Do You Know Your Sweat Rate?

Now that we are smack dab in the middle of summer, there is no escaping the heat. For those of you who are exercising outdoors, it is extremely important that you stay well hydrated. Why does this matter? Because becoming dehydrated can have significant negative impacts on your performance. It can also lead to painful cramps, fainting, and heat exhaustion.

A general rule of thumb is to make sure that your urine is very pale yellow (almost clear) through out the day. However, note that if you are taking a high dose of B vitamins, the first few times you urinate after taking these vitamins your urine will likely be a very bright yellow. This does not mean you are dehydrated, it merely indicates that you are excreting “excess vitamins” that your body cannot use.

Many athletes will become dehydrated in competition because they often forget to drink enough fluids during their event. The longer the event, the higher the risk of becoming dehydrated. This can happen in training as well, particularly if the athlete has to carry their own fluids.

How do you avoid becoming dehydrated while training, as well as in competition? First of all, make sure you are well hydrated before you start exercising. This means just before you begin training or competition, your urine should be very pale yellow.

The next step is to hydrate according to your sweat rate. How do you know what your sweat rate is? There is a fairly easy way to calculate this. Pick a day when you can train for at least an hour in conditions similar to what you would normally be training in. Weigh yourself in the nude and record your weight. Proceed to run/cycle/swim, etc. for an hour or longer. Keep track of how much fluid you drank while you were working out. Try to avoid urinating immediately after your workout. After completing your workout, dry yourself off as much as possible then re-weigh yourself, again in the nude. Now subtract your post-workout weight from your pre-workout weight and add the weight of the fluids you drank. Divide this number by the amount of time you trained and you have your sweat rate. Remember that 16 ounces equals one pound.

Here is an example: Jack weighs 165 pounds prior to his workout. He then goes and runs for 1.5 hours, during which time he drank 16 ounces of water (or 1 pound). He gets home and weighs himself again. He is down to 163 pounds. His sweat rate is (165 – 163 ) + 1 divided by 1.5 hours or 2 pounds per hour.

This means Jack needs to drink 32 ounces of fluid per hour when he is working out. If he does not, he is going to have to make sure he replaces this much fluid as soon as possible afterward.

If you regularly compete in completely different conditions than you train in, it is helpful to try to mimic those conditions at least once for the purposes of calculating your sweat rate to guide your hydration needs for competition.