In the past few weeks I’ve highlighted the over-emphasis that pop culture has placed on water intake on any ol’ day. However, when in a situation that causes you to sweat profusely, increasing your fluid intake is extremely important. Here’s what firefighters (who may have to fight a fire at any given time) and very active people need to do to stay hydrated:

1. Before: Get pre-hydrated

When you’re exercising or working very hard, especially if you’re in a hot environment, your sweat rate is likely higher than the rate at which you can take fluid in. Therefore, it’s important to have a lot of water in your system before the event. For firefighters that means using the methods I discussed last week to really make sure you’re properly hydrated at all times. For people engaging in heavy exercise, it is recommended that you drink 14 to 20 oz (400 to 600 mL) 2 hours before exercise and 5 to 12 oz  (150 to 350 mL) every 20 minutes or so during exercise.

2. During: Take in sodium

Although far less common than dehydration and heat illness, hyponatremia is a potentially fatal condition. It is caused by diluting all the sodium out of your cells – usually produced by a combination of extreme sweat loss and rehydrating by drinking only water, or by water drinking competitions. Female marathon runners exhibit the highest incidence of hyponatremia. Headache and nausea are the initial symptoms which develop into confusion/hallucinations, seizures and brain swelling.  For this reason, it’s important that you take in fluid or food containing sodium during and after any event that causes you to sweat profusely. (But remember, drink a sports drink only when you’ve sweated a good amount. Otherwise you’re just taking in unnecessary calories.)

3. After: Calculate your sweat losses

To decipher how much fluid you need to rehydrate, first calculate your sweat loss (formula shown below). You will need to weigh yourself before and after the exercise. If you have sweated heavily you will likely weigh less after the exercise than before. Since a handful of hours is not enough time to lose weight in fat, 100% of this weight loss is simply body water.

Sweat loss= (preexercise body weight – postexercise body weight) + fluids consumed during the exercise

Let’s say you usually weigh 180 lbs.* While fighting a fire you consume a bottle of water (typically ~ 16 oz). Back at the station you weigh yourself and you’re now 178 lbs. *See example below for metric system

If you convert lbs to ounces (1 lb = 16 ounces), your sweat loss equates to: 32 oz (2 lbs of weight loss) + 16 oz consumed during = a total of 48 oz.

To replenish, drink about 1.5 times what you lost in sweat (to account for urine production). If you didn’t drink any fluid during exercise, simply drink 24 oz for every pound of weight loss (or 1500 ml for every Kg of weight loss).

Here’s an example using the metric system: preexercise body weight: 82 Kg; postexercise body weight: 81.5 Kg; fluids consumed during the exercise: 400 ml

convert Kg to ml (1 Kg = 1000 ml) and sweat loss = 500 ml (.5 kg of weight loss) + 400 ml consumed during = a total of 900 ml.

Hydration needs vary significantly based on work intensity and environment

Note that there are specific fluid intake recommendations for wildland firefighters since the work and heat environment are unlike structural firefighting or athletic events.

Also, hydration tends to be a guess and check system for people who participate in endurance training and competitions. The right combination of water to sodium to carbohydrate (glucose) is highly specific to an individual and is also beyond the scope of this article.

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