Know Your 'Sweat Rate' to Stay Properly Hydrated

The thousands of runners taking part in this Sunday’s Fitbit Miami Marathon and Half Marathon are likely already familiar with their “sweat rate,” a relatively simple formula to help long-distance runners and other endurance athletes stay properly hydrated.

Even in January, South Florida temperatures in the morning can warm up quickly, requiring runners to hydrate with water or sports drinks. Fluid needs are highly dependent on an individual runner’s physical conditioning, as is the need for digesting anything stronger, such as those widely-available “energy” or “power” gels that carry about 100 calories of carbs, with sodium, potassium and sometimes caffeine.

Occasionally, first-time marathoners tend to overdo it, drinking too much and creating stomach upset. Research has found that most runners’ stomachs can empty only about 6 to 7 ounces, or 180 to 210 milliliters (ml), of fluid every 15 minutes. That’s about 24 to 28 ounces (720 to 840 ml) per hour.

The carbs in sports drinks can help restock the body’s spent energy. Most of these drinks will also replace the electrolytes lost to sweat, helping prevent hyponatremia (low blood-sodium level caused by excessive water intake). Sports drinks could also diminish the need for gels, which are more likely to cause stomach upset.

“Over-hydration is health a risk, too,” said Thomas San Giovanni, M.D., orthopedic surgeon at Miami Orthopedics & Sports Medicine Institute. “So how do you know how much you need to consume? Know your sweat rate.”

Your sweat rate will help you determine how much fluid you lose during exercise, and how many ounces you should drink, he explains.

Here’s how to find your sweat rate:

  1. Before you exercise, weigh yourself without any clothes on.
  2. Perform your exercise activity (running, biking, swimming, etc.), making note of exactly how many fluids you consumed during activity, and exactly how long you exercised.
  3. Immediately after you exercise, weigh yourself without any clothes on.
  4. Calculate the difference between your pre-exercise weight and your post exercise weight in ounces (Note: 1 lb = 16 oz). Add to this number the ounces of fluids you drank.
  5. Finally, divide by the duration of activity (in hours).

For example:
» Joe, a long-distance runner, wants to find out his sweat rate when running.
» Weight before exercise = 150
» Weight after exercise = 149
» Fluids consumed during run = 20 oz
» Exercise duration = 2 hrs

The numbers: 
» 150 lb – 149 lb = 1 lb (1 lb = 16 oz)
» 16 oz + 20 oz (fluid consumed) = 36 oz
» 36 oz ÷ 2 hrs = 18 oz
» Sweat rate = 18 oz per hour.

In this example, Joe’s sweat rate is 18 ounces per hour. Obviously, other factors come into play during endurance events, such as marathons and half-marathons, including the weather, a runner’s intensity level and overall physical conditioning. At the very least, sweat rates offer a good starting point for those in training for long-distance events or for those starting a regular exercise program.

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