Every IRONMAN competitor anticipates, experiments, and maybe stresses a bit (or a lot) about race day nutrition, starting with the foundation of fluid replacement. They appreciate that dehydration can slow you down, wreak havoc with your gastrointestinal system, and lead to overheating. On the flip side, over hydrating increases your risk for developing exercise associated hyponatremia, a fluid overload condition characterized by abnormally low levels of blood sodium that can be serious and even fatal. To find your own hydration sweet spot for race day—not too much and not too little—you can set up a process to check your sweat rate throughout your training season.
You may have heard recently that thirst should drive your drinking. While this is safe and prudent for the recreational exerciser to prevent fluid overload, we can all appreciate that Ironman events are not casual, no matter your race day goals. Just as you have a planned race day pace, you should also have a paced plan for hydration during the bike and run. However, endurance athletes will need to be prepared to adapt and adjust on race day as well.
Start by collecting your own sweat data. You can start anytime, but should do this throughout the training season to capture changes in your sweat rate that occur with increased fitness, acclimatization, increased training intensity, and of course the full smorgasbord of potential weather conditions. Creating a flow sheet for tracking is wise as this allows you to review data from the past as you develop and refine your hydration. Race day weather can differ greatly from recent training conditions due to travel to other climates and venues famous for labile weather conditions. Looking back at your records allows you to confidently tweak your race nutrition plan accordingly as the race day weather forecast emerges.
How to check your sweat rate
Begin to think of your sweat losses as an hourly rate specific to the bike and to the run. Fueling guidelines are also described at carbohydrates per hour, so this is a good base for your full race nutrition plan development.
While you are preparing for a long race, it is best to check sweat losses during shorter workouts- about 60 to 90 minutes. That’s because you burn stored fuel or muscle glycogen during exercise, contributing to the weight loss. Longer workouts mean more glycogen and water loss, so it throws off the data.
It is also important to be well hydrated prior to workouts when you are completing a sweat check. Weighing sweaty clothes and hair also throws off your calculations as does consuming solid or semi-solid products during the workout, so stick with liquids.
1) Weigh yourself nude right before a run (or bike).
2) Run at race pace for one hour, keeping track of how much you drink (in ounces) during the run (or bike).
3) After the run, strip down, towel off any sweat, and weigh yourself nude again.
4) Subtract your weight from your prerun weight and convert to ounces. Then add to that number however many ounces of liquid you consumed on your run (or bike).(1 pound = 1 pint or 16 oz. of water)
(For example, if you lost a pound and drank 16 ounces of fluid, your total fluid loss is 32 ounces.)
5) To determine how much you should be drinking about every 15 minutes, divide your hourly fluid loss by 4 (in the above example it would be 8 ounces).
- Many factors affect the sweat rate! Check out the Electrolyte podcast for further educational information!
- Because the test only determines your sweat losses for the environmental conditions you run in that day, you should retest on another day when conditions are different to see how your sweat rate is affected. You should also redo the test during different seasons, in different environments (such as higher or lower altitudes), and as you become faster, since pace also affects your sweat rate.