intense  exercise  in  the  heat,  it  is  typical  to  see  sweat  rates  of  1.0-­‐2.5  L/h.1–5  

• Dehydration  causes  in  a  decrease  in  sweat  rate  during  exercise,  which  results  in  a  decreased  ability  for  the  body  to  cool  through  evaporation.  

• As  environmental  temperature  and  relative  humidity  rise,  the  sweat  rate  of  the  exercising  individual  is  increased.  As  sweat  rate  increases  there  is  an  increase  in  sweat  sodium  concentration  as  the  body  can  only  reabsorb  sodium  at  a  set  rate.1  

• Heat  acclimatization  leads  to  an  increased  sweat  rate  during  exercise  (especially  in  the  heat),  smaller  reduction  in  plasma  volume,  and  a  lower  sweat  sodium  concentration.1,6,7  

• During  exercise  in  the  heat  most  athletes  do  not  replace  the  fluids  that  they  are  losing  through  sweat.  The  dehydration  that  results  causes  a  cascade  of  events  that  results  in  increased  cardiovascular  strain,  increased  core  temperature  as  a  result  of  the  reduced  plasma  volume  and  the  impairment  of  evaporative  heat  loss  due  to  decreased  sweat  rate  and  decreased  exercise  performance.1,8–12  

Change  in  sweat  rate  with  increasing  rectal  temperature  at  varying  levels  of  hypohydration.13  

  References  1.    Sawka  MN,  Montain  SJ.  Fluid  and  electrolyte  supplementation  for  exercise  heat  stress.  Am  J  Clin  Nutr.  2000;72(2  Suppl):564S–72S.  2.    Murray  B.  Hydration  and  physical  performance.  J  Am  Coll  Nutr.  2007;26(5  Suppl):542S–548S.  3.    Sawka  MN,  Burke  LM,  Eichner  ER,  Maughan  RJ,  Montain  SJ,  Stachenfeld  NS.  American  College  of  Sports  Medicine  position  stand.  Exercise  and  fluid  replacement.  Med  Sci  Sports  Exerc.  2007;39(2):377–390.  doi:10.1249/mss.0b013e31802ca597.  4.    Coyle  EF.  Fluid  and  fuel  intake  during  exercise.  J  Sports  Sci.  2004;22(1):39–55.  doi:10.1080/0264041031000140545.  5.    Maughan  R.  Physiological  responses  to  fluid  intake  during  exercise.  In:  Maughan  R,  Murray  R,  eds.  Sports  Drinks:  Basic  Science  and  Practical  Aspects.  Boca  Raton,  FL:  CRC  Press;  2001:129–52.  6.    Maughan  R,  Shirreffs  S.  Exercise  in  the  heat:  challenges  and  opportunities.  J  Sports  Sci.  2004;22(10):917–927.  doi:10.1080/02640410400005909.  7.    Hargreaves  M.  Physiological  limits  to  exercise  performance  in  the  heat.  J  Sci  Med  Sport  Sports  Med  Aust.  2008;11(1):66–71.    8.    Sawka  M,  Pandolf  K.  Effects  of  body  water  loss  on  physiological  function  and  exercise  performance.  In:  Gisolfi  C,  Lamb  D,  eds.  Perspectives  in  Exercise  Science  and  Sport  Medicine.Vol  3.  Caramel:  Benchmark  Press;  1990:1–38.  9.    Sawka  MN.  Physiological  consequences  of  hypohydration:  exercise  performance  and  thermoregulation.  Med  Sci  Sports  Exerc.  1992;24(6):657–670.  10.    Montain  SJ,  Latzka  WA,  Sawka  MN.  Control  of  thermoregulatory  sweating  is  altered  by  hydration  level  and  exercise  intensity.  J  Appl  Physiol  Bethesda  Md  1985.  1995;79(5):1434–1439.  11.    Sawka  MN,  Francesconi  RP,  Young  AJ,  Pandolf  KB.  Influence  of  hydration  level  and  body  fluids  on  exercise  performance  in  the  heat.  Jama  J  Am  Med  Assoc.  1984;252(9):1165–1169.  12.    Casa  DJ.  Exercise  in  the  Heat.  II.  Critical  Concepts  in  Rehydration,  Exertional  Heat  Illnesses,  and  Maximizing  Athletic  Performance.  J  Athl  Train.  1999;34(3):253–262.  13.  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.      

Practical  Applications  • Athletes  should  maintain  an  appropriate  level  of  hydration  prior  to  the  start  of  exercise  

in  the  heat  and  aim  to  minimize  fluid  losses  during  the  event  to  attenuate  any  performance  decrements.  

• Athletes  should  know  their  individual  sweat  rates  and  fluid  needs  in  order  to  minimize  fluid  losses  during  exercise,  especially  in  the  heat.  

• As  environmental  temperature  and  humidity  increase,  athletes  will  need  to  consume  more  fluid  during  exercise  as  their  sweat  rates  will  increase  to  aid  in  evaporative  heat  loss.  Heat  acclimatization  further  increases  an  athletes  sweat  rate  during  exercise  in  the  heat  so  additional  fluids  will  be  necessary  to  prevent  dehydration.  

• Maintaining  an  appropriate  level  of  hydration  during  exercise  will  assist  in  maintaining  plasma  volume  levels,  which  will  help  enhance  cardiovascular  and  thermoregulatory  function  during  exercise.  

Looking  Ahead  • Further  research  is  needed  to  examine  the  magnitude  in  which  sweat  rates  change  during  

different  environmental  conditions  and  hydration  status  in  exercising  athletes.  This  may  assist  in  determining  recommended  fluid  intake  levels  during  exercise  during  different  environmental  conditions.  

• Further  research  is  needed  looking  at  the  adolescent  population  in  regards  to  factors  affecting  sweat  rates  during  exercise  and  the  extent  of  which  those  affect  exercise  performance.