Dehydration, if severe enough can slow you down. Drinking can prevent this, but if you need to drink and how much depends on your sweat rate. Therefore, it is useful to know how much you sweat and this will be very different for each individual in a particular situation. In the literature we have recently seen a few great publications on sweat rates in different athletes. Some athletes may sweat up to 3 liters per hour in hot conditions!
Very few athletes know how much they sweat but it is actually very easy to calculate! This article will guide you through that process. The most important factors that determine how much you sweat are:
How much power you produce or how fast your run.
The temperature and humidity as well as cooling
The clothes you wear
Your level of acclimation and your training status (better trained and more acclimated will allow you to sweat more and regulate your body temperature better)
Your genetic make-up (some people sweat more than others)
It is important to realize that you probably need to do several measurements in different conditions to get a good idea of your sweat rate: easy training and hard training, hot conditions and cool conditions and so forth. The more you measure the better you will be able to predict your sweat rates.
Here is a brief step by step guide on how to figure out your sweat rate. Use the overview at the top of this page to do your calculations. White boxes are for values you measure and grey boxes for values you calculate.
1. Empty your bladder and record your weight (ideally nude body weight) (A)
2. Perform your workout, race or competition and record/memorize exactly how much you drank. This is easy if you drink from a bottle. You can weigh your bottle before (X) and after (Y) and record the difference (1 gram = 1 milliliter). (Z)
If you use different measurement units (fl oz) you need to convert all values to liters. Make sure all units are in kg or liters.
3. After exercise: Towel dry, empty your bladder and then record your weight (nude). (B)
4. Ideally, measure total urine production (U) after pre-weight recording. If that is not possible estimate it by using 0.3L per visit to the loo.
5. Now subtract your post-exercise weight from your pre-exercise weight to get the weight you lost during exercise.
6. Also subtract the weight of the bottle or bottles before (X) and after (Y) to obtain the volume you consumed (Z).
Weight lost C = A-B
Volume consumed = X-Y
7. You can now calculate your sweat rate: (C+Z-U) / time (in hours, calculated as number of minutes divided by 60).
A = weight before (kg)
B = Weight after (kg)
C = Weight loss (kg)
X = Weight of bottle(s) before (full bottles)
Y = Weight of bottle(s) after
Z = Weigh difference before and after
U = urine list
Your sweat rate estimations will never be 100% accurate, but they will give you a much better idea how much to drink than simply relying on thirst.
With the above calculation you make an assumption that all weight lost if sweat loss. This is not entirely correct. It is important to realize that here are other reasons for weight loss as well (you will use some carbohydrate and fat (up to around 1 lbs or 0.5kg during exercise of 2 hours or longer). During very prolonged exercise these losses may even be even greater.
Therefore, a small percentage weight loss 2-3% (1.5-2 kg for most athletes) is not a problem, but knowing how much you need to replace helps to prevent larger weight losses as a result of dehydration. Those larger losses can impact performance, especially in hot conditions. Knowing your sweat rate will also prevent you from overdrinking because you will know that there is no need to drink more than you sweat.
Knowing your sweat rate gives you important insights into how your body works and how different it will be in different conditions. Not everyone has access to laboratory facilities but this is something that is easily done at home with very simple equipment. Perform these measurements in different conditions and you will start to see trends and start to be able to predict your sweat rate!
References
Baker LB. Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Med. 47(Suppl 1):111-128, 2017.
Gonzalez et al. Expanded prediction equations of human sweat loss and water needs. J Appl Physiol 107(2):379-88, 2009.