Sweat loss rate test

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Sweat loss rate test

-Written by Curt Chesney

Are you the type of triathlete that doesn’t seem to run as well during a race as your training indicates you should be running? That was me the first 3 years I raced. Once I started to measure my sweat loss rate and track it during training I saw a dramatic improvement in my racing performance. A general rule of thumb is to see a 10% drop in performance with every 2lbs of liquid lost and not replaced. The first thing I look at with an athlete looking to run faster is their nutrition and hydration approach for their triathlons.

So many people that race triathlons don’t have a plan for hydration during the event. Even more don’t have a clear picture of how much they should be drinking in training or racing. Simply drinking “lots” or when you are thirsty is not a good plan. I always tell athletes that if they don’t know how much they are losing how do they know how much to drink. The only way to know how much you need is to perform a sweat rate loss test.

A lab is not necessary to measure your sweat loss rate. Weigh yourself without clothes before a run. Run an hour, preferably in the heat. You can drink during the run just make sure you keep track of exactly how much. With an empty bladder weigh again with no clothes on. If you drank during the run subtract that amount too. There are 16oz to a pound so if you drank 24oz subtract 1.5lbs from your weight. If your loss was 3lbs that’s 48 oz per hour. Take into consideration the temperature. For example if it’s 90 degrees then your sweat loss rate is probably 45 to 52 oz per hour max. If it is 68 degrees and cloudy then your loss rate is going to be more than 48oz on a hot day. Get a measurement on the bike as well. Shoot for a 2-hour bike test for more accuracy. A good practice is to measure this regularly at different temperatures and keep a log so you have an idea of your needs specific to race day conditions.

Your performance will decrease 10% with every 2lbs (32oz) of liquid lost if not replaced. As an example lets say your loss rate is 50oz per hour on a hot day and you are only drinking 30 oz per hour. After 2 hours you are about 40oz behind which would be over 12% reduction in performance. You can do the math for yourself and see how easy it is to get behind. In my opinion this is why so many people don’t seem to be happy with their run on 1/2 IM and longer races.

There are many variables that can also come into the equation. An athlete’s sweat rate can change with fitness and heat acclimation. Although if you have a high rate it will always be on the high side. Also, your sweat rate will be less on the bike. Usually the bike portion is not as hot as the run or at least the first half of the bike. So adjustments to your hydration plan must be made to not over hydrate. If an athlete’s loss rate is above 60oz per hour the stomach can have difficulty in absorbing more than this. As the intensity goes up the stomach will have more difficulty absorbing liquids. Which is why the bloated stomach is so common in racing as compared to what we have happen in training.

As you can see it is difficult to get an exact number to work with especially with some of these variables. Being lean or heavy has nothing to do with being a heavy sweater or not. I am 6 ft and 150lbs with a loss rate of over 64oz per hour. This is why knowing my rate has helped me so much. I was originally drinking about 24oz per hour on the bike when I needed at least 48oz. And would always have to jog the run. Now my 1/2IM run pr is very close to my 1/2 marathon pr. Remember that all hydration and nutrition plans need to always be practiced in training, which will also help your recovery. I can guarantee that if you at least get a ballpark number to work with and know if you are on the high or low end you will start to see improvements in your racing and training.

Curt Chesney
Curt Chesney
Curt Chesney combines five years of coaching experience with twenty years of racing experience to help his athletes reach their potential.
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