Hydration and Performance in the Heat: What You Need to Know
By Doug Stewart, Performance Coach, & Rebecca Dent, High Performance Dietitian
In a previous blog post, we discussed the impact of heat on endurance performance. Often, one of the key reasons for increasing body temperature and heart rate in the heat is linked to hydration status. In this article, we’ll look at popular advice on hydration and running in the heat to see how it holds up to scrutiny.
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 (as discussed in the previous blog). “The extent of physiological strain imposed by hyperthermia and dehydration relates to the magnitude of thermal strain and body water loss, as well as the prevailing ambient conditions and mode and intensity of exercise being performed.” (Periard et al., 2021. P1900). For clarity, dehydration describes the process of losing body water, whilst hypohydration refers to the uncompensated deficit of body water. Hyponatremia refers to when your blood sodium levels are abnormally low, which can be caused by drinking too much water (and thus over diluted).
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 running intensities. 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 (if looking at sweat rate for a race, run at race intensity and in race kit)
- If a longer run and taking bottles of fluid with you, then weigh your bottles prior to your run
4. After you get back, get undressed and dry yourself with a towel, then weigh yourself
- If you drank during your run, 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 running, 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.
ETT athlete Meryl during a sweat test at the Loughborough University Sports Services lab
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 a runner 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 runner, 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, where the finish time may be more similar to those racing trail and ultramarathons, 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).
In ultramarathons, the average decrease in bodyweight has been found to be c. 5%, but has been recorded as high as 11% (Kao et al., 2008, 2015). At the 100-mile Western States Endurance Run, the faster finishing athletes (on average) varied from 1% gain to 6% loss (Hoffman et al., 2013). But, as you can see below, there is a broad range across the runners, with some gaining and others losing close to +10% to -10%, respectively (Hoffman et al., 2013).
Relationship of percentage change in body weight (postrace minus prerace) with relative performance for 887 observations at 161-km ultramarathons. “Relative performance” was determined by dividing the difference in the individual’s finish time from the winning time for that year by the race time limit (30 h) and presenting as a percentage. Source Hoffman et al., 2013
Conclusion
Overall, there is a trend of the faster finishers losing more weight than the slower finishers. There is also a smaller spread of changes in weight among faster finishers, by comparison to slower finishers. However, bodyweight changes being used to estimate sweat loss may have errors (Maughan et al., 2007). For example, over the 100 mile distance, a runner could theoretically lose around 4.5% to 6% of their bodyweight without a reduction in body water (Hoffman et al., 2016).
This is worth expanding upon: Hoffman and his colleagues based their calculations on a 68.8kg ultrarunner and estimated that they would use between 300mg and 600mg of stored liver and muscle glycogen, whilst each gram of glycogen is linked to 1g to 3g of water. If the runner were to burn 14,500kcals and consume 8,228kcals (81% carb, 12% fat and 7% protein), with the assumption that all kcals consumed are used during the race, plus some other considerations (such as the mass of the bladder prior to and after the race), then this would see around a 3kg to 4kg of weight loss not associated with hydration. There a number of assumptions made to generate this number, but it certainly is worth considering for longer events. Further research by Hoffman et al., suggests that those competing in events lasting between 25 and 30 hours would see 1.9% to 5% loss in body mass to maintain their body water balance (Hoffman et al., 2018).
Therefore, the evidence suggests that maintaining bodyweight variation within 2% during ultrarunning events is not a practical recommendation. Moreover, as briefly mentioned in this blog, sodium is a component of sweat and often discussed as a key electrolyte for endurance athletes. We discuss this and other considerations for hydration, such as altitude, here.
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.
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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.
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