Do You Sweat When You Swim?

My first thought was no, why would you? You have all the sweat you need in the form of water all around you.

Of course it also helps that people have actually studied this question.¹Although many of the studies focused on either competitive swimmers or prepubescent Australian surfer lifeguards The short answer is that yes, you do sweat when you swim.

So how do you go about measuring sweat secretion from someone who is not only submerged in a fluid very similar to sweat but also constantly moving? Sweating is often measured by assuming the difference in a swimmer’s mass before and after swimming is due to sweat (adjusting for urination and fluid ingested). Additional corrections using methods for calculating mass loss from substrate oxidation and respiratory losses are also sometimes used. If all you care about is local sweat collection (as opposed to whole body), there is hygrometry or gravimetry.²Baker, “Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability”

Here is the best data I could find.

Water temperature	Sweat	Core temp change	Notes
29.8 ± 2.8° C	Sweat rate: -0.5kg/hr	1.0 ± 1.0° C	Nine young competitive male swimmers who covered 9,000 m in 180 minutes. (Soler et al., “Thermal Responses and Body Fluid Balance of Competitive Male Swimmers During a Training Session”
>26.5° C	Sweat rate: -1.99 ± 0.34 L/hr	Not measured	45 elite swimmers (22 females, 23 males). González-Custodio et al., “Sweat rates during international open water competitions and the importance of feedings in elite swimmers”
20° C to <26.5° C	Sweat rate: -1.14 ± 0.36 L/hr	Not measured	Ibid.
<20° C3With a wetsuit in cold water	Sweat rate: -1.48 ± 0.49m L/hr	Not measured	Ibid.
23° C	Sweat rate: 0.4 ± 0.28 L/hr	Pre: 37.1 ± 0.3° C Post: 37.2 ± 0.6° C	Nine male competitive swimmers. Macaluso et al., “Effects of three different water temperatures on dehydration in competitive swimmers”
27° C	Sweat rate: 0.76 ± 0.36 L/hr	Pre: 37.0 ± 0.3° C Post: 37.9 ± 0.5° C	Ibid.
32° C	Sweat rate: 1.25 ± 0.37 L/hr	Pre: 36.9 ± 0.4° C Post: 38 ± 0.4° C	Ibid.

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Heat generation is mediated by many factors:⁴Hall, Guyton and Hall Textbook of Medical Physiology

1. Basal metabolic rate
2. Additional rate of metabolism due to activity
3. Metabolism from thyroxine release
4. Metabolism from catecholamine release
5. Metabolism from increased intracellular activity as cellular temp increases⁵How this differs from #2 I have no idea
6. Thermogenic effect of food

Dissipation of heat is mediated by “two factors: (1) how rapidly heat can be conducted from where it is produced in the body core to the skin and (2) how rapidly heat can then be transferred from the skin to the surroundings.”⁶Hall, Guyton and Hall Textbook of Medical Physiology Core-to-surface heat transport seems to be a function of body surface area,⁷Sinclair et al., “Thermoregulatory responses of junior
lifesavers wearing protective clothing”, fat,⁸McMurray & Horvath, “Thermoregulation in swimmers and runners”, sweat gland density,⁹Baker, “Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability.” The author is employed by the Gatorade Sports Science Institute (a division of PepsiCo®!). and cardiac output.¹⁰Schey et al., “Skin temperature and core-peripheral temperature gradient as markers of hemodynamic status in critically ill patients: A review.” If heat dissipation relies on peripheral vasodilation, then cardiac output needs to increase. If blood pressure falls, the body needs to increase systemic vascular resistance to maintain an adequate blood pressure. This means constricting peripheral vessels. Sweat gland density is inversely proportional to BSA.

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Sweating rates are proportional to water temperature (R² 0.9758). Interestingly, people who same in 23 degree water still sweat even if their core temp did not change much. Another study showed that sweating also happened in 20° C water (0.076 L/hr).¹¹McMurray and Horvath, “Thermoregulation in swimmers and runners”

Although sweating rates were taken from different studies with different protocols, it seems like the equilibrium point might be around 16° C to 19° C.¹²Pulling this out of my ass

Given the amazing heat capacity of the surrounding water, peripheral vasodilation should be able to do the trick. Why the sweating? Ideally, the way to investigate this would be to take two groups of people, make it so half of them can’t sweat, have them swim. Unfortunately, nobody has investigated this because of the ethical issues inherent in injecting botulinum toxin throughout every skin surface and forcing participants to swim for their lives.

My hunch is that the sweating response in swimming is just because, as far as the hypothalamus is concerned, increased core temp is increased core temp, regardless of whether or not you are in a situation where you are submerged in what is essentially a wet heatsink.