How To Calculate Your Hydration Loss During Exercise And Strategies To Rehydrate For Recovery

How To Calculate Your Hydration Loss During Exercise And Strategies To Rehydrate For Recovery

Ashley Thomas ·
Water makes up around 60% of our body composition and must be precisely regulated every second of the day to maintain balance or our life could be at risk. Such a critical substance yet our requirements for water intake are often not considered or not well understood by the general sporting population. We might place some thought into what we are going to drink before or during exercise, however, post-exercise hydration is just as important in order to recover and perform subsequent training sessions, but how much do we need and should It just be pure water?

When we exercise we cool ourselves down via excreting water and electrolyte droplets onto the exterior skin which act as our own air conditioner, commonly known as sweating. The amount of sweat and the chemical makeup of our sweat is individualised. For example, we have two athletes both racing a 10km running race who are the same gender and did the same overall time. Athlete ‘A’ loses 0.5kg of sweat over the race with a high sodium and chloride concentration, some potassium and even smaller amounts of calcium and magnesium. However, athlete ‘B’ loses 1.5kg of sweat over the course of the race with a very low sodium and chloride concentration and minuscule amounts of potassium, magnesium and calcium. Both athletes raced the same race in the same climate, however, had two very different patterns of sweat loss and therefore their post-exercise hydration strategies would be different.

What do we lose in sweat?
When we sweat we are losing water and electrolytes. The electrolytes include sodium, chloride, potassium, calcium and magnesium (1). We generally lose these in the sequential order mentioned above. However, the number of electrolytes we lose is individual to the person's physiology, diet, total body plasma or if we are hyper-hydrated. If you eat a high-salt diet you are likely to sweat out more sodium, if we have just gone to a warmer climate we generally lose more electrolytes and if we are unfit, we tend to lose more electrolytes as well. Then there are other factors such as, you're genetic physiology and how many sweat glands you have and pre-menstrual or menopausal women. Many factors to consider.

The rate of sodium and chloride reabsorption by the body is flow-dependent which means there is a direct relationship between our sweat rate and sodium and chloride sweat loss (1). Therefore the more someone sweats the more sodium and chloride they will lose through sweat (1). This is only relative though to the individual and we cannot assume that because our friends sweat more, they will lose more sodium and chloride than another who sweats less. To make it even more complicated, potassium doesn’t follow the same regulation system and is indirectly related to sweat flow rate (2,3).

How do we test sweat loss?
The simplest and most accurate method to assess how much our whole body loses through sweat is via changes in body mass during exercise (4). This is easily calculated by weighing yourself before a workout and again after. However, you do need to consider if you have drank or eaten anything whilst in training as this would need to be added to your total weight loss.

If you have gone to the toilet you would need to minus this amount from your total weight loss. Once you have your weight loss number you then need to replace 125-150% of this fluid that you’ve lost during the session over the 4-6 hours after you stop exercising (4). You will continue to lose fluids through sweating and urine losses after you finish exercising, this is why we increase the percentage of fluid replacement. For example, I weighed 65kg before I went for an hour run, and I weighed 64.5kg when I returned home and drank 200mL of water from the water fountain on the run and didn’t go to the toilet. My hydration loss is 0.5kg + 0.2kg- 0mL= 0.7kg. Therefore I would want to drink 875-1005mL over the next 4-6 hours to replace my fluid loss.

Other ways to test fluid and electrolytes lost in sweat is via patch sweat testing or pee tests. I have used both before with athletes but I find the later more indicative and an immediate indication of your current hydration status. You can buy pee sticks from your local chemist.

How do we rehydrate?
Rehydration after exercise can only be achieved if the electrolytes lost in sweat, as well as the lost water, is replaced (5). Consumption of a volume of fluid in excess of the sweat loss and replacement of electrolyte, particularly sodium, losses are essential (6). Since we don’t just sweat out pure water alone we need to ensure we don’t just rehydrate with pure water. We lose electrolytes too, so we need to rehydrate with a solution that contains water and electrolytes. This can be easily made by adding an electrolyte tablet to your water bottle such as SaltStick Electrolyte Drink MixHammer Nutrition - Endurolytes Fizz, Science In Sport (SIS) - Go Hydro Tablets or Huma Gel - Hydration Drink Mix

It is handy to note that pure water actually isn’t hydrating as we need sodium present for water to stay in the body. If we are dehydrated and already have low sodium concentrations in our blood then drinking pure water will dilute our blood plasma even more and cause more water and salt to be excreted via our urine. This process can actually result in further dehydration.

Take Home Points
Next time you go out for a solid training session, conduct a little experiment on yourself by weighing yourself before and after training, adding any hydration drank and subtracting any urine. Times by 125-150% and start rehydrating with water and electrolytes over the next 4-6 hours.

References (1) (4)

(2) Jéquier E, Constant F. Water as an essential nutrient: the physiological basis of hydration. Eur J Clin Nutr. 2010 Feb;64(2):115-23. doi: 10.1038/ejcn.2009.111. Epub 2009 Sep 2. PMID: 19724292.

(3) (6) Shirreffs SM, Armstrong LE, Cheuvront SN. Fluid and electrolyte needs for preparation and recovery from training and competition. J Sports Sci. 2004 Jan;22(1):57-63. doi: 10.1080/0264041031000140572. PMID: 14971433. (

4) (5)Maughan RJ, Shirreffs SM. Recovery from prolonged exercise: restoration of water and electrolyte balance. J Sports Sci. 1997 Jun;15(3):297-303. doi: 10.1080/026404197367308. PMID: 9232555.

(5) (1)Baker LB. Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Med. 2017;47(Suppl 1):111-128. doi:10.1007/s40279-017-0691-5

(6) (3)Baker LB, Stofan JR, Hamilton AA, Horswill CA. Comparison of regional patch collection vs. whole body washdown for measuring sweat sodium and potassium loss during exercise. J Appl Physiol (1985). 2009 Sep;107(3):887-95. doi: 10.1152/ japplphysiol.00197.2009. Epub 2009 Jun 18. PMID: 19541738.

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