“Dehydration” Is Not Always Bad, “Hydration” Is Not Always Good
This is an important idea to comprehend for anyone (really, anyone) with plans to complete a half marathon, marathon, or any endurance event lasting more than an hour. I am not going to give you any easy answers. That’s not because I don’t like you or because I want to see you fail. It’s because I want you to have to think about it. To be open to new ideas. To experiment. To develop an understanding of what works best for you, and a realization that “what works best for you” can be different depending on the day. If you remember nothing else remember this: “Dehydration” is not automatically bad, and “hydration” is not automatically good.
Why should you drink during exercise?
When you exercise, your body warms up. When your body warms up, you sweat to cool yourself down. When you sweat, you lose body water. The water in your body has important functions for survival, health and performance (Click here for some great background on this…thanks Matt!). If you lose too much water during exercise, your health and/or performance could be at risk. Drinking fluids helps replenish some of that lost body water. However, if you drink too much fluid, your health and/or performance could also be at risk. So a balance must be struck.
How much sweat do you lose?
The answer? It depends. On a lot of stuff:
Individual variation: Our lab has been conducting hydration analyses for endurance athletes for years. Sweat losses have ranged from about 500mL to 2.5 litres per hour.That is a big range!
Environmental conditions: As temperature increases, so will sweat rate. For the same group of athletes in our lab exercising at the same intensity, sweat rate increased by an average of 400mL per hour in hot (30 degrees Celsius) versus normal (22 degrees Celsius) conditions. In those same athletes, sweat rate decreased by an average of 500mL per hour from windless to high-wind conditions (They sweat less in the wind). Think about how much better you feel when you’re running inside on a treadmill and you turn on a fan. A study examining humidity and sweat rate found a 500mL per hour increase from 24% to 80% relative humidity.
Exercise intensity: The harder you go, the more you sweat. A 2011 study showed that slower runners sweat less per hour and lose less body mass than faster runners. A marathoner finishing in the back of the pack may sweat 500mL per hour (or less), while an elite marathon runner could lose up to 3.6 litres per hour!
Exercise duration: On the surface, this one seems obvious. The longer you exercise, the more sweat you lose. True, but, as was just mentioned, longer duration generally means lower intensity, and lower hourly sweat losses. So you can’t expect the same rate of fluid loss for a marathon as you would for a 5k.
What are the current recommendations?
Many athletes and industry experts still subscribe to outdated or overly simplified recommendations. The default idea is that “dehydration is bad” and “hydration is good”. In reality, severe dehydration is bad, and overhydration is just as bad. Recommendations of a certain amount of fluid per hour are often given, but with so many factors affecting how much an athlete sweats, an individual approach is best.
Current hydration advice gives credence to the individual nature of hydration, and the value of perception of thirst. The American College of Sports Medicine recommends that athletes drink enough during endurance exercise to lose no more than 2-3% of their initial body mass (3% of 80kg is 2.4kg). This stems from research showing that dehydration during exercise can result in higher core temperature, heart rate and fatigue, and reduced performance.
Some researchers contend that hydration guidelines should not depend on body mass at all, and that the most important determinant of when to drink is the perception of thirst. They contend that overhydration is a more common problem than dehydration, and that dehydration (body mass loss) of less than about 5% is not always dangerous to the average athlete exercising for long periods of time.
Both of these viewpoints warrant consideration. Thirst is a finely tuned physiological mechanism and should be utilized by all athletes in order to avoid extreme dehydration or overhydration. But for runners targeting longer races, body mass loss provides a more personal baseline to start from.
Putting it into practice:
If you’re running a 5k or 10k, drinking to thirst is your best bet, as long as you areproperly hydrated before you start. If it’s a hot day, you may benefit from taking in a bit more fluid, and from simply pouring fluid on your body. After all, skin temperature is a big determinant of how hot you feel when exercising.
For half marathon or marathon runners, you still want to pay attention to thirst, but it’s a good idea to measure body mass change on some of your long training runs. Body mass change is a reliable estimate of how much sweat you’re losing. Before your run, weigh yourself on a digital scale (preferably naked). When you get back, weigh yourself again (preferably naked and dry). If you gained weight after a 2- or 3-hour run (or if you had to pee 5 times along the way), you drank too much. If you lost more than 2-3% of your body mass over the same amount of time, you might benefit from drinking more. We have seen many people like this in the lab, and have received reports from some that an increase in fluid intake has helped. For others, it may lead to stomach problems and discomfort and may require more practice in order to be beneficial. It is a cost/benefit anaylsis, where if the benefits of reducing dehydration do not outweigh the cost of increasing fluid intake, then it may not be worthwhile or practical.
Make sure your practice sessions mimic the expected race conditions. If it’s going to be hot on race day, practice hydration in the heat. If it’s going to be cold, practice hydration in cool temperatures. Try to figure out how much fluid you lose per hour when you’re running at your race pace. You’ll lose a lot less at an easy pace. Be ready to adapt.
If your race is coming up and you didn’t pay particular attention to hydration in training: Did you have any problems? If not, then don’t change anything for the race! Don’t be afraid to let your perceptions of thirst and fullness dictate your hydration.Avoid following rigid, generalized hydration plans, and avoid the temptation of defaulting to the “more is better” approach.
I could go on like this for days, but I will end things there. I welcome your comments and questions. While you don’t want to blindly follow the advice of others, it’s still valuable to discuss, share experiences and gain new insights.
References: provided as links within the text.
Why should you drink during exercise?
When you exercise, your body warms up. When your body warms up, you sweat to cool yourself down. When you sweat, you lose body water. The water in your body has important functions for survival, health and performance (Click here for some great background on this…thanks Matt!). If you lose too much water during exercise, your health and/or performance could be at risk. Drinking fluids helps replenish some of that lost body water. However, if you drink too much fluid, your health and/or performance could also be at risk. So a balance must be struck.
How much sweat do you lose?
The answer? It depends. On a lot of stuff:
Individual variation: Our lab has been conducting hydration analyses for endurance athletes for years. Sweat losses have ranged from about 500mL to 2.5 litres per hour.That is a big range!
Environmental conditions: As temperature increases, so will sweat rate. For the same group of athletes in our lab exercising at the same intensity, sweat rate increased by an average of 400mL per hour in hot (30 degrees Celsius) versus normal (22 degrees Celsius) conditions. In those same athletes, sweat rate decreased by an average of 500mL per hour from windless to high-wind conditions (They sweat less in the wind). Think about how much better you feel when you’re running inside on a treadmill and you turn on a fan. A study examining humidity and sweat rate found a 500mL per hour increase from 24% to 80% relative humidity.
Exercise intensity: The harder you go, the more you sweat. A 2011 study showed that slower runners sweat less per hour and lose less body mass than faster runners. A marathoner finishing in the back of the pack may sweat 500mL per hour (or less), while an elite marathon runner could lose up to 3.6 litres per hour!
Exercise duration: On the surface, this one seems obvious. The longer you exercise, the more sweat you lose. True, but, as was just mentioned, longer duration generally means lower intensity, and lower hourly sweat losses. So you can’t expect the same rate of fluid loss for a marathon as you would for a 5k.
What are the current recommendations?
Many athletes and industry experts still subscribe to outdated or overly simplified recommendations. The default idea is that “dehydration is bad” and “hydration is good”. In reality, severe dehydration is bad, and overhydration is just as bad. Recommendations of a certain amount of fluid per hour are often given, but with so many factors affecting how much an athlete sweats, an individual approach is best.
Current hydration advice gives credence to the individual nature of hydration, and the value of perception of thirst. The American College of Sports Medicine recommends that athletes drink enough during endurance exercise to lose no more than 2-3% of their initial body mass (3% of 80kg is 2.4kg). This stems from research showing that dehydration during exercise can result in higher core temperature, heart rate and fatigue, and reduced performance.
Some researchers contend that hydration guidelines should not depend on body mass at all, and that the most important determinant of when to drink is the perception of thirst. They contend that overhydration is a more common problem than dehydration, and that dehydration (body mass loss) of less than about 5% is not always dangerous to the average athlete exercising for long periods of time.
Both of these viewpoints warrant consideration. Thirst is a finely tuned physiological mechanism and should be utilized by all athletes in order to avoid extreme dehydration or overhydration. But for runners targeting longer races, body mass loss provides a more personal baseline to start from.
Putting it into practice:
If you’re running a 5k or 10k, drinking to thirst is your best bet, as long as you areproperly hydrated before you start. If it’s a hot day, you may benefit from taking in a bit more fluid, and from simply pouring fluid on your body. After all, skin temperature is a big determinant of how hot you feel when exercising.
For half marathon or marathon runners, you still want to pay attention to thirst, but it’s a good idea to measure body mass change on some of your long training runs. Body mass change is a reliable estimate of how much sweat you’re losing. Before your run, weigh yourself on a digital scale (preferably naked). When you get back, weigh yourself again (preferably naked and dry). If you gained weight after a 2- or 3-hour run (or if you had to pee 5 times along the way), you drank too much. If you lost more than 2-3% of your body mass over the same amount of time, you might benefit from drinking more. We have seen many people like this in the lab, and have received reports from some that an increase in fluid intake has helped. For others, it may lead to stomach problems and discomfort and may require more practice in order to be beneficial. It is a cost/benefit anaylsis, where if the benefits of reducing dehydration do not outweigh the cost of increasing fluid intake, then it may not be worthwhile or practical.
Make sure your practice sessions mimic the expected race conditions. If it’s going to be hot on race day, practice hydration in the heat. If it’s going to be cold, practice hydration in cool temperatures. Try to figure out how much fluid you lose per hour when you’re running at your race pace. You’ll lose a lot less at an easy pace. Be ready to adapt.
If your race is coming up and you didn’t pay particular attention to hydration in training: Did you have any problems? If not, then don’t change anything for the race! Don’t be afraid to let your perceptions of thirst and fullness dictate your hydration.Avoid following rigid, generalized hydration plans, and avoid the temptation of defaulting to the “more is better” approach.
I could go on like this for days, but I will end things there. I welcome your comments and questions. While you don’t want to blindly follow the advice of others, it’s still valuable to discuss, share experiences and gain new insights.
References: provided as links within the text.
