In previous blogs we’ve discussed what sodium is and how it might be helpful during exercise. We’ve also discussed that the amount of salt lost in sweat varies significantly from person to person. In this article, we’ll take a closer look why that is, and examine if salty sweaters are simply people who eat a lot of salt.
How sweat glands work
To understand how sodium losses vary from person to person, it’s important to have a basic understanding of how sweat is produced, and released onto our skin surface. Our skin is an organ, one of the largest in the body. It has many roles, protecting our insides from the environment, acting as the first layer of defence for our immune system, and of course helping us maintain a stable body temperature. During exercise, we produce plenty of heat, and the water in sweat evaporating from the skin surface removes some of this heat to the atmosphere.
Sweat itself is produced in our sweat glands, from the fluid that surrounds it. This fluid exists outside of cells, so it is considered extracellular fluid, and is almost the same in its concentration of electrolytes to the blood. The sweat that is first produced therefore has the same sodium concentration as the surrounding fluid that it came from, and the blood. However, as sweat travels up through the gland towards the skin surface, some of the sodium and chloride ions are reabsorbed back into the body through special channels. This means that the sweat that ends up on the skin surface will always have a lower sodium and chloride concentration than the blood. This is important because it means that we lose proportionally more water than sodium when we sweat. The result is that the concentration of sodium in our blood will always increase as a result of sweating, assuming no fluid has been consumed.
Retaining sodium – the effect of sweat rate and diet
Whilst the amount of sodium that is first produced into sweat depends solely on the rate of sweat production, the amount reabsorbed back into the body can vary substantially. Firstly, as the rate of sweating increases, sweat is forced through the glands more quickly, and less of the sodium has a chance to be reabsorbed by the channels that line the gland. So for any given person on any given day, how salty your sweat is will be at least partly influenced by your sweat rate.
But the effectiveness of these channels to reabsorb sodium is also regulated. As far back as the late 1930s, scientists could observe that the amount of sodium and chloride lost in sweat could be altered by changing the amount of salt in people’s diets. Often though these studies were conducted using extremes of salt intake (often very low compared to normal or very high), over several days or weeks, in people who were often not exercising and not producing sweat at the same rate as would be expected by athletes during training or competition.
For this reason, we revisited this concept during my PhD. We were interested if changes in salt intake for only 3 days could significantly impact sodium losses during exercise. This time period was chosen to reflect the time that many athletes alter their diet in final preparation for a marathon, triathlon or other endurance event. If athletes had their sweat sodium losses measured in training, could this not be reflective of race day losses if they alter their salt intake in the days before a race? We took runners and cyclists and provided them a very low salt diet, then gave them salt or placebo capsules so that their diet was either very high or very low in salt. They also did another trial following their usual diet, which was almost exactly in the middle of the two extremes.
From two hours running or cycling at a moderate intensity in 35°C heat, we could clearly see an effect, where the sweat sodium concentration, despite the same sweat rate in each trial, increased as sodium intake in the three days before exercise increased. However, what we did see is that three days of altering salt intake only increased the sweat sodium concentration an average of ~10% from low to normal salt diet, and ~11% from normal to high salt diet. So whilst an effect is there, it’s probably not great enough to have any practical significance for athletes.
Given what we know about sweat gland function, it is most likely that this effect is the result of the sodium and chloride channels adapting to be more effective at retaining sodium when faced with less in the diet, and less effective when faced with excess sodium. This function is similar to what occurs in the kidneys for urine production, except that the kidneys respond within an hour or two to changes in sodium, whereas the sweat glands seem to take at least 12 hours, possibly more than a day to respond.
Heat acclimation and sweat sodium losses – an indirect effect of sodium balance
A final scenario where much larger changes in sweat sodium concentration are observed is during a period of heat acclimation. When non-heat acclimated athletes exercise in hot temperatures for consecutive days, the sweat sodium concentration falls by as much as 30% after 10-14 days. This effect is believed to be the same one as the dietary effect. The process of heat acclimation (i.e. exercising in the heat) causes significant sweat sodium losses, which if unreplaced, cause a deficit of sodium. The sweat glands therefore respond by trying to conserve more sodium. Interestingly, if sodium intake is deliberately increased during heat acclimation to prevent the sodium deficit, then the sweat sodium concentration does not change.
Summary
Sweat sodium concentrations vary considerably between individuals, and even within the same individual in different circumstances. The sweat rate, and the amount of sodium in the diet can both influence sweat sodium concentration within an individual. The process of becoming heat acclimated also reduces sweat sodium concentration, but only if the large sodium losses during the acclimation process are not adequately replaced.
References
McCubbin AJ et al. Impact of 3-day high and low dietary sodium intake on sodium status in response to exertional-heat stress: a double-blind randomized control trial. Eur J Appl Physiol. 2019; 119(9):2105-2118.
