August 12, 2024
Proper hydration during exercise is important, mostly during high intensity exercise in warm to hot circumstances. It helps you to peak performance and avoid muscle cramps.
During high-intensity sports like cycling and running, substantial amounts of water and electrolytes can be lost because of thermoregulatory sweating. The degree of these sweat and sodium losses are strongly dependent on factors like the intensity of exercise, climate temperature and humidity. Some individuals can even lose up to 3 liters per hour in hot circumstances, while others might only lose less than half a liter per hour. A body weight loss of 2-5% loss due to sweat loss can have a major negative impact on your performance.
In this article we explain with examples how much fluids you should take and how you can verify your own personal sweat rates and maximum level of acceptable dehydration – such that it still not significantly impacts your performance.
It starts with your sweat rates
Personal sweat rate
Will you perform your activity in warm(er) circumstances? You might want to have a good understanding of your personal sweat rate, to understand how much fluids you should aim for during your race. Your sweat loss is highly dependent on several factors including temperature and humidity, but also level of acclimatization and genetics play a role. Some people simply sweat more than others.
The first step is to get to know your sweat rate in different circumstances. Perform multiple sessions, as sweat losses will differ per day based on the above-mentioned factors. It’s best to do so during training sessions between 1-2 hours, as during longer training the influence of urination and glycogen loss will play a bigger role. This might affect the accuracy of your sweat rate measurements. Although measuring your sweat rate is still an estimation, it’s much better than having no guideline at all. As soon as you have a view of your sweat rates, you can estimate your expected sweat loss on your activity day. This is simply your sweat rate multiplied by the expected duration of the race.
Know your sodium loss
If your expected activity time is above 4 hours and you believe you are a ‘salty sweater’, you could consider testing your sodium sweat loss. White stripes on your jersey / bibs or a salty taste may indicate that you are a salty sweater. Sodium sweat loss is a highly individual number which is mostly decided by genetics. There are simple test for this which do not fluctuate much, with a very valuable outcome. Is your sweat sodium concentration over 1000 mg / L sweat? Then it’s advised to replace around 30-60% of your sodium losses during races over 4 hours.
Acceptable weight loss
Lastly, you need to determine how much weight loss is acceptable at the finish line. It seems that dehydration up to 5-6% of body weight loss could potentially be within normal ranges during ultra endurance events. However, aiming for 2% would be safer. E.g., if your start weight is 80 kg, your safe body weight loss of 2% at the finish line would be 78.4 kg. You can add around 0.5 kg extra as acceptable loss during long endurance events because this is weight loss from fuel depletion, as every gram of glycogen saved in our muscle and liver binds with some fluids. That is also directly the explanation why you might be a bit heavier in weight after taking excessive amounts of carbohydrates.
Calculating fluid intake requirements
Knowing all the necessary information beforehand, you can now calculate your required fluid intake during your race;
Step 1. Multiply the expected total time of your race by the expected sweat rate
Step 2. Subtract your acceptable weight loss of around 2%. The outcome is the volume you need to consume in total.
Step 3. Divide this number by the total time of the race to get your hourly intake.
Example
Given a 80 kg cyclist with an expected sweat rate of around 1.4L per hour during a mid-summer race.
- Acceptable weight loss: 2% of 80 kg = 1.6 kg. Add to this an additional 0.5 kg for fuel loss. This leads to a total of 2.1 kg.
When he performs a cycling race of 4 hours:
- Total expected sweat loss: 4 h x 1.4 L = 5.6 L.
- Required fluid intake = total expected sweat loss = acceptable weight loss = 5.6 kg – 2.1 kg = 3.5 L
- Required hourly intake = 3.5 L / 4, which is approximately 900 mL / hour.
This example illustrates that aiming to refuel approximately half of your fluid and sodium losses might already be challenging enough. Drinking according to your total losses would require you to drink amounts that would be much greater than your body’s instinct. Choose mostly isotonic drinks in combination with water during warm weather situations, ideal for hydration as well as it ensures that carbohydrates and electrolytes are more easily absorbed. During cold weather circumstances, you could consider variating with hypertonic drink; a drink that has a higher number of solutes than in human blood, such that water must first move out of the bloodstream back into the intestine to dilute the solution in the intestines. This in fact, dehydrates the body and increases the risk of stomach and intestinal problems. A hypertonic drink is the best choice to meet your needs for carbohydrates when energy is needed quickly, and temperatures are rather low.
Most importantly: listen to your body and digestive system; if you experience digestive complaints, reduce the intensity temporarily where possible, as you will benefit from it later in the race or during your activity.
Gather data through EatMyRide app
EatMyRide allows you to take and save your sweat loss data into the application via using the button Sweat rate test. For each test you simply add your naked weight just before the start of your activity. As soon as your activity is finished, you enter your post-activity naked weight, fluid intake and urination stops. An overview of your tests will be shown so you know how your sweat loss depends on the intensity and temperature of your activity. You see this by clicking on the progress button on the homepage.
It comes with 2 benefits for you:
- After any future activity you will see your estimated fluid loss, taking into account the intensity and temperature
- If you use a Garmin head-unit with touchscreen, you can both see your real-time sweat loss and track in real-time your fluid intake
The more sweat loss tests you performed, the better the EatMyRide algorithm can estimate your fluid loss for your next activity. You might notice the completeness score in the app: this is telling you whether you took enough tests within different circumstances.
Example sweat rate profile. The color of the dot shows the intensity.
Based on your sweat rate tests you will see your sweat loss for every future exercise (and if you add data as well your sodium loss)
You can even use your sweat rate test results to see real-time sweat loss estimates on your head-unit
All numbers… what is the practical advice?
Low sweat rate (≤ 1 L / hour)
It’s advisable to drink around 300-500 ml per hour. Ranging more towards sweat losses of 0.5 L / hour? You could drink to thirst, as it is unlikely that you would benefit from drinking much more. Over time, your total fluid and sodium losses are still relatively low.
Medium sweat rate (1 – ≤ 1.5 L / hour)
An intake around 750 ml / hour will meet your required fluid amounts. Consider an isotonic drink to maintain electrolyte balance.
High sweat rate (≥ 1.5 L / hour)
You will benefit from proper nutrition strategies aiming to be ‘frontrunner’ of the dehydration depletion curve. Concrete, aiming to replenish 1 liter per hour, or roughly 50% of your sweat rate during your training or race would usually be a realistic guideline. This may require training! Be aware of sodium losses and consider an isotonic drink to maintain electrolyte balance.
Setting your fluid requirements in the app
Next time you create an in-ride plan in EatMyRide app and reach the page where you add drinking products, click on the box 500 ml/hour and customize this number corresponding to whether you have low/medium/high sweat rate.
It is important to realize that the goal should not be to 100% replace sweat losses during a competition. This would require you to drink amounts that would likely be much higher than your body’s instinct and can lead to several negative side effects. Use these sweat rate numbers as a guideline, as in race situations you will very likely not be able to follow your original hydration plan due to race dynamics.
References
- Baker, B., & Jeukendrup, A (2014, April). Optimal composition of fluid-replacement beverages. Retrieved from https://pubmed.ncbi.nlm.nih.gov/24715561/
- McCubbin AJ. (2022, June 13). Modelling sodium requirements of athletes across a variety of exercise scenarios – Identifying when to test and target, or season to taste. Retrieved from https://pubmed.ncbi.nlm.nih.gov/35616504/
