While a casual rider likely has little concern regarding how much they’re actually sweating, if you’re an endurance cyclist or ride competitively, knowing your sweat rate is a critical component for achieving optimal performance. If you start your long rides strong but tend to fade a couple of hours in and can’t seem to take in enough fluids towards the end, you might have a problem with your hydration plan.
Hydration is one of the critical variables that can affect how the body responds to the environment. Since the body perspires to cool itself during exercise, replacing fluids is critical to keep it going. A finely tuned hydration plan can be the key to breaking through performance plateaus and recovering quicker. Failure to hydrate properly will most certainly affect your performance, causing the body to suffer tremendously; you might even end up in the hospital.
What is a Sweat Rate?
Sweat rate refers to how quickly an athlete loses fluids due to sweating. It’s a “moving target,” meaning that it will change as environmental conditions change. If you know the rate at which you’re losing fluids, you will know the rate at which to replenish them. Failure to replenish fluids could result in a variety of illnesses, including dehydration or the dreaded phenomenon of “bonking” during a ride. On the other hand, overhydrating is almost as bad as it can result in water intoxication, which occurs when the amount of salt and electrolytes in the body becomes too diluted. The key is to hydrate at the right level, at the right time, with the right stuff.
Why you want to know your Sweat Rate
It’s not unusual for a cyclist to lose two or three pounds in a one hour ride. Considering that an average human stomach can hold 32 ounces of water, if those fluids weren’t replenished during that ride, the body would be dipping into its reserves and facing dehydration. Knowing your sweat rate is essential because it offers a suggested amount of fluids that need to be replenished in order to keep your body functioning properly. This data should be used to design personalized hydration and recovery plans that will increase the efficiency of each ride and improve your overall performance.
What is a Sweat Rate test?
A sweat rate test isn’t as intimidating as it sounds; it just refers to a series of steps that have to be taken and specific data that must be collected. The test takes about an hour and should be repeated once a season, when the weather changes. Once you’ve completed the test you will have highly personalized information to help put together hydration and recovery plans.
When to test
Optimally, the sweat rate measurement should be done in the specific context, such as a preparation for a race or a planning a training block. It’s worth sweat testing more than once. In fact, you should be sweat testing throughout your training season. Test whenever you find yourself in a new environment or when the weather starts to change. If you’re training for an upcoming event or race, you can tweak your plans based on your data, but you also can perform another sweat test. If you’re able to train in the same kind of environment and at the same intensity level that you anticipate you’ll be at during an upcoming race or event, you should absolutely conduct a sweat test.
When Sweat Rate works...
The sweat rate method is most reliable when conducted in the environment in which you are most accustomed to riding. In order for data to be an accurate representation of your regular sweating patterns, it’s recommended that you collect data on a day where the weather is “comfortable;” certain weather conditions can cause your body to go into overdrive just to be able to maintain “safe” internal conditions.
...and when it doesn’t
While the sweat rate method is a reliable way to determine your hydration needs, there are times where you won’t be able to rely on it alone.
If you find yourself in extreme temperatures or are still acclimating to a new environment, it’s best to wait until the weather has changed or until your body has adjusted to the environment, because these factors will affect your results. Because your body has a bit of a defensive response in these circumstances, it is unlikely that data collected will be a good representation of your true sweat rate in more “normal” circumstances. This means that extreme cold or heat are not appropriate settings in which to determine your sweat rate.
Riding in environments especially high in humidity can also prove to be even more challenging because it inhibits the sweating process. More specifically, high moisture levels make it harder for sweat to evaporate from the surface of your skin. All that resting sweat ends up trapping in the very heat that it’s supposed to be dispersing! Not only does riding in high humidity decrease the accuracy of your data, but it also increases the likelihood of developing heat-related illnesses.
Another scenario where you shouldn’t count on accurate test results is if you go to the bathroom during your sweat test. If you have to stop to go to the bathroom during your ride, it’s best to shelf the testing for the day and start over another day because it will skew your post-ride weight.
Calculating your Sweat Rate
As simple as a sweat rate test is, it’s imperative that you follow each step and take each measurement as carefully as possible. Mistakes and shortcuts will affect your results and what you thought would help you could wind up hurting you in the end.
What you need
Sweat tests are simple and don’t require any particularly special equipment, but there are a few key items you’ll need that might already be in your home:
- a bathroom scale
- a kitchen scale
- dry towels
A bathroom scale will be used to take your pre and post ride weights, whereas the small kitchen scale is needed to weigh bottles. A digital scale would be best since you will actually care about the numbers after the decimal point. While you can certainly choose to not consume water at all, it doesn’t mean it’s safe. Because it’s likely that you’ll be sopping wet with sweat after your ride, you’ll also need dry towels to dry off before jumping on the scale.
Before getting started
Going into a sweat rate test well hydrated is extremely important. Start hydrating the day before with some extra fluids or by eating foods that are more hydrating. If you can’t get ahead the night before, then try to start increasing your fluids intake at least four hours before your ride. Empty your bowels and urinate prior and make sure to weigh yourself wearing only undergarments, or if possible, nothing at all.
If you aren’t properly hydrated prior to weighing yourself, it’s best to skip the ride. It’s never recommended that you ride when dehydrated.
How to do it
Before we begin, make sure to switch your scale units to kilograms and measure your fluids in liters, since metric units of weight and volume simplify this calculation. Follow the steps below and be sure to record necessary data, details, and notes pertaining to your session.
1. Fill the bottle(s) and weigh them on the kitchen scale. This will be your number X.
2. Weigh yourself before the ride. If possible, weigh yourself naked. Otherwise, do so with as little clothing as possible. The point is to be as accurate as possible, so added weight, such as clothes, can throw off your results. This will be your number A.
3. Ride for an hour at the same intensity level and in the same weather conditions you normally would.
4. After completing your ride, take your kit off and dry yourself off with a towel. Weigh yourself on a bathroom scale. This will be your number B.
5. Weigh your bottles again on the kitchen scale, even if you finished all the fluids. This will be your number Y.
6. Subtract your post-test weight (B) from your pre-test weight (A) to get the weight you lost during the session. This will be your number C.
7. Subtract the weight of the bottle(s) before (X) and after (Y) to obtain the amount you consumed (Z).
8. To calculate the sweat rate add lost weight (C) to fluids consumed (Z) and divide it by the time spent riding (1 hour).
SR=(C+Z)/time
How to use Sweat Rate data
Far too often, athletes use their data to support bad decisions, such as using sweat rate to adopt a predetermined, inflexible strategy for fluid and electrolyte replacement. Instead, use your sweat rate to give you a better understanding of how your body works and as a “ballpark figure” for your bodies’ needs. Consider that you will sweat more at harder efforts and less at easier efforts so using your one-hour sweat rate to design a hydration plan for a crit race or an Ironman isn’t the best of ideas. A full replacement of fluids also requires drinking beyond the natural thirst instincts which can be very dangerous and is generally considered unrealistic.
As long as you go into your training session or a race well hydrated, the body can handle quite a bit of dehydration so you should never aim for total fluid replacement. How much dehydration is acceptable is very subjective but in 2007, the American College of Sports Medicine has published a guideline that stated:
"The goal of drinking during exercise is to prevent excessive (>2% body weight loss from water deficit) dehydration and excessive changes in electrolyte balance to avert compromised performance."
A sweat rate test is a good opportunity to be more self-aware of how you ride and sweat, but it’s also a good time to pay attention to other things, such as how your sweat smells. Different sweat smells indicate different things. For example, caffeine specifically stimulates sweat glands in the scalp, groin, and underarms that results in a fatty, odiferous sweat. Diets that focus on extra low levels of carbohydrates cause the body to break down proteins and fats instead, which produces acetone and causes a strong ammonia-like odor.
Putting your sweat rate to use
Knowing your sweat rate helps you make all kinds of important decisions about your training, all of which will affect your overall health and performance. However, knowing your sweat rate proves to be invaluable when it comes to putting together your hydration and recovery plans.
Hydration Plan
Staying hydrated while riding isn’t necessarily the easiest thing to do, but it’s a lot easier if you develop a Hydration Plan for yourself. A Hydration Plan refers to how an athlete meets their hydration needs and should include pre, during, and post-ride hydration considerations, as well as be nutritionally balanced. Perhaps one of the most important things you can do to prepare yourself for a ride is to start hydrating in advance. Sipping 12 - 16 ounces of fluids four hours before riding, and doing so again two hours before your ride, is a simple way to ensure you’re well hydrated. If it’s going to be a long ride or if you’re dealing with hot, humid weather, then you might want to add an electrolyte supplement to your water.
Once you start pedaling, you’re unlikely to reach for your water bottle until you actually feel thirsty. At that point, you’ve already fallen behind on your hydration. Setting a reminder on your bike computer to alert you and remind you to drink at a preset interval is a good way to make sure you follow through with your plan during your ride.
Example: The sweat rate tests showed that you lose 1 liter of fluid per hour of exercise. Since total fluid replacement is not recommended, it’s best to aim for a conservative 80%. This means that you need to take in 1000 ml * .8 = 800 ml of fluid per hour, which translates to 800 ml / 29.574 (ml/oz) = 27.05 fl oz. A standard cycling bottle is 22 fl oz (650 ml) and a tall bottle is 26 fl oz (768 ml), so think of it this way: you need to take in a full tall bottle or 1 and a quarter of a standard bottle to properly hydrate.
Consider that an average sip size for men is 25 ml and for women is 20 ml, a man will need to take 800/25=32 sips during the hour and a woman 800/20=40 sips. If you set your bike computer to alert you every 10 minutes, you will be alerted 6 times per hour and every time you get an alert, you need to take 5 and 6 sips respectively.
| Variable | Description | Calculation | |
|---|---|---|---|
| A | Sweat rate (ml/hr) | 1000 | |
| B | Fluid replacement goal (%) | .8 | .8 = 80% |
| C | Fluid replacement (ml/hr) | 800 | A*B |
| D | Fluid replacement (oz/hr) | 27.05 | C/29.574 |
| E | Standard bottles per hour | 1.22 | D/22 |
| F | Large bottles per hour | 1.04 | D/26 |
| G | Drinking frequency (every X min) | 10 | |
| H | Drinks per hour | 6 | 60/G |
| Sips per drink (male) | 5.33 | C/25/H | |
| Sips per drink (female) | 6.66 | C/20/H |
It’s imperative that your hydration plan is nutritionally balanced; your body requires both water and electrolytes to function properly. Sodium is the primary mineral lost during the sweating process, so drinking the appropriate amount of fluids but failing to account for the electrolyte deficit can result in hyponatremia, commonly referred to as low sodium concentration or water intoxication. Hyponatremia is a direct result of replacing lost fluids with fluids that do not contain any electrolytes.
Knowing when to add electrolytes and how much is another important element to a well-balanced plan. Workouts that last more than an hour or that are in hot, humid weather are two instances where going for the sports drink or popping in an electrolyte tablet is the better option. Excessive sweating, especially if it’s salty, and just not feeling satisfied or quite “right” on water alone are also two indicators. In some cases, you might even find that you need to replace depleted potassium, too.
In general, it’s recommended that for workouts lasting under two hours, supplements containing 120-160 mg of sodium and 50-100 mg of potassium per 16 oz serving. Endurance athletes often require significantly higher levels: 400-500 mg of sodium and 400-500 mg of potassium per 16 oz serving. Other factors, such as high temperatures, humidity levels, and personal fitness and activity level. The amount of sodium you can take in is best determined experimentally: start with the recommended dose and monitor your heart rate during exercise. If your heart rate is higher during an otherwise normal training session, you should consider lowering your sodium intake. Beware that there are supplements on the market with exceptionally high sodium content, known to cause alarming heart rate spikes in some athletes.
Recovery Plan
A solid recovery plan is essential to getting back to “normal.” If you effectively used your sweat rate to stay well-hydrate during your race, then it’s likely that the recovery process will go quite a bit smoother for you than for someone who didn’t. Because you know how much you generally sweat on that kind of ride, you know what your body needs in order for it to feel and function normally. While fluids tend to be the main source of hydration and nutrition during intense rides, your body will likely need some solid foods during the recovery period.
Timeliness in replacing lost fluids and nutrients is of paramount importance for recovery. The idea is to start the process as soon as possible, within 20 to 40 minutes if possible. If your training ride ends at your car rather than your home, it’s wise to keep a recovery drink or two in a cooler inside the car so you can start the process immediately.
What to eat and drink
Some athletes prefer recovering with specially made products that are designed to provide well-balanced nutrition to aid in a speedy recovery. There are all kinds of recovery drinks and foods out there, so you’ll have to do some research and determine what will work best for you, but they’re supposed to do the same thing: replace protein and vital nutrients that are essential in rebuilding muscles after prolonged physical activity.
If recovery drinks and snacks aren’t for you, then you can always replace all of those lost calories, electrolytes, and fluids with a well-balanced meal instead. Just be sure that you’re including foods that are nutritious and beneficial to your body's healing instead of loading up with greasy foods and beverages loaded with sugar and don’t forget to hydrate with a balanced beverage.
How much to eat and drink
The amount of calories and fluids you need to replace will depend upon what your body depleted during your ride. Since it isn’t realistic to aim to replace all lost fluids during your ride, it’s good to aim for 80 percent. Generally, whatever you aren’t able to replace during your ride should be replaced during recovery.
If you weigh 180 lbs (81 kg) and you’re aiming to stay under the 2% body weight loss during your ride, you’re expected to lose 180 * 0.02 = 3.6 lbs (1.62 kg). Since 16 oz of water weighs 1 lbs, you need to replenish 3.6 lbs * 16 oz/lbs = 57.6 oz (1.62 liters). Drinking that much fluid in one sitting is unrealistic and dangerous so consider this to be the minimal amount of fluids you need to take in by the end of the day, one sip at a time.