Sweating is an important cooling mechanism in most sporting conditions. So the most important thing you lose when you sweat is heat! However, sweating also results in fluid losses (which can result in dehydration), and along with this loss of fluid we will lose electrolytes (hence the salty taste of sweat). It is also obvious that some athletes sweat a lot more than others. Sweat rates can vary from almost nothing to 5 litres per hour… In this article we will discuss what factors determine how much you sweat as well as what normal sweat rates are. We will also address electrolyte losses in sweat and in particular sodium losses.
What factors determine sweat loss?
The most important factor to determine sweat loss is probably the amount of heat produced and this is dependent on the exercise intensity. The more power we produce (the more and the forceful our muscles contract) the more heat will be produced. In fact, for every kcal of energy produced we will produce roughly 4kcal of heat. The second factor is the weather. Hot and humid weather will increase sweat rate. Another factor is of course clothing. How trained you are and how used you are to exercise in hot conditions are other factors. One factor we can’t influence is a genetic factor. We are born with a certain number of sweat glands and the exact number will vary from person to person. This could be one of the reasons why there are such large differences between individuals. Having said that the scientific literature seems to suggest that it is not the number of sweat glands, but mostly how well these sweat glands work, that is responsible for differences between individuals.
How much fluid do we lose through sweating?
As already mentioned, sweat losses may vary from person to person and situation to situation. In a study with a very large number of athletes in a wide variety of sports, sweat rates were mapped out (1). The infographic above shows the ranges in sweat rates observed in different sports (a rough reflection of the data obtained in the paper). The most obvious observation is the large variation within a sport. Another observation is that sweat rates in some sports can be extremely high. For example, in American football sweat rates are extremely high because the conditions are often extreme (hot weather), the players wear a lot of protective clothing, and the large muscle mass and explosive efforts means a lot of heat production.
The take home message here is that working with average sweat rates is meaningless. We really need to work out sweat rates of individual athletes. (In a previous blog we discussed how this can be done).
How much sodium do we lose?
The amount of sodium we lose depends on sweat rate, duration of exercise and sweat sodium concentration. If any of these are very low, the total losses are likely to be very low. Only if a “salty sweater” will exercise for long enough, will the losses accumulate to something meaningful. In the study we referred to above the average sweat sodium concentration was 36.1 mmol/L or 0.8 grams per litre. There was substantial variation with some losing as little as 0.25 grams per litre/L and some as much as 2 grams per litre. This accumulated into hourly losses of just under 1 gram per hour on average, but in the extreme cases losses were several grams per hour. This is the exception though…It is also unlikely that such high sweat rates could be maintained for very long because to achieve them the intensity needs to be very high, and conditions need to be very hot.
For example, many sweat rates measured in this study (1) were obtained during soccer or football practice, and the duration of these sessions is usually around 90 min. It is unlikely that the same intensity could be maintained (or would need to be maintained) for many hours. So, the total loss would only be a few grams of sodium at most.
What about other electrolytes?
Sodium and chloride are the two electrolytes that have the highest concentrations in sweat and are lost the most. However, there are also other electrolytes in sweat, albeit at much lower concentrations. Potassium losses are 160-320mg per litre of sweat and magnesium losses are only 4-15mg per litre of sweat.
It is important to note that the concentration of most electrolytes in sweat is always lower than the concentration in blood. This means that you will always lose more fluid relative to these electrolytes. In one of the next blogs we will come back to this point because it is a way the body protects itself against hyponatremia.
This blog was just about reporting how much athletes sweat and how much of various electrolytes is lost through sweating. Although the assumption is quickly made that such losses are a problem, this may not be the case as we will see in the next blog.
Barnes KA, Anderson ML, Stofan JR, Dalrymple KJ, Reimel AJ, Roberts TJ, Randell RK, Ungaro CT, Baker LB. Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes: An update and analysis by sport. J Sports Sci. 2019 Oct;37(20):2356-2366.