Why do we sweat?
Humans perspire more (per unit of skin surface area) than just about any creature on the planet. And, as athletes, this is a very good thing because it gives us a competitive edge when it comes to exercising for prolonged amounts of time in hot environments.
The reason we’re so sweaty is pretty simple. It all comes down to thermoregulation - the ability to keep your core body temperature under control - in the face of high temperatures and the high levels of heat production in your working muscles.
A key consequence of exercise is that your core body temperature starts to rise pretty quickly. That’s because a by-product of getting your muscles to contract vigorously is the production of excess heat. Your core temperature at rest is usually about 37C. If it goes above about 40 degrees you can be in real danger of developing debilitating or even deadly heatstroke.
Sweating - or more accurately the evaporation of sweat from your skin’s surface into the air - is an incredibly efficient means of removing heat, with each gram of evaporated sweat drawing out about ~540 calories of heat from the body. The more we can sweat (to an extent), the harder and longer we can work out without our core temperature overstepping the mark.
It’s widely believed that we developed our ability to sweat so effectively because we evolved on the hot, dry African Savannah where we were not fast enough over short distances to hunt prey animals like gazelle and kudu by out-pacing them in a flat out sprint, but where we could run them down over several hours in the heat of the day (a practice called ‘persistence hunting’).
Tracking and harassing the animals from a distance over multiple hours eventually caused them to overheat and collapse with heatstroke, whilst we remained cool enough to keep moving in large part due to the fact we’d been sweating, which had allowed us to keep our core body temperature within an acceptable range.
We still have the basic human biology of the persistence hunter hardwired into our DNA and it comes to the fore when taking part in any sort of endurance exercise, especially in warm weather.
That’s why if you go out running with a dog, you’re more likely to beat it over a long distance in the summer than in the winter. Dogs (like most other animals) lack the ability to sweat much so they have to pant heat out of their mouth, which is much less effective.
How does sweating work and are there different types of sweat glands?
You have anywhere between 1.5 and 5 million sweat glands on your body, but, not all sweat glands are the same; there are actually two main types: eccrine and apocrine sweat glands.
Your eccrine glands produce most of the sweat you can see when you exercise. They’re tiny, numerous (you can have as many as 700 of these per square centimetre of skin!) and are found almost everywhere on the body, although they’re most concentrated on your palms, soles and face.
Why does sweat smell?
Apocrine glands secrete a slightly different type of sweat into a hair follicle which then makes its way up to your skin’s surface. Because of this, apocrine glands are bigger and are only found in your armpits and a few other places where hair grows.
These glands secrete a thicker solution that’s largely to blame for the distinctive odour associated with sweating. Sweat itself doesn’t actually smell at all, the smell happens when bacteria on the skin and hair begin to break it down.
Men tend to have significantly more apocrine sweat glands than women, which perhaps lends some credibility to the stereotype that men smell more than women!
Eccrine glands are the ones predominantly involved in cooling us down, so these are the ones we’ll focus on here.
What gets your sweat glands going?
There are a number of things that cause us to get a sweat on (including emotion and our response to a spicy curry), but the main cause - and the one we’re interested in as athletes - is an increase in body temperature.
Neurones in your brain that are sensitive to heat detect temperature changes and stimulate your sweat glands. This causes a complex ‘cascade reaction’ that results in the secretion of sweat.
What’s in your sweat?
Sweat is made up largely of water, but it also contains valuable minerals and electrolytes and this electrolyte content is of particular significance to us athletes.
Sodium is the most abundant electrolyte in sweat. Even people who have very dilute sweat usually lose at least 250 mg of sodium per litre of sweat. And some people lose upwards of 2,000 mg per litre!
Other important electrolytes like potassium, magnesium and calcium are also lost in your sweat, but in much smaller amounts. Most people only lose up to about 150mg of potassium per litre of sweat, for example.
Why do you need to replace what’s lost in your sweat?
You can lose water and electrolytes for a while before your performance suffers. In fact our bodies can tolerate more dehydration than you might think.
But at some point you need to start replacing the fluid and electrolytes you’re losing, otherwise your blood plasma volume decreases and you start to become significantly dehydrated.
For short activities like a sprint triathlon, where your total sweat losses are lower, you can simply not worry about drinking anything much at all when you’re on the move. Following your body’s instincts (i.e. drinking some water to thirst as and when you feel you need to) and rehydrating and replenishing electrolytes through eating food afterwards will usually suffice.
For slightly longer activities like an Olympic distance triathlon, there’s a need to drink some water to replace some of what’s being lost to prevent your blood volume getting too low and your performance suffering as a result.
During longer activities like a half or full distance triathlon - or if you lose a lot of salt in your sweat like I do - replacing some of the sodium along with the water you’re losing can be very beneficial in helping maintain blood plasma volume.
This is because, beyond a certain level of loss, you can run the risk of diluting your blood sodium levels (a condition called hyponatremia) and the body tries to defend against that by limiting how much water you absorb.
It’s important to realise that you don’t need to replace 100% of what you lose in your sweat during exercise. Your body is very well equipped to function well despite a certain level of loss and besides, it’s usually pretty difficult to replace 100% of your losses anyway.
With that said, figuring out how much (and what) you might need to drink during a triathlon is an important piece of the puzzle for serious triathletes to understand in order to maximise performance, especially in the heat.
It’s also something that is highly individual - with a lot of variables coming into play including sweat rate, duration and how salty your sweat is - so a level of self-experimentation is always recommended in order to get things nailed.
Our free online Sweat Test helps you get started with personalising your hydration strategy through some good old-fashioned trial and error in training.
Andy Blow has a few top 10 Ironman and 70.3 finishes and an Xterra World Age Group title to his name. He founded Precision Hydration to help athletes solve their hydration issues. He has a degree in Sport and Exercise Science and was once the Team Sports Scientist for Benetton and Renault F1 teams.