Running - Knowledge

Running Basics: Calculating Sweat Rate to Create a Hydration Plan

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In order to keep up a proper hydration routine and maintain the perfect balance of nutrition and fluid intake, you will have to know the basics of sweat and sweat rates. It is vital to understand the way in which the body takes in water and sweats it out to plan out your own diet plan as most of your biological processes require water. Staying hydrated is key. Most of your body, around 60%, is composed of water and you cannot possibly go even more than a few days without it. When running or during any kind of physical activity, water is what allows you to reach your full potential.

If an inadequate amount of water is taken in then the runner can be exposed to fatigue and exertional heat illness. This overall has a negative impact on the performance of the runner due to dehydration. The easiest loss of water from the body is through sweat and when the fluid lost is not replenished in a balanced amount then dehydration can occur. In order to avoid this decline in performance, you can measure your sweat rate. You can then determine the appropriate amount of water to drink per run or workout session. This is the quantity of fluid you lose during a run and by a simple formula, you can be calculating sweat rate in no time.

What is Sweat Rate

The rate at which fluid is lost through sweat in a set period of time is what is called a sweat rate. Since the smallest amount, starting at just 2%, of dehydration can inhibit your performance you might want to be hydrated when you start your run. Again this can differ from person to person, as some need more fluids to sustain themselves from the perspiration they experience during a run or workout than others. This depends on the rate at which you sweat. By monitoring and calculating sweat rate you can estimate the amount of water you need during exercise to prevent getting dehydrated. Or how much more water you require post training to replenish yourself.

sweat rate
Know your sweat rate (source)

Calculating Sweat Rate: Steps to Follow

In order to be accurately measuring and calculating sweat rate, follow these steps:

  • Start out with a warm-up which will be energizing enough to make you sweat a small amount
  • Urinate if you need to be this before you make a note of your weight
  • Use an accurate weighing scale to measure your weight
  • Begin your workout or training session and try to schedule it for around an hour 
  • If you cannot do this for an hour then make it 30 minutes and multiple the sweat rate by two to give you an hours calculation of the sweat rate
  • Keep yourself hydrated and drink an appropriate amount of any beverage during this workout
  • Make sure to not urinate during or right after the workout
  • Now weigh yourself again, in the same manner, you did before the workout wearing the same clothes
  • Record this data along with the initial weight data

Sweat Rate Calculator

To get the best results use kilograms and milliliters when calculating sweat rate. The conversions are given below if required.

A. Body Weight pre‐exercise [lb/2.2 = kg]

B. Body Weight post‐exercise [lb/2.2 = kg]

C. Change in Body Weight Grams [kg x 1000 = g] (A - B)

D. Volume of fluid consumed mL [oz x 30 = mL]

E. Sweat Loss  mL [oz x 30 = mL] (C + D)

F. Exercise time [min or hr]

G. Sweat Rate     [mL/min or mL/hr] (E / F)

If you want to convert Sweat Rate (G) back into ounces: G/30 = oz

The final value should be G, which is your sweat rate. It represents the amount of fluid which is lost when you sweat for over a specific amount of time, usually an hour.

Sweat Loss = (Pre Exercise- Post Exercise)+ Fluid Consumed during Exercise

Sweat Rate = Sweat Loss/ Exercise Time

Now that you understand calculating sweat rate, you can use this hydration calculator information to figure out how much fluids you should intake while training and right after to avoid dehydration or fatigue.

sweat rate calculator
Sweat Rate Calculator (source)

Key Points to Remember

It is important to take in the factor of temperature and climate when calculating sweat rate. When exposed to more heat for about ten to fourteen days, the sweat rate can increase. This means you should use your sweat rate calculator again after you have acclimatized to the temperature change. You can see very high sweat rates in men and women that are extremely fit.

According to the climate change and seasons, you should be calculating sweat rate whenever you feel an increase or decrease in heat. There is a higher amount of sodium being lost when you initially begin your training session in the heat or warmer climates. Due to this, you need to just increase the sodium intake in your diet by a very small amount until you get used to the change in temperature. This is usually after ten to fourteen days.

What is Hyponatremia

When you suffer from low sodium in the blood as a consequence of overhydration by drinking too much water, this is called Hyponatremia. When too many fluids are consumed then the salt level in the body gets diluted. The symptoms you will experience include nausea, vomiting, headache, fatigue, poor appetite, muscle spasms, and even seizures. If you develop acute hyponatremia then this can be even more dangerous leading to brain disease, cardiac arrest, cerebral edema, seizures, coma, and death. This is why hyponatremia is very important to prevent and runners should be wary to maintain a balance between fluid and food intake.

One of the easiest ways to get this condition is by drinking too many fluids on a run and not including sodium in your diet. Exercise-associated hyponatremia can occur as a result of any form of physical activity. You need to be careful of both not drinking enough and drinking too much.

How Much Water Should I Drink

Hyponatremia is an extremely serious condition, however, most of the cases are rare. The less severe symptoms are more prevalent for hyponatremia but this does not mean you take your diet and fluid regime lightly. 

So you must be thinking, how much water to drink while running?

Well, we have compiled a guide for the amount of water that is appropriate for your body weight and temperature of the region you live in.

According to research by dieticians, the adequate fluid intake for an average athlete is around 500 ml per hour or 17 oz/hr and for most climates. A limit of 1 liter per hour will most likely lead to water intoxication and symptoms of hyponatremia. You are not supposed to drink more than 500 ml to 740 ml per hour unless you predict a substantial loss of fluid within that time span. Here is a list of fluid intake for body weight and temperature per hour:

Average athlete, average temperature

For the typical athlete, approximately 500 ml to 740 ml per hour (20 to 25 oz/hr) is an adequate amount of fluid intake in most types of climates.    

Lighter athletes or cooler temperature

For lighter weight athletes, approximately 473 ml to 532 ml per hour (16 to 18 oz/hr) is the right amount of fluids. This works well for athletes in cooler climates too.

Heavier athletes or hotter temperature

For athletes of heavier weight, they should consider approximately up to 830 ml per hour (28 oz/hr). This is appropriate for athletes that live in hotter climates.

how much water to drink
Water intake is essential for good running

Summing Up

Now that you have the basic information on sweat, fluid intake, dehydration, hyponatremia, and calculating sweat rate, you can finally make a fluid plan for yourself. Make sure you keep calculating sweat rate according to the change in temperature and measure the appropriate amount of fluids to replenish yourself. With these in mind, you should have some great training sessions ahead of you and unleash your full running capacity in your next marathon. Don’t let dehydration, or hypohydration, hold you back from the finish line!

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