First things first – You CANNOT only consume water during longer bouts of exercise.
YOU MUST ALSO TAKE IN ELECTROLYTES in order to avoid HYPONATREMIA – a potentially deadly condition caused when electrolyte imbalances occur and motor functions start to shut down as a result.
Sorry for the internet yelling…we like you too much to let you die out there.
(You can read even more info about battling the heat and hyponatremia HERE.)
Now that we have that out of the way…onward…
There are a lot of people out there who would love to hear about losing 6 pounds or more of 
body weight in 2 hours. They would likely pay hundreds of dollars to find out the “secret.” The “secret” in this scenario is improper hydration during higher temperature outdoor training – and it is our goal to make sure this DOES NOT happen to you.
As most of the people reading this post have already figured out what I’m going to tell you – it is HOT outside! With this heat comes increased sweating. Increased sweating leads to what can be a rapid loss of water weight. This is NOT good for your performance or your health.
We have numerous athletes that we coach who have dropped 6+ pounds during a 2 hour training session. As the mercury rises, so too should your fluid intake.
But by how much?
The answer: Calculate Your Sweat Rate
Everyone is going to have different needs when it comes to staying hydrated. The temperature is going to play a huge factor in this as well. Intensity and duration of activity are also factors. The best thing for you to do is to estimate your “sweat rate” during different training conditions so that you can better dial in your hydration plan.
Here is what we have our athletes do:
Step 1. During a more intense training session (think intervals) of 1-1.5 hours in length, we have our athletes weigh without clothes prior to their workout. Note: we typically use a shorter training session so increased food intake does not have to be factored in.
Step 2. Record the volume of fluid ounces consumed during this training session as well as the duration of the training session. Now we know the fluid volume consumed (“In”) and minutes exercised.
Step 3. Weigh post workout, again without clothing on. Now we have a good estimate of volume lost.
Step 4. Add up the amount of fluid ounces consumed, as well as the amount of ounces of body weight lost (16 oz is equal to 1 lb, so if you lose 2 lbs then add 32 oz).
Step 5. Divide this by the number of minutes exercised.
Step 6. Multiply this by 60 (to get rid of minutes and to find out how much you should be consuming per hour).
BAM! You now have a pretty good estimation of how much fluid you should be consuming to try and keep up with the amount of fluids you are losing over the course of a hot ride.
Don’t be surprised if you come up with a really high number. Of our athletes that we have tested, most are falling in the 32-60 oz per HOUR range during higher intensity, higher temperature training/racing. We’ve seen sweat rates as high as 4L per hour, but that is on the extreme end of the spectrum.
We recommend you test this often over a variety of conditions (intensity, duration, temperatures, etc). The more data points you have, the better you are going to be at dialing in your fluid consumption for optimum performance!
Note 1: some weight loss is likely inevitable over the course of a longer training session, the goal is to avoid the extremes.
Note 2: If you are gaining weight during a training session then you are likely over-hydrating or overdoing your sodium intake.
Want more info on avoiding a heat beatdown? Follow this link for our 10 Heat Beatdown Avoidance Tips
For information on what you should be hydrating with – contact us. Also, tune-in for an upcoming blog post on hyponatremia – something every endurance athlete should be aware of!
Test this out and then leave us a comment on what your sweat rate ended up falling at. We would love to compile even more data points. Just make sure you include the heat index and intensity of your training in the comments.