What's Cool In Road Cycling

Sweat Bomb: How Much Do You Sweat?

Here's How You Can Tell

Sweating helps us cool down but also drops our fluid balance. Is it important to figure out your own individual sweat rate, and how might you go about it?

water

Introduction
Southern Ontario has been hot and humid for the last several weeks. The high heat and humidity while out riding had me re-thinking about proper hydration strategies for cyclists in the heat. However, without having an idea about how much water I’m losing while exercising, it can be difficult to put together a hydration plan. In this week’s Toolbox article, I will discuss the importance of sweating, and help you calculate your own sweat rate.

Athletes lose both water and electrolytes as a by-product of our body’s thermoregulation; that is our body’s method to maintain a relatively constant internal temperature. Sweating is our body’s most efficient method to lose excess heat. Water is excreted out of sweat glands on the skin & is then evaporated into the air.

As a side note: wiping sweat off your skin does not cool you down, even if it feels good to get the moisture off your skin. Only the evaporation of sweat off your skin provides effective heat loss.

Sweat Rate Basics
In simple terms, sweat rate represents how quickly an individual loses water during exercise. The value is typically displayed in liters per hour (L/hr). In the scientific community, there are several methods used by researchers to determine an athlete’s sweat rate. These include hygrometry & gravimetry, which use either ventilated sweat capsule to measure changes in air temperature & humidity (hygrometry) or collect sweat directly from the skin’s surface (gravimetry). Both those methods have their strengths, but also cannot be performed very easily by athletes in the field.

The most accurate method to measure whole-body sweat rate in the field is through changes in nude body weight before and after exercise. One important consideration for this method is to account for all non-sweat related changes in body weight, mainly fluid intake & urine output. Let’s investigate how this can be done below:

sweat

How to Calculate Your Sweat Rate
Calculating your sweat rate is surprisingly easy. This guide was adapted from Asker Jeukendrup and Andy Blow from Precision Hydration.

To get started, you’ll need an accurate bathroom scale & a towel. A smaller kitchen scale might also be helpful to weigh your cycling bottles. Record things in kg and hours to simplify things.

  1. Use the washroom & then record your body weight (in kg). It’s best to weigh in without any clothing on to avoid issues with sweat-soaked clothing. If possible, it’s best to avoid peeing during these sessions.
  2. Next, weigh your water bottle(s) (in kg) before getting started with your ride.
  3. Do your workout session! Keep track of the duration of the workout (in hours).
  4. After the session, weigh your water bottle(s) again, even if they’re empty.
  5. Next, dry yourself with a towel and record your post-ride weight (in kg). Once again, it’s best to weigh in without clothes.
  6. Next, we’ll calculate the difference in weight from pre-exercise to post-exercise:

Weight lost (kg) = preride weight – postride weight

7. We also need to account for the water that you consumed during the session:

Volume consumed (kg) = postride water weight – preride water weight

Finally, with that information known, we can calculate your sweat rate:

Sweat rate (L/h) = (Weight lost (step 6) + Water consumed (Step 7)) / Exercise duration (h)

How much do athletes sweat?
There are numerous factors that influence an athlete’s sweat rate, including inter-individual differences such as gender, age, etc., as well as ambient weather conditions, workout intensity, acclimatization status, etc. Based on a meta-analysis by Baker (the open access article is linked at the bottom of this article), the mean sweat rate was approximately 1.25 L/hr, with a range from 0.5 L/hour up to about 3.5 L/hr (with a few outliers in the 4+ L/hr range). The distribution was calculated from a group of 461 athletes (369 male, 92 female) across a variety of sports.

In general, a “normal” sweat range is around 1-1.5 L/hr. A low-volume sweater is < 1 L/hr, while a high-volume sweater would be classified above 2.0 L/hr. However, keep in mind that this calculation doesn’t consider body size/weight into consideration. For example, a small athlete with a 1.5 L/hr sweat rate might be a high-volume sweater.

What can you do with this information?
Having some knowledge of your sweat rate can assist you in developing a ballpark idea of how much water you’re losing under a variety of exercise intensity levels and ambient weather conditions. Keep in mind that replacing exactly the amount of water lost isn’t the goal (i.e. if your sweat rate is 1 L/hr, you do not need to drink 1 L/hr of water). Our bodies can tolerate a moderate amount of dehydration. In fact, Dr. Stephen Cheung’s lab found in a 2015 paper that dehydration up to 3% did not affect submaximal cycling performance in the heat. Rather, the idea is to have a general idea of exercise intensities (or weather conditions) that might require more hydration than usual.

That’s all for this month. I hope you learned something about sweat rates and can now try calculating your own sweat rate. For additional amazing resources on this topic, check out the Precision Hydration sweat test, as well as mysportsscience, from Asker Jeukendrup.

Ride fast, stay safe, and I’ll see you next month.

No Tour of Qatar in 2017, news in EUROTRASH. The riders might not miss the heat, but it was a good way to start the season. Pic:CorVos/PezCyclingNews.

References:

Baker LB. Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Med. 2017;47(Suppl 1):111-128. doi:10.1007/s40279-017-0691-5

Cheung SS, McGarr GW, Mallette MM, Wallace PJ, Watson CL, Kim IM, Greenway MJ. Separate and combined effects of dehydration and thirst sensation on exercise performance in the heat. Scand J Med Sci Sports. 2015 Jun;25 Suppl 1:104-11. doi: 10.1111/sms.12343. PMID: 25943661.

 

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