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This article is part of a series:
Why has the simple act of drinking water become the most confusing topic in sports nutrition?
→ Part 1 — What we know
→ Part 2 — What we aren’t sure about
→ Part 3 — What we don’t know
→ Part 4 — What you can do
Why has the simple act of drinking water become the most confusing topic in sports nutrition?
→ Part 1 — What we know
→ Part 2 — What we aren’t sure about
→ Part 3 — What we don’t know
→ Part 4 — What you can do
“Drowning” in hydration. Part 2 of 4:
What we aren’t sure about. Hydration during exercise — Debate and controversies.
Thomas Solomon PhD.
22nd August 2021.
In Part 1 of this series, I discussed what we know about hydration. Of course, for hydration, just like many things in life, there is a lot we aren’t sure about. There is debate and there are controversies. Today, I will drown you in that hullabaloo...
Reading time ~15-mins (3500-words).
Or listen to the Podcast version.
Or listen to the Podcast version.
Having followed Part 1, you now know that water and electrolyte balance is one of the most tightly regulated processes in the human body. During exercise, because your body burns more energy and produces more heat, you sweat to cool down. But this removes (dehydrates) water from your “tank”. A feeling of thirst develops after your body has attempted to restore balance following a disturbance within your total body water (your “tank”). Therefore, a feeling of thirst indicates that dehydration has occurred — water has left the tank — and that you need to drink (hydrate) to replenish levels in the tank.
You also learned in Part 1 that starting exercise in a hypohydrated (“dehydrated”) state and becoming further dehydrated during exercise is rather common in athletes. However, I also presented evidence to show that the serious adverse health effects of dehydration during exercise are not at all common. All that said, the evidence makes it absolutely clear that starting exercise with a poor hydration status (hypohydrated) and/or not having fluid available during exercise, will amplify the performance-diminishing effects of dehydration during exercise, especially on a hot day. Therefore, staying hydrated day-to-day is important. But, no drama… Optimal daily hydration can very likely be achieved by simply drinking fluid when you feel thirsty.
But, if you are training every day or even twice a day, staying euhydrated can become more challenging. Likewise, if you’re amidst a long-duration race, possibly taking place in warm/hot conditions, you will be losing a lot of body water (dehydrating) and, therefore, you need to put some water back into the tank. To tackle dehydration during exercise, you can drink fluid. To remedy dehydration after exercise, you can drink fluid. Spot the common theme? It’s not rocket science. But, with the bonanza of “hydration” products being thrown at us in a marketing bombardment, there is a burning question that comes to mind:
Should you “drink to thirst” or “drink to a schedule”?
As I thoroughly explored in Part 1, the systematic reviews of all studies show that greater body weight loss (as a biomarker for dehydration) during exercise is not associated with poorer performance. Yes, laboratory studies that expose folks to dehydrating protocols (including heat and/or prolonged exercise or diuretic drugs) prior to an exercise test, show that dehydration reduces aerobic endurance performance, particularly when the exercise test is conducted in hot and humid conditions. But, ecologically valid studies that use “real world” in-the-field conditions do not support these conclusions, showing that dehydration only impairs endurance performance when air temperature is greater than ~25-30°C.
No matter what expert opinions say about thirst, you are the only person who knows if and when you feel thirsty. Plus, even if you are thirsty, stomach discomfort or lack of access to water may prevent you from drinking. Personally, I can recall several occasions when I was sweating balls deep into a hot race but wasn't thirsty plus a whole bunch of other occasions when I was sweating balls deep into a hot race and my thirst drive was sky-high. And... I can recount many occasions when I have felt very thirsty during a long session or race but, because of logistics, had no access to fluid. I never needed medical attention nor have I ever died of thirst. So, then I wonder...
Drinking to thirst adequately supported hydration status during 2-hours of Easy-effort running in 22, 30 and 35°C, in endurance-trained runners. This strategy has been supported in trained runners during racing, where self-selecting fluid intake with 473±234 mL/h (millilitres per hour) (while maintaining high carbohydrate availability with gels) did not impair half-marathon performance when compared to drinking to a schedule with 1,557±182 mL/h (while maintaining high carbohydrate availability with carbs in the fluid). And, planned drinking to a schedule (1,380 ± 320 mL/h) offered no benefit to half-marathon running performance over drinking to thirst (384 ± 180 mL/h), in trained runners. Plus, in the context of ultrarunning, several studies have found that drinking to thirst, even under hot ambient conditions (≥30°C), supports adequate hydration without impairing performance (see here, here, here, here, and here).
But, if your nerd brain is on, you may wonder whether I have just cherry-picked experimental studies to confirm a bias? Fear not, because a 2011 systematic review and meta-analysis of all studies by Eric Goulet found that drinking to thirst (+5.2±4.6%) and drinking more than thirst (+2.4±5.0%) both improve endurance performance compared to fluid intake at a rate lower than that dictated by thirst but that drinking more than thirst does not further improve performance. In other words, there is no need to drink more than you feel like you want to drink. Similarly, a 2019 systematic review from Goulet and Martin Hofman examined the impact of drink to thirst vs. drink to a schedule on 1–2 h endurance cycling or running performance at moderate to high intensity (60-90% HRmax), under a range of ambient conditions (20-33°C). They found that athletes’ on a “drink to schedule” drink 1073±247 mL/h and those “drinking to thirst” drink 505±156 mL/h and that both strategies meaningfully improve performance albeit, compared with a planned schedule, drinking to thirst improves performance negligibly (~1%) better (median superiority = 0.98%; 95% confidence interval 0.11–1.84%).
Figures from Eric Goulet & Martin Hofman’s Impact of Ad Libitum Versus Programmed Drinking on Endurance Performance: A Systematic Review with Meta-Analysis.. Sports Medicine (2019).
Wise words? Perhaps. But, this is a small study of middle-aged trained cyclists racing for 6 hours in extreme heat so more work is needed. And, “more work” can of course mean being your own experiment to see what works for you. Besides, you probably haven’t ever thought about drinking because you feel like it vs. drinking because you are thirsty because you probably want to drink because you are thirsty. Ah, the confusion of hydration...
In reality, “drinking ad libitum” — what you want, when you want, in whatever volume desired — will be fine unless you hyperhydrate, gain weight, dilute your plasma sodium, and die of hyponatremia (plasma sodium ≤135 mM). So, “drink to thirst”; it's probably safer and probably as effective.
“Drinking to thirst” and “drinking to a schedule” improve performance (compared to no fluid intake) during long-duration moderate to high-intensity exercise. Statistically, drinking to thirst is about 1% better. Whether that 1% is meaningful is debatable. Practically, the two strategies are very similar and the choice to use either strategy will be determined by your personal preferences and the logistics on the day. For these reasons, it is perhaps best not to consider “drinking to thirst” (or ab libitum drinking) and “drinking to a schedule” (planned drinking) as opposing sides of a debate but rather as two separate tools for your hydration toolbox.
Therefore, drinking to thirst will work for most athletes but also having some idea of your body water loss during exercise (dehydration) is sensible to use as a guide but not a rule. So, the next important question is...
As you learned in Part 1, your body dehydrates via water loss in your sweat, breath, tears, pee, poop, breast milk, blood, and puke. But, during exercise, sweat accounts for most of the water leaving your body and, during a session or race, changes in body weight can be a useful biomarker for this. So, knowing your approximate sweat rate isn’t a terrible idea.
To estimate your sweat rate you can weigh yourself naked having peed and pooped immediately before a run. Head out for a run, noting the volume of any fluids/gels you drink during the run. Immediately after the run, go for a pee (measuring the volume of how much you pee), towel-dry your body, then re-weigh yourself naked. Now, subtract your post-run bodyweight (kilograms) from your pre-run bodyweight (kg), add on the volume (1 litre = 1 kg) of fluids/gels you drank during the run, and then subtract the volume (1 L = 1 kg) of urine you produced after the run. This value (kg) is an estimate of the amount of water (1 kg = 1 L) your body lost during the run. Now divide the amount (L) by the duration of your run (minutes) and multiply by 60 to derive an estimate of your sweat rate in litres per hour.
But, this is a guide, not a rule: Sweat rates are highly variable within people day to day, even on days with similar environmental conditions! Sweat rate is also influenced by exercise intensity (the hotter the furnace, the more heat you generate) and ambient conditions (heat, humidity, wind speed). Therefore, to increase the accuracy of your sweat rate estimate, you would need to repeatedly measure and record your sweat rate under several environmental conditions and at a range of intensities. Since You are the only you — plan, prepare, practice, and refine, ideally under conditions that model expected conditions on race day. But, don’t stress over the precision! Because...
Yikes.
Besides these sources of error, body weight changes during exercise also don’t necessarily reflect “water leaving the tank”. For example, observations of athletes racing in the 21 and 56 km events at the 2009 Two Oceans Marathon in South Africa showed that total body water (measured using the “gold standard” isotope dilution method) decreased during the race but to a lesser extent than the decrease in body weight. And, data from a 100 km ultra race in Switzerland showed that runners maintained total body water and plasma sodium concentrations despite losing weight (1.5 ± 1.1 kg) during the race. So...
What accounts for the additional weight loss?
After adjusting for urine loss and food or fluid intake, during a long race, weight loss will be largely caused by dehydration (loss of body water) in your sweat and breath, and tears/blood/puke/piss/poop if things get hairy — you know this. However, at a higher intensity you burn energy at a higher kcal/min rate and as the duration gets longer you burn more total kcal energy. So, as a race gets longer, weight loss will also be caused by energy metabolism — the breakdown of stored fuel, fat and glycogen, which is “burned” and exhaled as CO2 along with water vapour in your breath. But, at the same time, the biochemical reactions of energy metabolism that produce ATP (energy) and CO2, also produce water. And, since glycogen stores ~1 to 3 molecules of water for every molecule of glucose, glycogen breakdown (glycogenolysis) during exercise also produces water. This water released from energy metabolism and glycogenolysis joins the total body water tank and is distributed between the intra- and extracellular pools accordingly. In simple words, during exercise, water and CO2 leave your body and cause weight loss but “metabolic” water is added back into the total body water tank, which means that the drop in total body water during exercise will likely be less than body weight loss might suggest.
These sources of error were thoroughly discussed in 2018 by Martin Hoffman (chief doc at Western States 100), Eric Goulet, and Ron Maughan and in Ron Maughan’s theoretical analysis — “Errors in the estimation of hydration status from changes in body mass” — in which he found that respiratory water losses can be as high as 0.18 litres per hour during heavy exercise and that weight loss due to energy expenditure is about 0.23-0.24 grams per kcal “burned” (or ~0.24 kg per 1000 kcal “burned”). Furthermore, Pastene and colleagues, who studied 6 athletes running a treadmill marathon, calculated that 557 g of fuel (glycogen and fat) was “burned” during which 402 g of water was released from metabolism and 1280 g of water was released from stored glycogen breakdown — that’s the potential for 2.2 kg of weight loss without any loss of body water. Given these estimates, it means that you can lose up to 3% body weight during a marathon without any change in hydration status.
We’re not sure exactly how best to hydrate during exercise. The reason we’re unsure is that in real-world ecologically valid settings, loss of body weight during exercise (as a biomarker for dehydration) is not associated with poorer performance. Furthermore, loss of body weight during exercise is not entirely due to body water loss (dehydration); some weight is lost in expired CO2 from energy metabolism aka “burning” stored fuel. And, loss of body weight during exercise can occur without changes in hydration status because “burning” stored fuel produces water in our cells. Therefore, “drinking to a schedule” to match body weight loss is unnecessary and also potentially dangerous because drinking too much fluid during exercise (hyperhydration; indicated by weight gain) can “dilute” sodium in your blood causing hyponatremia (plasma sodium ≤135 mM).
But, fear not… Despite the uncertainties, there is also a lot we do know:
The overall risk of a dehydration-related adverse health event or symptomatic hyponatremia during exercise is extremely low (see Part 1), so don’t get overly anxious.
If you start exercise euhydrated, dehydration (water leaving your body) during exercise is highly unlikely to affect your performance. But, if you start exercise hypohydrated (aka “dehydrated”) and do not have fluid available during exercise, dehydration during exercise will likely impair your performance, especially on a hot day. Therefore, doing your best to stay hydrated day-to-day is an essential component of your training toolbox, and this can very likely be achieved by simply drinking fluid to quench your thirst when that feeling arises.
I hope I am beginning to shed some light on what some folks consider a “dry” topic… Today was all about the debate and controversies concerning hydration during exercise. Next up is the biggest unknown in the world of exercise hydration… Sodium! But before things get too salty, that will have to wait until next time...
Thanks for joining me for another “session”. Until next time, keep drinking smart.
You also learned in Part 1 that starting exercise in a hypohydrated (“dehydrated”) state and becoming further dehydrated during exercise is rather common in athletes. However, I also presented evidence to show that the serious adverse health effects of dehydration during exercise are not at all common. All that said, the evidence makes it absolutely clear that starting exercise with a poor hydration status (hypohydrated) and/or not having fluid available during exercise, will amplify the performance-diminishing effects of dehydration during exercise, especially on a hot day. Therefore, staying hydrated day-to-day is important. But, no drama… Optimal daily hydration can very likely be achieved by simply drinking fluid when you feel thirsty.
But, if you are training every day or even twice a day, staying euhydrated can become more challenging. Likewise, if you’re amidst a long-duration race, possibly taking place in warm/hot conditions, you will be losing a lot of body water (dehydrating) and, therefore, you need to put some water back into the tank. To tackle dehydration during exercise, you can drink fluid. To remedy dehydration after exercise, you can drink fluid. Spot the common theme? It’s not rocket science. But, with the bonanza of “hydration” products being thrown at us in a marketing bombardment, there is a burning question that comes to mind:
Should you “drink to thirst” or “drink to a schedule”?
“Drinking to thirst” and “drinking to a schedule” produces considerable debate.
The debate between “drinking to thirst” (sometimes also referred to as ad libitum drinking) and “drinking to a schedule” (aka planned drinking) is complex because one side argues towards one goal — performance — while the other side argues towards two goals — health and performance. On one side of the debate are folks who advocate “drinking to a schedule” to replace sweat losses and prevent more than 2% weight loss because, they argue, dehydration (as indicated by weight loss) is associated with impaired performance. Some of these folks also advocate drinking to “stay ahead of thirst” because the neurophysiology that drives the feeling of thirst can become impaired under certain conditions (old age, temperature extremes, after-exercise) and that thirst can come too late. On the other side of the fence are the folks who argue that drinking to a schedule is unnecessary because, they argue, dehydration (weight loss) is not associated with impaired performance and that weight loss during exercise is not all water leaving the tank. These folks argue that “drinking to thirst” is sufficient to maintain performance and that it is safer because drinking to “stay ahead of thirst” is dangerous when too much water is added to the tank (hyperhydration) and “dilutes” plasma sodium levels, causing hyponatremia (plasma sodium ≤135 mM), which is fatal if untreated.As I thoroughly explored in Part 1, the systematic reviews of all studies show that greater body weight loss (as a biomarker for dehydration) during exercise is not associated with poorer performance. Yes, laboratory studies that expose folks to dehydrating protocols (including heat and/or prolonged exercise or diuretic drugs) prior to an exercise test, show that dehydration reduces aerobic endurance performance, particularly when the exercise test is conducted in hot and humid conditions. But, ecologically valid studies that use “real world” in-the-field conditions do not support these conclusions, showing that dehydration only impairs endurance performance when air temperature is greater than ~25-30°C.
No matter what expert opinions say about thirst, you are the only person who knows if and when you feel thirsty. Plus, even if you are thirsty, stomach discomfort or lack of access to water may prevent you from drinking. Personally, I can recall several occasions when I was sweating balls deep into a hot race but wasn't thirsty plus a whole bunch of other occasions when I was sweating balls deep into a hot race and my thirst drive was sky-high. And... I can recount many occasions when I have felt very thirsty during a long session or race but, because of logistics, had no access to fluid. I never needed medical attention nor have I ever died of thirst. So, then I wonder...
Does “drinking to a schedule” improve performance over “drinking to thirst”?
Drinking to thirst is likely inadequate to restore body water during exercise when you start exercise hypohydrated. This further highlights the necessity to maintain adequate hydration status on a daily basis, between sessions, in prep for your next session — something you learned in Part 1. During exercise, there have been several attempts to unravel this question. A 2019 study by Perreault-Briere and colleagues found that neither drinking to a schedule nor drinking to thirst offered any advantage over no fluid intake during a 1-hour cycling time-trial in 30 °C heat in trained endurance athletes. Suggesting that fluid intake during a 1-hour race is unlikely to have any impact on your performance (so long as you start euhydrated). But, when looking at longer-duration exercise, running specifically, the picture changes a little…Drinking to thirst adequately supported hydration status during 2-hours of Easy-effort running in 22, 30 and 35°C, in endurance-trained runners. This strategy has been supported in trained runners during racing, where self-selecting fluid intake with 473±234 mL/h (millilitres per hour) (while maintaining high carbohydrate availability with gels) did not impair half-marathon performance when compared to drinking to a schedule with 1,557±182 mL/h (while maintaining high carbohydrate availability with carbs in the fluid). And, planned drinking to a schedule (1,380 ± 320 mL/h) offered no benefit to half-marathon running performance over drinking to thirst (384 ± 180 mL/h), in trained runners. Plus, in the context of ultrarunning, several studies have found that drinking to thirst, even under hot ambient conditions (≥30°C), supports adequate hydration without impairing performance (see here, here, here, here, and here).
But, if your nerd brain is on, you may wonder whether I have just cherry-picked experimental studies to confirm a bias? Fear not, because a 2011 systematic review and meta-analysis of all studies by Eric Goulet found that drinking to thirst (+5.2±4.6%) and drinking more than thirst (+2.4±5.0%) both improve endurance performance compared to fluid intake at a rate lower than that dictated by thirst but that drinking more than thirst does not further improve performance. In other words, there is no need to drink more than you feel like you want to drink. Similarly, a 2019 systematic review from Goulet and Martin Hofman examined the impact of drink to thirst vs. drink to a schedule on 1–2 h endurance cycling or running performance at moderate to high intensity (60-90% HRmax), under a range of ambient conditions (20-33°C). They found that athletes’ on a “drink to schedule” drink 1073±247 mL/h and those “drinking to thirst” drink 505±156 mL/h and that both strategies meaningfully improve performance albeit, compared with a planned schedule, drinking to thirst improves performance negligibly (~1%) better (median superiority = 0.98%; 95% confidence interval 0.11–1.84%).
×
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Is “drinking to thirst” too distracting?
There’s a tonne of things to think about during a race. Minimising them will help focus all resources on forward progress. If the complex neuroendocrine systems governing water and electrolyte balance are autonomic (self-regulating), should you have to “think” about thirst? Another “uncertainty” in the “hydration pool”, something that is rarely and/or poorly considered, is that “drinking to thirst” is interpreted by some folks, but not by all, as “drinking what you want, when you want, in whatever volume desired” (aka drinking ad libitum). One 2014 study by Armstong and colleagues examined this concept during a 164 km cycling road race in the heat (36.1°) in 24 trained cyclists, 12 of whom were told to drink however much they wanted whenever they wanted, while the other 12 were instructed to drink only when they felt thirsty. Total fluid intake during the race was not influenced by these instructions (folks who “drank to thirst” imbibed 5.6±2.6 Litres during the race; folks who “drank ad libitum” drank 6.0±2.4 L). Neither was any other physiological or psychological variable, including race finish time (~6.7 hours), hydration status (body weight, urine specific gravity), caloric intake, thirst, pain, RPE, or thermal sensation. The authors concluded that “if athletes drink ad libitum, they can focus on training and competition rather than being distracted by ongoing evaluation of thirst sensations”.Wise words? Perhaps. But, this is a small study of middle-aged trained cyclists racing for 6 hours in extreme heat so more work is needed. And, “more work” can of course mean being your own experiment to see what works for you. Besides, you probably haven’t ever thought about drinking because you feel like it vs. drinking because you are thirsty because you probably want to drink because you are thirsty. Ah, the confusion of hydration...
In reality, “drinking ad libitum” — what you want, when you want, in whatever volume desired — will be fine unless you hyperhydrate, gain weight, dilute your plasma sodium, and die of hyponatremia (plasma sodium ≤135 mM). So, “drink to thirst”; it's probably safer and probably as effective.
So, should you “drink to a schedule” or “drink to thirst”?
Remember that drinking (to thirst or ad libitum or to a schedule) is easy in a lab because water is on-tap. Yes, drinking to thirst, a regulatory mechanism that evolved with us, will likely fulfil your wildest hydration dreams on race day. And, even though thirst develops after dehydration and is alleviated before complete rehydration is achieved, thirst is your only reliable subjective sensation that indicates dehydration has occurred. But, when outdoors on race day, fluid availability is different. Fluid is not “on tap” during many races, so having a rough idea of your hourly fluid needs during exercise may help you drink an appropriate volume when the opportunity arises. Plus, your thirst mechanism might become impaired during a long, intense race, especially in warm/hot conditions, in which case, drinking to thirst might not provide you with sufficient fluid. Therefore, it might be prudent to have some idea of how much water your body loses during exercise to help provide you with an estimate of your fluid needs. Lots of mights. Lots of uncertainties.“Drinking to thirst” and “drinking to a schedule” improve performance (compared to no fluid intake) during long-duration moderate to high-intensity exercise. Statistically, drinking to thirst is about 1% better. Whether that 1% is meaningful is debatable. Practically, the two strategies are very similar and the choice to use either strategy will be determined by your personal preferences and the logistics on the day. For these reasons, it is perhaps best not to consider “drinking to thirst” (or ab libitum drinking) and “drinking to a schedule” (planned drinking) as opposing sides of a debate but rather as two separate tools for your hydration toolbox.
Therefore, drinking to thirst will work for most athletes but also having some idea of your body water loss during exercise (dehydration) is sensible to use as a guide but not a rule. So, the next important question is...
How can you monitor dehydration during exercise?
Just like monitoring your daily hydration status, to assess changes in your hydration status during exercise, you can measure total body water and/or plasma and urine osmolality or specific gravity (aka density). These methods are accurate and reliable assessments under most conditions but require expensive specialist equipment and expert interpretation and are not practical on a daily basis. So, once again, you need a simple biomarker.As you learned in Part 1, your body dehydrates via water loss in your sweat, breath, tears, pee, poop, breast milk, blood, and puke. But, during exercise, sweat accounts for most of the water leaving your body and, during a session or race, changes in body weight can be a useful biomarker for this. So, knowing your approximate sweat rate isn’t a terrible idea.
To estimate your sweat rate you can weigh yourself naked having peed and pooped immediately before a run. Head out for a run, noting the volume of any fluids/gels you drink during the run. Immediately after the run, go for a pee (measuring the volume of how much you pee), towel-dry your body, then re-weigh yourself naked. Now, subtract your post-run bodyweight (kilograms) from your pre-run bodyweight (kg), add on the volume (1 litre = 1 kg) of fluids/gels you drank during the run, and then subtract the volume (1 L = 1 kg) of urine you produced after the run. This value (kg) is an estimate of the amount of water (1 kg = 1 L) your body lost during the run. Now divide the amount (L) by the duration of your run (minutes) and multiply by 60 to derive an estimate of your sweat rate in litres per hour.
×
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Simple.
But, this is a guide, not a rule: Sweat rates are highly variable within people day to day, even on days with similar environmental conditions! Sweat rate is also influenced by exercise intensity (the hotter the furnace, the more heat you generate) and ambient conditions (heat, humidity, wind speed). Therefore, to increase the accuracy of your sweat rate estimate, you would need to repeatedly measure and record your sweat rate under several environmental conditions and at a range of intensities. Since You are the only you — plan, prepare, practice, and refine, ideally under conditions that model expected conditions on race day. But, don’t stress over the precision! Because...
Body weight change during exercise isn’t all “water leaving the tank”.
As you probably know, every time you measure anything, there is an error of measurement consisting of bias (consistent errors) and noise (variable errors) caused by you or the tools you are using. For example, the scale you use to measure your body weight might be terribly imprecise — giving a markedly different value for repeated measures — or terribly inaccurate — giving a totally misleading value. A bad scale will contribute to error when using body weight change as a biomarker of dehydration. If you are actively trying to gain or lose weight, the error of measurement of using bodyweight changes as a biomarker for dehydration will also increase — body weight changes will only reflect body water if you are in energy balance from day to day. And, some water can also be lost from the total body water “tank” into the contents of your stomach and intestinal lumen to help “dilute” overly concentrated food/gels etc — while this water leaves the tank (dehydration) it also doesn’t leave the body mass you weigh on that scale, causing measurement error.Yikes.
Besides these sources of error, body weight changes during exercise also don’t necessarily reflect “water leaving the tank”. For example, observations of athletes racing in the 21 and 56 km events at the 2009 Two Oceans Marathon in South Africa showed that total body water (measured using the “gold standard” isotope dilution method) decreased during the race but to a lesser extent than the decrease in body weight. And, data from a 100 km ultra race in Switzerland showed that runners maintained total body water and plasma sodium concentrations despite losing weight (1.5 ± 1.1 kg) during the race. So...
What accounts for the additional weight loss?
After adjusting for urine loss and food or fluid intake, during a long race, weight loss will be largely caused by dehydration (loss of body water) in your sweat and breath, and tears/blood/puke/piss/poop if things get hairy — you know this. However, at a higher intensity you burn energy at a higher kcal/min rate and as the duration gets longer you burn more total kcal energy. So, as a race gets longer, weight loss will also be caused by energy metabolism — the breakdown of stored fuel, fat and glycogen, which is “burned” and exhaled as CO2 along with water vapour in your breath. But, at the same time, the biochemical reactions of energy metabolism that produce ATP (energy) and CO2, also produce water. And, since glycogen stores ~1 to 3 molecules of water for every molecule of glucose, glycogen breakdown (glycogenolysis) during exercise also produces water. This water released from energy metabolism and glycogenolysis joins the total body water tank and is distributed between the intra- and extracellular pools accordingly. In simple words, during exercise, water and CO2 leave your body and cause weight loss but “metabolic” water is added back into the total body water tank, which means that the drop in total body water during exercise will likely be less than body weight loss might suggest.
These sources of error were thoroughly discussed in 2018 by Martin Hoffman (chief doc at Western States 100), Eric Goulet, and Ron Maughan and in Ron Maughan’s theoretical analysis — “Errors in the estimation of hydration status from changes in body mass” — in which he found that respiratory water losses can be as high as 0.18 litres per hour during heavy exercise and that weight loss due to energy expenditure is about 0.23-0.24 grams per kcal “burned” (or ~0.24 kg per 1000 kcal “burned”). Furthermore, Pastene and colleagues, who studied 6 athletes running a treadmill marathon, calculated that 557 g of fuel (glycogen and fat) was “burned” during which 402 g of water was released from metabolism and 1280 g of water was released from stored glycogen breakdown — that’s the potential for 2.2 kg of weight loss without any loss of body water. Given these estimates, it means that you can lose up to 3% body weight during a marathon without any change in hydration status.
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So, the relationship between body weight change and hydration status during exercise is not a simple one. When using body weight loss to assess your sweat rate during exercise, always remember that it provides a rough estimate of body water loss (dehydration). During very long duration exercise, a lot of fuel is “burned” and this alone can account for a large proportion of body weight loss during the event. Therefore, in this context, a planned “drinking to a schedule” strategy to prevent weight loss could be dangerous since it will cause you to drink more fluid than is actually lost from the tank (hyperhydration), increasing the risk of “diluting” your plasma sodium levels causing hyponatremia (plasma sodium ≤135 mM).
What can you add to your hydration toolbox?
With hydration, as you are probably now coming to terms with, there is a lot we aren’t sure about:
We’re not sure exactly how best to hydrate during exercise. The reason we’re unsure is that in real-world ecologically valid settings, loss of body weight during exercise (as a biomarker for dehydration) is not associated with poorer performance. Furthermore, loss of body weight during exercise is not entirely due to body water loss (dehydration); some weight is lost in expired CO2 from energy metabolism aka “burning” stored fuel. And, loss of body weight during exercise can occur without changes in hydration status because “burning” stored fuel produces water in our cells. Therefore, “drinking to a schedule” to match body weight loss is unnecessary and also potentially dangerous because drinking too much fluid during exercise (hyperhydration; indicated by weight gain) can “dilute” sodium in your blood causing hyponatremia (plasma sodium ≤135 mM).
The overall risk of a dehydration-related adverse health event or symptomatic hyponatremia during exercise is extremely low (see Part 1), so don’t get overly anxious.
If you start exercise euhydrated, dehydration (water leaving your body) during exercise is highly unlikely to affect your performance. But, if you start exercise hypohydrated (aka “dehydrated”) and do not have fluid available during exercise, dehydration during exercise will likely impair your performance, especially on a hot day. Therefore, doing your best to stay hydrated day-to-day is an essential component of your training toolbox, and this can very likely be achieved by simply drinking fluid to quench your thirst when that feeling arises.
Thanks for joining me for another “session”. Until next time, keep drinking smart.
Disclaimer: I occasionally mention brands and products but it is important to know that I am not affiliated with, sponsored by, an ambassador for, or receiving advertisement royalties from any brands. I have conducted biomedical research for which I have received research money from publicly-funded national research councils and medical charities, and also from private companies, including Novo Nordisk Foundation, AstraZeneca, Amylin, A.P. Møller Foundation, and Augustinus Foundation. I’ve also consulted for Boost Treadmills and Gu Energy on their research and innovation grant applications and I’ve provided research and science writing services for Examine — some of my articles contain links to information provided by Examine but I do not receive any royalties or bonuses from those links. These companies had no control over the research design, data analysis, or publication outcomes of my work. Any recommendations I make are, and always will be, based on my own views and opinions shaped by the evidence available. My recommendations have never and will never be influenced by affiliations, sponsorships, advertisement royalties, etc. The information I provide is not medical advice. Before making any changes to your habits of daily living based on any information I provide, always ensure it is safe for you to do so and consult your doctor if you are unsure.
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About the author:
I am Thomas Solomon and I'm passionate about relaying accurate and clear scientific information to the masses to help folks meet their fitness and performance goals. I hold a BSc in Biochemistry and a PhD in Exercise Science and am an ACSM-certified Exercise Physiologist and Personal Trainer, a VDOT-certified Distance running coach, and a Registered Nutritionist. Since 2002, I have conducted biomedical research in exercise and nutrition and have taught and led university courses in exercise physiology, nutrition, biochemistry, and molecular medicine. My work is published in over 80 peer-reviewed medical journal publications and I have delivered more than 50 conference presentations & invited talks at universities and medical societies. I have coached and provided training plans for truck-loads of athletes, have competed at a high level in running, cycling, and obstacle course racing, and continue to run, ride, ski, hike, lift, and climb as much as my ageing body will allow. To stay on top of scientific developments, I consult for scientists, participate in journal clubs, peer-review papers for medical journals, and I invest every Friday in reading what new delights have spawned onto PubMed. In my spare time, I hunt for phenomenal mountain views to capture through the lens, boulder problems to solve, and for new craft beers to drink with the goal of sending my gustatory system into a hullabaloo.
Copyright © Thomas Solomon. All rights reserved.
I am Thomas Solomon and I'm passionate about relaying accurate and clear scientific information to the masses to help folks meet their fitness and performance goals. I hold a BSc in Biochemistry and a PhD in Exercise Science and am an ACSM-certified Exercise Physiologist and Personal Trainer, a VDOT-certified Distance running coach, and a Registered Nutritionist. Since 2002, I have conducted biomedical research in exercise and nutrition and have taught and led university courses in exercise physiology, nutrition, biochemistry, and molecular medicine. My work is published in over 80 peer-reviewed medical journal publications and I have delivered more than 50 conference presentations & invited talks at universities and medical societies. I have coached and provided training plans for truck-loads of athletes, have competed at a high level in running, cycling, and obstacle course racing, and continue to run, ride, ski, hike, lift, and climb as much as my ageing body will allow. To stay on top of scientific developments, I consult for scientists, participate in journal clubs, peer-review papers for medical journals, and I invest every Friday in reading what new delights have spawned onto PubMed. In my spare time, I hunt for phenomenal mountain views to capture through the lens, boulder problems to solve, and for new craft beers to drink with the goal of sending my gustatory system into a hullabaloo.
Copyright © Thomas Solomon. All rights reserved.