Fluid Balance in Heat and Humidity

Ilana Katz, MS, RD, LD

With the change of season, we are soon going to be seeing hot and humid temperatures. A key to athletic success is avoidance of a state of under-hydration. There are however many complexities of efficient fluid balance due to environmental conditions. For example, the higher the temperature, the more athletes sweat. For sweat to have a cooling effect, it must evaporate off the skin. Therefore, the higher the humidity, the more athletes sweat but with reduced natural cooling efficiency. Furthermore, clothing can trap sweat against the skin and further reduce the body's natural cooling effect. As an athlete becomes progressively dehydrated, sweat rates reduce, and core body temperature rise, resulting in heat exhaustion.

Evidentially, fluid balance is complicated, not only by environmental conditions, but also by the conditioning of the athlete. Better conditioned athletes may have a more natural cooling system because they have developed efficient sweat systems. This allows better conditioned athletes to perform longer but it also means that they need to consume more fluids. The better the fluid balance, the more sweat potential there is. It is thus especially important in the heat and humidity to monitor fluid balance during training and events, no matter how well conditioned the athlete may be.

Granted, it is difficult to consume sufficient fluids during a hard physical workout, and therefore athletes should have a PLAN of how to remain in fluid balance. This is not as easy as it may seem, because many athletes rely on �thirst' as the alarm bell for when to drink. Thirst, however, is a delayed sensation that does not occur until the athlete has already lost 1 to 2 liters of fluid. Because of this, athletes should learn to consume fluids on a fixed time interval rather than relying on thirst for when to drink.

If an athlete loses one quart of water per hour , he/she should find a way to drink over four glasses of water per hour. Again, it may be difficult to know precisely how much water you are losing during activity, but doing the following simple exercise can help estimate how much fluid is lost, and how to recover.

One pint of water weighs approximately one pound. (One liter weighs 2.2 pounds). Knowing this simple fact, an athlete can estimate their fluid loss rates and can plan their water consumption during strenuous physical activity.

Planning your fluid balance (preferably do this exercise in typical conditions for your sport, e.g. if you are an endurance runner, this exercise should be done on a typical hot and humid day, during one of your training runs):

Note the starting time of your physical activity session.
Note your body weight before starting your event (preferably with no clothing)
Get through your event and monitor how much fluid you consumed within this time.
On completion of the event, note the time it took from beginning to end. (Subtract start time from end time).
Remove clothing and dry of any excess sweat.
Calculate your sweat loss: Subtract your finishing body weight, from your starting body weight.
You should plan on consuming extra fluid (over ands and above what you consumed during this event), equivalent to one pint of fluid for each pound lost.

- Volumes of fluid will range, for example 2 to 5 ounces in time intervals, every 10 or 20 minutes.

Use the following example to individualize the plan for you:

David weighs 150 lbs at the beginning of his 3 hour endurance training run. He drinks 1 pint (2 cups) of fluid during the event. At the end of the run, he weighs 148 lbs. He needs an additional 2 pints (one pint per each pound lost) during an event lasting 3 hours (for a total of 3 pints (6 cups or 48 ounces)). There are 9

20-minute increments in 3 hours, so he should plan on sipping on at least 5 (and a bit) ounces of fluid every 20 minutes to equal his 48 ounces (9 X 5 = 45 ounces).

NOTE: you should not go more than 20 minutes without some fluid replacement.

If David cannot tolerate drinking that much fluid, he should begin training his body to drink more by gradually increasing his fluid consumption over several weeks to achieve an equalized pre and post training weight.

The main point is that any fluid amount greater than the current amount consumed is necessary if weight loss is experienced during your physical activity events to stay in fluid balance and reduce the risk of dehydration and heat exhaustion.

Tip of the Month

Do not rely on thirst as an indicator of fluid needs. You should become accustomed to consuming fluids without feeling �thirsty�. Enough fluid should be consumed before exercising so that your urine is a clear color. Dark urine is a sign that low-volume concentrated urine is being produced which suggests the body is retaining fluid because of underhydration. Fluid consumption is more likely to occur if it is available, therefore make sure you do not have to go looking for water on a training day or at an event.

Some general guidelines:

Before Training and Competition	Drink adequate fluids the day before Drink at least 2 cups (17-20 ounces) of fluid 2 to 3 hours before exercise or competition

During Training and Competition	Replace Sweat Losses Drink 7-10 oz every 10 to 20 minutes

After Training and Competition	Monitor sweat losses. Drink 3 cups (24 oz) for every 1 lb weight loss through sweat. (You should replace 150% of sweat losses, because you are continuing to sweat.)
	Rehydrate within 2 hours post-exercise

Ilana Katz has a masters degree in dietetics with an emphasis in sports nutrition. Her work has ranged from elite to recreational athletes. She specializes in body composition and weight management specific to individual goals and needs