With a record number of days reaching at least 100 degrees in Houston this year, this week’s Medical Monday focuses on staying hydrated at a level that will help you get the most out of your body while remaining safe.
Written by Dr. John Cianca, MD
Medical Director, Houston Marathon Committee
Your body generates heat when its muscles are working. The harder they work, the more heat you generate. Higher temperatures outside just add to the heat you are already generating. Your body vents heat by sweating – as sweat evaporates, it cools your body. If it’s hot and humid outside, however, evaporation is not as effective. Since we live in a climate that is both hot and humid, overheating is a real concern. Staying hydrated is a key step in combating the weather conditions in Houston.
As you exercise, you lose fluid and sodium (salt) from sweat, so replacing these substances important to function normally. Determining how much fluid to replace is an individual process because people sweat differently with respect to volume and content. Being under-hydrated or over-hydrated is not healthy and will impair your performance.
The best way to estimate your needs is to use your body weight as a guide. Weight change during activity is almost entirely due to sweat loss. If you weigh yourself before and immediately after (making sure to remove sweat-soaked clothes) you can have a fairly accurate estimate of the amount of fluid that needs to be replaced. For every pound of weight lost, replace with 16 ounces of fluid, preferably fluid that has some sodium content. (While drinking H20 replenishes lost water, it does not address the sodium that was lost, which is why sports beverages were created). If you do not lose weight, don’t replace fluid other than to quench thirst. If you have gained weight, do not consume fluid until you are urinating regularly – weight gain during exercise implies fluid overload, and this must be reduced before consuming more liquids.
You can estimate your sweat rate by using the weight lost during exercise in similar conditions, divided by the length of time exercising. For example, if you lose 3 pounds (48 ounces) in 2 hours of exercise in 75 degree, 85% humidity weather, you would divide 3 pounds by 2 hours to find your sweat rate: 1.5 pounds (24 ounces) per 1 hour for those temperature conditions. Replace fluid at roughly this rate to restore your normal hydration level for similar conditions.
The color of urine also can be an indicator of your hydration status. Normal urine is pale yellow, while urine that is dark means that you are under hydrated. If it is clear, you are over hydrated.
As you become more fit, your sweat rate will change, so you should continue to check your weight pre and post workouts to best understand how your body is reacting. If you are exercising for less than an hour, you can safely and effectively rehydrate after your run. When exercising for longer than an hour, consider replacing fluids during exercise.
Even while keeping track of your hydration status and fluid needs, be careful and conservative on hot days. Adequate fluid levels enable the sweating mechanisms which help you stay cool, but the best way to immediately cool yourself is to decrease activity intensity and move to a cooler area. Ultimately, training is most effective when your body is challenged, but not overwhelmed.
Using these tips, you can stay hydrated during your training runs in the many hot days sure to come! Check out more tips in this article from Running Times about training and running during the summer months.
Dear Dr John,
You said, “Your body vents heat by sweating – as sweat evaporates, it cools your body”. But you also lose heat by breathing, which is another tool your body use to regulate your metabolism. This is actually more important in high-humidity and high-temperature environment like Houston.