Calculating your Sweat Rate / Hydration Considerations for Triathlon

Science has known for a long time that the sensation of thirst is not the best indicator of how much you need to drink when training and racing[1]. Studies have also shown that losing as little as 2% of your body weight due to dehydration impairs physical and mental performance and losing 5% of your body weight by dehydration can reduce your work capacity by 30%[2]. And here we are, most of us, training in Central Texas in summer. So how do we know how much we should drink to stay hydrated in extreme heat and humidity?

We need to determine our sweat rate in conditions that will mimic the warmest race temperatures that we'll face. Luckily, we don't need to see a doctor or go to a lab to find this out. Here's a simple protocol to determine your sweat rate and plan for hydration before, during and after training:

Select a training session where the temperature and humidity (as well as your intensity) are like what you'll experience racing. At the conclusion of your pre-training bathroom routine, weigh yourself without clothes. Go out and complete a run or cycling training session. Don't eat or go to the bathroom during your ride or run. Take note of how much fluid you drank during your training session. Go back to the scales, get out of all your sweaty clothes, dry off, and weigh yourself again. Now you can calculate your sweat rate. Here's a notional example:

Pre-workout weight: 175 lbs. You go out at about noon (to replicate the heat you'll experience near the end of the 56-mile bike leg of the 70.3 triathlon) and ride 36 miles in 2 hours. During the ride you drink 32 ounces. 16 oz = 1 lbs, so you took on 2 lbs of liquid during your ride. Post-workout weight is 173. That's only 2 lbs, but you must account for the 2 lbs of fluid you drank, so that's a total loss of 4 lbs. That's 64 oz lost for 36 miles (1.77 oz./mile or 32 oz/hour). Now you have a good idea of how much to drink while racing and training at varying distances. Since the physical demands and convective cooling of running are different, you should also do this protocol for running and arrive at an oz./mile running sweat rate.

So great, simple enough, but dang, that's a lot of fluid. Do you actually need to replace every bit of fluid I lose? If possible, yes! Studies indicate that we should aim to replace all of our fluid losses to maintain our best performance[3].

Are all fluids equal, though? No, straight water does not hydrate you as well as a fluid with electrolytes[4]. "Sports drinks" like Gatorade and Powerade, in addition to being expensive, have too much sugar to be what I would recommend for hydration for training and competition longer than an hour. I have gotten very good results (and totally cured my cramping issues) with "Nuun" tablets which you drop into a water bottle. You can order Nuun online or purchase it in many stores, including your local bike/running stores and even Wal-mart.

Note that hydrating yourself for performance does not just happen during the event. You should aim to “pre-hydrate” as early as 24 hours before a hot event or big training day.

Triathlon offers us some interesting twists in the hydration problem since we cannot drink while swimming[5]. For those warm weather/warm water swims, we can bring an extra water bottle all the way to the waterline and keep drinking right up to the start. We can also have an additional bottle in transition and gulp down a few ounces in T1. Since the bike is a tremendous vehicle, consider “overconsuming” on the bike to catch up with what you lost in the swim and preloading for the demands of the run. It is easier to drink large amounts of fluid perched securely on a bike seat than striding along on a run.

 

 

 

 


[1] https://www.utphysicians.com/whats-the-difference-between-thirst-and-dehydration/#:~:text=Thirst%20or%20dehydration,fatigue%2C%20irritability%2C%20and%20headaches.

[2] https://sportscardiologybc.org/the-effects-of-hydration-on-athletic-performance/

[3] https://pubmed.ncbi.nlm.nih.gov/9303999/

[4] https://pubmed.ncbi.nlm.nih.gov/15212747/

[5] https://www.triathlete.com/nutrition/race-fueling/yes-you-can-get-dehydrated-while-swimming/


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