Is sodium in sweat simply a reflection of the salt in your diet?

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.

Dietary sodium and sweat infographic

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.