Hydration and Performance in the Heat: An Introduction
by Doug Stewart
In 1944 Pitts et al., detailed how dehydration resulted in an increasing heart rate and body temperature, and a reduction in sweat rate that saw the subjects feel worse the more dehydrated they got. Those that drank water were much less likely to end up in the Zone of Impending Exhaustion!
Source: Pitts et al., 1944. The effect of water consumption on marching in the heat. 6 subjects in 37.8°C and 35 to 45% relative humidity
Pitts et al., highlighted that drinking water helped reduce these symptoms and that, “the more nearly water intake approximates sweat loss, the better the subjects remains.” (Pitts et al., 1944. P256)
Three years later, Adolph highlighted the fact that the impact of dehydration results in a decrease in blood volume and diminishing cardiovascular performance (Adolph, 1947). This leads to increasing physiological strain and perception of effort when exercising.
Sweat is a combination of water and electrolytes, and when discussing thermoregulatory sweating, as we are here, it is designed to help control the body’s temperature through cooling of the skin. Sweat rate and electrolyte composition varies greatly between individuals, but also for each individual depending on environmental conditions and the intensity of their training/race. Research has shown that sweat rate typically sits between 0.5 and 2.0 litres per hour, but some athletes have recorded sweat rates at over 3 litres per hour (Baker, 2017). Sodium concentration, a key electrolyte, has a similarly large spread, with ranges typically varying from around 20mmol per litre to 70mmol per litre, but with some athletes recording over 100mmol per litre (Baker et al., 2016).
Source: Baker et al., 2016. Sweat rate and sweat sodium concentration (from the forearm). The black vertical line represents the average in both graphs.
Calculating Your Sweat Rate
Calculating your sweat rate is relatively easy to do, and given the wide range of different sweat rates, it is important that you test yourself as it is so individualised. Additionally, it is important to appreciate that you will likely require multiple tests in different environmental conditions (for example, on hotter days versus cooler days) and at various intensities, and for different sports (e.g. cycling and running). Clothing and (if wearing) a race vest can also have an impact on sweat rates (by making you warmer or cooler). Therefore, trying to replicate typical race conditions (weather, altitude and clothing) will be helpful for testing.
Here’s how to calculate your sweat rate:
1. Go to the bathroom and empty your bladder
2. Weigh yourself naked and record your bodyweight
3. Go for your run or ride (if looking at sweat rate for a race, train at race intensity and in race kit)
- If a longer run/ride and taking bottles of fluid with you, then weigh your bottles prior to your training
4. After you get back, get undressed and dry yourself with a towel, then weigh yourself
- If you drank during your training, weigh your bottles and record their weight post-run
5. Subtract your post-run weight from your starting weight. If you did not drink, the loss in grams = ml of sweat loss
If you drank during the run, subtract the finishing weight of your bottles from their starting weight. Then add this number to the body weight loss.
For example, if an athlete ran for 2 hours and lost 1.3kg (1300g) while drinking 500ml (500g), that would mean total sweat loss was 1800ml. To then work out hourly sweat rate, divide the total sweat loss number by minutes = 1800/2(hours) = 900ml per hour.
The above description does simplify things a little as not all weight loss is from sweat. When you are exercising, you will also be burning carbohydrates and fat. However, knowing your approximate hourly sweat rate will be helpful when looking to avoid dehydration or, importantly, over drinking when training and racing.
TMR Coaching & Elite Trail Team athletes getting their sweat rate assessed in the lab at Loughborough University © Steve Ashworth Media
So, should you drink to replace 100% of your sweat loss?
In 1996, the American College for Sports Medicine released a position stand paper on exercise and fluid replacement that advised the following:
“During exercise, athletes should start drinking early and at regular intervals in an attempt to consume fluids at a rate sufficient to replace all the water lost through sweating (i.e., body weight loss), or consume the maximal amount that can be tolerated.”
This could be read and understood to mean 100% fluid replacement. But, for an athlete sweating 3 litres per hour, this will be extremely uncomfortable and difficult to achieve! Moreover, this advice received a lot of negative feedback and attention. It was argued that this narrative was promoted by sports drinks companies and put athletes at risk of hyponatremia (Noakes, 2012).
Research suggests that elite marathon runners may typically drink between 200ml/hour to 800ml/hour (Noakes, 2003), whilst others have found the range amongst elite marathoners to be 300ml/hour to 1,090ml/hour (Beis et al., 2012).
In 2007, the American College for Sports Medicine released an updated position stand, stating that the aim of consuming fluid during exercise is to prevent a greater than 2% loss of bodyweight, and stop any large changes in electrolyte balance – thus helping to avoid negative impact on performance. Taking a 72kg athlete, then a 2% loss in bodyweight would be 1.44kg – or a sweat loss of 1.44l.
In a recent study, it was found that when subjects lost around 2% of body weight, they were around 6% slower on a 3km time trail, compared to when they drank water to replace 95% of sweat loss. However, outside of the lab, the results seem to be more varied. One study of elite marathon runners found that the average loss in bodyweight over an average finish time of 2:06:31 was 8.8%, with the winner in Dubai 2009 losing 9.8% (Beis et al., 2012). Another study showed a relationship between those with a faster finish time being positively associated with the larger percentage of bodyweight loss (Zouhal et al., 2011). In an iron distance triathlon there was again a positive association between body weight loss and finish times (Sharwood et al., 2004). And this was also shown to be the case in an ultra marathon – where those that lost the most bodyweight performed the best (Kao et al., 2008).
However, the athletes may have pre hydrated before the race, which could result in elevated body weight at the start. Moreover, these findings do not automatically suggest that losing bodyweight when racing is performance enhancing, but they do indicate that capping the weightloss at 2% compared to pre-race is not essential for endurance performance (Goulet, 2012).
Therefore, the evidence suggests that maintaining bodyweight variation within 2% during longer endurance events is not a practical recommendation. But understanding your sweat rate and having a hydration plan is likely beneficial for racing performance.
References:
Adolph, E. F. (1947). Physiology of Man in the Desert. Physiology of Man in the Desert.
American College of Sports Medicine; Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007 Feb;39(2): 377-90.
Baker, L. B., Barnes, K. A., Anderson, M. L., Passe, D. H., & Stofan, J. R. (2016). Normative data for regional sweat sodium concentration and whole-body sweating rate in athletes. Journal of sports sciences, 34(4), 358-368.
Baker, L. B. (2017). Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability. Sports Medicine, 47, 111-128.
Beis, L. Y., Wright-Whyte, M., Fudge, B., Noakes, T., & Pitsiladis, Y. P. (2012). Drinking behaviors of elite male runners during marathon competition. Clinical Journal of Sport Medicine, 22(3), 254-261.
Funnell, M. P., Embleton, D., Morris, T., Macrae, H. Z., Hart, N., Mazzotta, T., ... & James, L. J. (2023). Exercise-induced hypohydration impairs 3 km treadmill-running performance in temperate conditions. Journal of Sports Sciences, 41(12), 1171-1178.
Goulet, E. D. (2012). Dehydration and endurance performance in competitive athletes. Nutrition Reviews, 70(suppl_2), S132-S136.
Hoffman, M. D., Goulet, E. D., & Mughan, R. J. (2016). Don’t Lose More than 2% of Body Mass During Ultra-Endurance Running. Really? Abstracts for the 4th Annual Congress on Medicine & Science in Ultra-Endurance Sports, May 30, 2017, Denver, Colorado, International Journal of Sports Physiology and Performance
Hoffman, M. D., Goulet, E. D., & Maughan, R. J. (2018). Considerations in the use of body mass change to estimate change in hydration status during a 161-kilometer ultramarathon running competition. Sports Medicine, 48(2), 243-250.
Hoffman, M. D., Hew-Butler, T., & Stuempfle, K. J. (2013). Exercise-associated hyponatremia and hydration status in 161-km ultramarathoners. Medicine and science in sports and exercise, 45(4), 784-791.
Kao, W. F., Hou, S. K., Chiu, Y. H., Chou, S. L., Kuo, F. C., Wang, S. H., & Chen, J. J. (2015). Effects of 100-km ultramarathon on acute kidney injury. Clinical Journal of Sport Medicine, 25(1), 49-54.
Kao, W. F., Shyu, C. L., Yang, X. W., Hsu, T. F., Chen, J. J., Kao, W. C., ... & Lee, C. H. (2008). Athletic performance and serial weight changes during 12-and 24-hour ultra-marathons. Clinical Journal of Sport Medicine, 18(2), 155-158.
Maughan, R. J., Shirreffs, S. M., & Leiper, J. B. (2007). Errors in the estimation of hydration status from changes in body mass. Journal of sports sciences, 25(7), 797-804.
Périard, J. D., Eijsvogels, T. M., & Daanen, H. A. (2021). Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies. Physiological reviews.
Pitts, G. C., Johnson, R. E., & Consolazio, F. C. (1944). Work in the heat as affected by intake of water, salt and glucose. American journal of physiology-legacy content, 142(2), 253-259.
Noakes, T. (2003). Fluid replacement during marathon running. Clinical Journal of Sport Medicine, 13(5), 309-318.
Noakes, T. (2012). Waterlogged: the serious problem of overhydration in endurance sports. Human Kinetics.
Sharwood, K. A., Collins, M., Goedecke, J. H., Wilson, G., & Noakes, T. D. (2004). Weight changes, medical complications, and performance during an Ironman triathlon. British journal of sports medicine, 38(6), 718-724.
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Zouhal, H., Groussard, C., Minter, G., Vincent, S., Cretual, A., Gratas-Delamarche, A., ... & Noakes, T. D. (2011). Inverse relationship between percentage body weight change and finishing time in 643 forty-two-kilometre marathon runners. British journal of sports medicine, 45(14), 1101-1105.