By Alister Gardner, 6-8 minute read
We all know that we should stay hydrated on hot days and during exercise, especially during endurance events of 30 minutes or more, as dehydration can severely affect both our performance and our health.
Guidelines suggest 500-1000 ml per hour, which might be fine for shorter workouts of 60 minutes or so, but as we go for longer workouts, this may not directly correspond to the real amount we sweat. It is also recommended that we consume 500-700 mg of sodium per litre, but does that really replace what is lost in sweat and more importantly, does it maximise fluid absorption?
In this article we will look at how to ride (or run!) while maintaining that fine line between dehydration and hyponatraemia.
The effects of dehydration:
We get thirsty after losing just 1% of body mass through sweating, and at 2% there is a noticeable difference in both physical and mental performance, as well as an increased risk of nausea and stomach issues (2). By 5%, during physical activities, output may be reduced by as much as 30%. At 6%, our heart rate will increase and our VO2max will decrease. At 8-9% loss, dehydration becomes a much bigger health risk and we will most likely be experiencing dizziness, weakness and confusion. By 11% there is a dangerous risk of kidney failure. To make a long story short, staying hydrated is important!
The general recommendation by the American College of Sports Medicine is to not lose more than 2% body weight (3). So how can we measure our fluid loss from sweating?
We can get a very good idea of how much fluid we lose by simply weighing ourselves before and after a workout. The difference would be the amount of fluid lost through sweat. There are 3 factors that must also be considered when we do this:
Some weight loss will come from depleted glycogen reserves. Our bodies can store up to 500 g (2000 cal) of glycogen which can be depleted in under 2 hours.
Some weight loss will also come from fat burnt, especially in longer workouts. It can contribute to around half the energy burnt in a moderate intensity workout and varies from 0.3-1.5g/min or 36-180 g over 2 hours, depending on how well adapted you are to burning fat.
Fluids consumed during the workout: this will bring our weight back up (1 litre is 2.2 lbs).
How about an example from a very sweaty guy (me)?
I weighed myself before a 3-hour run in 29°C heat and was at 160 lb. Upon my return, I weighed 151 lbs, which is a difference of 9 lbs. During this time, I drank 1.3 litres of fluid, which equals 2.8 lbs, and so had a total loss of 11.8 lbs. Factoring in glycogen and fat burnt for fuel (approximately 1.5 lbs), that is a total fluid loss of 10.3 lbs (5 litres), or 1.6 litres per hour.
So this provides me with a great insight on how much to drink to stay hydrated during endurance races.
As we can see in the graph below (4), the average sweat rate is 1.2 litre per hour, which means I am an above average sweater. However, it appears some folks can sweat a lot more than me (5+ litres per hour!!).
Hyponatremia and maximising fluid absorption
Water is great. It is refreshing and goes down easily. However, in endurance events, we need to add sodium for our bodies to absorb the water from our intestines better and thus avoid the health risk associated with hyponatremia.
Hyponatremia is the dilution of sodium in our blood and can be dangerous for our health if levels get too low. It typically occurs when we are overly precautious about staying hydrated and we drink too much water. As a result some people actually gain weight during activity. However, it is possible to be both dehydrated and suffer from hyponatremia as it is specific to sodium balance — not just hydration. At the 2002 Boston marathon, of 488 runners tested, 13% were diagnosed with hyponatremia at the end of the race (5). It is typically more common among longer endurance events (4+ hrs) or stage races (6,7). It is very rare that it is fatal, but it can easily be avoided.
So how much sodium?
The American College of Sports Medicine suggests 500-700 mg of sodium per litre of water, which is probably calculated as a result of the amount of sodium already in popular sports drinks that were used for testing. However, real life tests show that our sweat can contain much more sodium than that, with well-trained endurance athletes losing an average 900 mg of sodium per litre and untrained athletes that are not adapted to hot and humid environments losing up to 1800 mg per litre (8, 9).
As for maximising absorption, in other words, not letting the water pass straight through and needing to go to the toilet, it has been demonstrated that drinks with over 1200 mg of sodium can retain fluid best (10).
Our tips for staying hydrated and performing better:
- Measure your fluid loss regularly, and in a variety of conditions. This will help you know your re-hydration needs for the different conditions you train and race in more accurately.
- In very hot and humid conditions you should adjust your expected pace or output slightly so that you do not overheat and fall short on your training objectives (you can speed up again when it gets cooler).
- Have a look at the sodium intake you get per total litre of water you drink.
- If you have 1 x 500 ml bottle of electrolytes and 1 x 500 ml bottle of water on your bike, then the electrolyte total is only half.
- Check to see how many electrolytes you get from the energy gels and chews you eat during the time you drink that litre and add that to your total.
- In ultra-endurance events there are often salty snacks at aid stations on the course. It is difficult to quantify the sodium you get here so prepare to have more with you.
What should you take with you when you train?
- Approximately 30 minutes – just water.
- Up to 60 minutes – water with electrolytes.
- 60 minutes and more – carbohydrates and water with electrolytes.
Identify the cause of your cramps: It has long been thought that the main cause of cramping is due to a loss of electrolytes; however, it seems to be more complicated than that. For example, some types of cramps appear in cold weather and can be fixed by massaging the area.
So two types of cramps should be recognised:
- Whole body cramping, possibly accompanied with headaches and nausea, is most likely due to dehydration and a lack of electrolytes.
- Localised cramping, for example, a calf muscle or hamstring. This is more likely due to the muscle being overworked, and by immediately stretching and massaging the area you can often alleviate the problem. Strength training will help reduce the risk in the future too (11).
The thing to note is that on race day, we are likely to push our muscles harder and for longer periods than normal AND sweat a great deal when we do it. When cramps occur it will help to know which type of cramp it is in order to remedy the problem for next time more effectively.
XACT developed XACT ELECTROLYTES with our sweat-rates in mind. We wanted to create a portable and easy-to-use tablet with enough sodium content to replace what is lost in sweat, while at the same time be palatable and refreshing.
Each tablet has 520 mg of sodium and is designed to go in 500 ml of water. That gives 1040 mg of sodium per litre. This is not precisely what we lose but in endurance sports we often eat gels, energy chews and on-course snacks that also have salt in them. If we enjoy what we drink then we can hydrate better, and if we can get the right amount of sodium with the right amount of fluid then we can perform better.
- R. Murray “Rehydration strategies--balancing substrate, fluid, and electrolyte provision.“ Int J Sports Med. 1998.
- Jeukendrup, Asker, Gleeson. “Dehydration and Its Effects on Performance.” Sport Nutrition : an Introduction to Energy Production and Performance. 2nd ed. Leeds: Human Kinetics, 2010.
- Sawka et al “Exercise and Fluid Replacement. American College of Sports Medicine position stand.” Med Sci Sports Exerc. 2007.
- Lindsay B. “Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability” Sports Med. 2017
- Almond et al. “Hyponatremia among Runners in the Boston Marathon.” N Engl J Med 2005.
- Hew-Butler, Loi, Pani & Rosner MH “ExerciseAssociated Hyponatremia: 2017 Update.” Front. Med. 2017.
- Rosner & Kirven “Exercise-Associated Hyponatremia” Clin J Am Soc Nephrol. 2007.
- Maughan “Fluid and electrolyte loss and replacement in exercise.” J Sports Sci. 1991.
- Verde, Shephard, Corey & Moore ''Sweat composition in exercise and in heat.'' 1982
- Maughan, Leiper “Sodium intake and post-exercise rehydration in man.” Eur J Appl Physiol Occup Physiol. 1995
- Bergeron “Muscle Cramps during Exercise - Is It Fatigue or Electrolyte Deficit?” Current Sports Medicine Reports 2008.
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