With summer upon us and temperatures climbing, it’s appropriate that we talk about an often-forgotten nutrient: water. When it makes up around two thirds of your body mass, it’s importance can’t really be overstated – especially if you want to maximise your performance.
First, some quick definitions:
Osmolality is a measure of how concentrated a fluid is with certain particles – eg. salty sea water has higher osmolality than tap water.
Euhydration is the state of being properly hydrated.
Hyperhydration is the state of being overly hydrated.
Hypohydration is the state of being underly hydrated.
Dehydration is a process that takes you from a higher level of hydration to a lower level of hydration.
Rehydration is a process that takes you from hypohydration to euhydration.
Heat and exercise can both lead to sweating. When you put them together, you’re probably going to get a lot of sweating. This sweat is water and some electrolytes that both come from blood. Since the concentration of electrolytes in sweat is lower than that of blood, when you sweat, not only does your overall blood volume decrease from the water loss, but blood osmolality also increases.
These changes trigger a response by osmoreceptors and stretch receptors which lead to changes in arginine vasopressin (AVP; also called anti-diuretic hormone) secretion. This is then picked up by the kidneys, which work to either reabsorb water rather than excreting it as they normally would or increase the rate of excretion. In the case of lots of sweating, water would be reabsorbed, slightly reducing the rate of dehydration.
However, the kidneys alone cannot bring blood volume back to where it should be. This is a problem because the heart must compensate for the reduced stroke volume by increasing pump rate. Even then, it struggles to match cardiac output. Along with heart rate, core body temperature also increases. As does thirst, thermal sensation, and perceived exertion. This means everything is going to feel an awful lot more uncomfortable, and you probably won’t even be able to match euhydrated performance. Hypohydration has been shown to decrease endurance, strength, cognitive and skill performance. Proper hydration may also have implications in muscle growth. Cells may initiate protein synthesis pathways in response to membrane stretch which happens when cells are well hydrated. Hypohydration, therefore, may drastically decrease the role of this mechanism in growth efforts.
What can we do about this?
The simple answer is “drink water”. But to those of us that care about performance, that answer isn’t good enough. Research has shown that ad-libitum fluid intake is often not enough to sufficiently rehydrate from intense exercise. This means there may be carryover effects to subsequent sessions since you would be training in a hypohydrated state.
Rather than relying solely on subjective feelings of thirst, we can take some measurements. Weigh yourself before and after training. Assuming you didn’t eat anything or change your clothing, the difference in mass will be from water loss. In the first hour after that training session, aim to drink 1.5x that water loss.
In addition to this, we can manipulate the content of the fluid consumed. The effect a fluid has on osmolality, and therefore AVP, will determine how much of that fluid is retained. Adding electrolytes, specifically salt, will increase osmolality, aiding rehydration further.
There is a reason why sports drinks generally all contain carbohydrate. On top of replenishing glycogen stores, carbohydrate can influence the rate of delivery of fluids into circulation. Higher carbohydrate content means slower delivery rates. This means that there is no sudden massive rise in blood volume which would trigger an AVP response similar to being hyperhydrated. Once again, then, more fluid is retained.
Consuming protein gives a unique opportunity to increase albumin levels. Increased circulating albumin aids to reabsorb water back into the blood, directly helping the rehydration process.
Measuring hydration at home
Unfortunately, there isn’t a simple rule for the amount of fluid to drink every day. And without a multitude of different tests, each with their own nuances, it can be tricky to accurately determine hydration levels. However, using a few data points, we can get a good idea.
Body mass: if you have experienced a sudden change in body mass that isn’t explained by gut content, it may well be water. Thus, this number can give us a rough estimate of hydration status compared to baseline.
Urine colour: it is not the case that ‘the clearer, the better’, and this generally only tells you whether you have had much to drink recently. However, darkness can give a good indication of hypohydration.
Thirst: you might have heard “if you’re thirsty, you’re already dehydrated” before. This isn’t necessarily true – thirst can be different for everyone. But generally, it is a response triggered because a change in markers of hydration have been detected.
Casa et al., 2010. Influence of hydration on physiological function and performance during trail running in the heat. Journal of Athletic Training, 45(2), p147-156.
Cheuvront et al., 2001. Thermoregulation and marathon running: biological and environmental influences. Sports Medicine, 31(10), p743-762.
Cheuvront et al., 2014. Dehydration: physiology, assessment, and performance effects. Comprehensive Physiology, 4(1), p257-85.
Clayton et al., 2014. Effect of drink carbohydrate content on postexercise gastric emptying, rehydration, and the calculation of net fluid balance. International Journal of Sport Nutrition and Exercise Metabolism, 24(1), p79-89.
Evans et al., 2017. Optimizing the restoration and maintenance of fluid balance after exercise-induced dehydration. Journal of Applied Physiology, 122(4), p945-951.
Funnell et al., 2023. Ad-libitum fluid intake was insufficient to achieve euhydration 20h after intermittent running in male team sports athletes. Physiology and Behaviour, 268, 114227.
Haussinger, D., 1996. The role of cellular hydration in the regulation of cell function. The Biochemical Journal, 313, p697-710.
James et al., 2011. Effect of milk protein addition to a carbohydrate-electrolyte rehydration solution ingested after exercise in the heat. The British Journal of Nutrition, 105(3), p393-399.
James et al., 2014. Effect of whey protein isolate on rehydration after exercise. Amino Acids, 46, p1217-1224.
Knepper et al., 2015. Molecular physiology of water balance. New England Journal of Medicine, 372, p1349-1358.
Low et al., 1997. Signaling elements involved in amino acid transport responses to altered muscle cell volume. FASEB Journal, 11(13), p1111-1117.
Maughan et al., 1995. Sodium intake and post-exercise rehydration in man. European Journal of Applied Physiology and Occupational Physiology, 71(4), p311-319.
Montain et al., 1992. Influence of graded dehydration on hyperthermia and cardiovascular drift during exercise. Journal of Applied Physiology, 73(4), p1340-1350.
Sawka et al., 2007. American college of sports medicine position stand. Exercise and fluid replacement. Medicine and Science in Sports and Exercise, 39(2), p377-390.
Schoenfeld, B.J., 2010. The mechanisms of muscle hypertrophy and their application to resistance training. The Journal of Strength and Conditioning Research, 24(10), p2857-2872.
Shirreffs et al., 1996. Post-exercise rehydration in man: effects of volume consumed and drink sodium content. Medicine and Science in Sports and Exercise, 28(10), p1260-1271.
Volpe et al., 2009. Estimation of prepractice hydration status of national collegiate athletic association division 1 athletes. Journal of Athletic Training, 44(6), p624-629.