The Science of Sweat
Thermoregulation – When our core body temperature rises we excrete sweat onto the surface of our skin. Our body heat is transferred to the fluid on the skin and it evaporates removing the heat from the body and cooling us.
The fluid we sweat not only removes water from our bodies, but also electrolytes such as:
Continued effective sweating is imperative for effective thermoregulation. Replacing fluid and electrolytes lost through sweating maintains homeostasis, aids performance and wellbeing and allows sweating to continue.
Hydration & Performance
Dehydration represented as % of initial body weight
+ 1% to -1% = Well hydrated
-1% to -3% = Minimal dehydration
-3% to -5% = significant dehydration
> -5% = Serious Dehydration
-15% to -20% = Probable death
American College Sports Medicine (ACSM) suggests the goal should be the avoidance of dehydration to any greater than 2% body weight fluid loss. Understandably, dehydration will be of greater chance and also of greater significance in hotter conditions.
*There are instances where high achieving athletes have significantly higher dehydration than 2% body weight loss, however consider that significant dehydration and electrolyte imbalance are likely to have negative effects on welfare and performance in most instances.
Hypohydration prior to exercise increases physiological strain, reduced performance. 1.5 – 2% dehydration reduced performance in 1500m, 5km, 10km track races (Armstrong et al. 1985)
In temperatures > 30⁰C, 2-7% dehydration consistently reduced endurance performance. For exercise duration > 90mins performance reduced with dehydration > 2% in temperate and hot conditions. (Cheuvront et al. 2003)
Even 50% replacement of fluid loss (orally) of a 4% dehydration during a 20min break was effective in increasing exercising capacity when compared to no rehydration. (Casa et al. 2000)
Accuracy and performance in a soccer dribbling task was impaired 5% at 2.4% dehydration. (Mcgregor et al 1999)
Cricket bowling accuracy was impaired at 2.8% dehydration (speed unaffected) (Devlin et al. 2001)
Performance in a soccer specific fitness test was worsened by 2.4% dehydration without supply of fluids vs fluid supplied group (0.7% dehydration). (Edwards et al. 2007)
In occupational settings (Bates et al. 1996), a dehydration of 1 – 2% of body weight resulted in a 6 to 7% reduction in physical work rate. A dehydration of 3 – 4% of body weight resulted in a 22% to 50% reduction in work rate, for “moderate” and “hot” environments respectively.
Mental performance (mental function, visual-motor skills and arithmetic tests) begins to decrease at 2% dehydration and thereafter is proportional to the degree of further dehydration. Sustained attention, error rate, response time, and task accuracy were all negatively affected during heat exposure, even when sedentary.
Physiology of Dehydration
- Decreased blood plasma volume
- Decreased blood pressure
- Decreased stroke volume
- Results in increased heart rate for given work output
- Overall greater cardiac demands / stress
*Heart rate increase 3-5bpm for every 1% dehydration (Montain et al.1992)
The inability to dissipate heat effectively through decreased sweat rate and reduced skin blood flow results in heat stress due to not being able to lower core body temperature.
Adequate fluid intake before, during and after activity and effective cooling methods can help avoid the negative effects of dehydration.
*Every 1% dehydration results in core temperature rising 0.15 – 0.2⁰C (Montain et al. 1992)