The sodium concentration of sweat when performing identical exercise in identical conditions is greater in winter than in summer due to heat acclimitisation factors, according to a 2008 Curtin University study.
The study analysed 29 healthy, male, manual outdoor workers aged between 18 and 50 who were working in a variety of trades.
“Sweat rates were higher and sodium concentrations were lower in the summer (acclimatised) than the winter (unacclimatised) trials,” the paper states.
This means workers may be sweating out more salts during cooler months, potentially increasing the need for electrolytes.
|Mean day 1||r
|Mean day 2||r
Researchers also found that “Sweat sodium concentration was reduced on the second day in summer but not winter… suggesting that one heat exposure in summer is sufficient to trigger an acclimation effect. In winter this difference was not present.”
Mean values of sweat sodium concentration (mmol.L-1) from individual arms and legs taken on two consecutive days in summer and winter and correlation (r) between right and left limb data. (n = 29 subjects). Note the similar sodium concentration across both days in winter and significant reductions in summer.
The paper also cites previous research into sweat sodium concentration which drew a similar conclusion.
“The data predict that sodium loss would be greater in the unacclimatised individual (winter data) even with a lower sweat rate due to the higher sweat sodium concentration,” the paper states, adding that replacing lost electrolytes is imperative to avoid possible impaired work performance.
The paper concludes that most “sports drinks” have too great a carbohydrate (sugar) concentration and that “fluid replacement beverages should have far less carbohydrate and ideally more than 15 mmol.L-1 of sodium”.
The 29 Curtin University participants had their sweat rates and sweat-sodium composition analysed when exercising in an environmental chamber on two consecutive days in both winter and summer.
The climate chamber was maintained at 35°C with a relative humidity of 50 per cent and minimal air flow. The Wet Bulb Globe Temperature (WBGT) was approximately 29.3°C and Thermal Work Limit approximately 180 W.m-2.
Typical summer temperatures in the study location were around 30 to 35°C, while winter temperatures average 15 to 20°C.
Dehydration also a danger in winter:
Based on the data in the Curtin University study, researchers predict that the mean sweat loss over a 10-hour shift would be 4.7 litres in summer – among heat acclimatised workers – and 4.1 litres in winter when workers were not heat acclimatised.
While sweat rates are noticeably less in the cooler months, there is still the need to implement hydration strategies in winter, in conjunction with electrolyte replacement.
In fact the importance of workplace hydration strategies may actually increase in winter. Workers are less aware of the need to hydrate due to sweat quickly evaporating in cold, dry air and therefore being less noticeable. Other factors that increase winter dehydration risks include:
- Lower temperatures suppress thirst
- Cold exposure signals the kidneys to conserve less fluid
- Breathing cold, dry air pulls moisture from the body