All it takes is one glance at your wilted front lawn on a South Texas summer afternoon to get a real world understanding of the scientific principle of hydration: living things need water to survive. LOTS of water.

The not-so-simple question is, how much is enough? You've heard to drink eight 8-oz glasses of water per day as long as you've heard to look both ways before you cross the street. But, as with most accepted tenets of our popular culture, the pendulum eventually swings back and the tried-and-true advice of our grandmothers is replaced with new-and-more-fashionable thinking. Now we're told to forget the formulas and just drink when we're thirsty. Ah, the progress of science. What's an athlete to do?

The old

Conventional wisdom has been that the average adult needs to consume eight cups of water per day to make up for fluid lost though normal body function. Where the 64 oz standard came from, no one is quite sure, but it should be enough fluid to prevent dehydration in most people, and it’s not too much to cause any harm if it’s not needed.

The exceptions to this rule are active people, particularly those in hot climates. For these folks, more water is lost due to perspiration, and therefore more needs to be replaced to keep the body well-hydrated. A common guideline for athletes is to consume 4 oz of fluids every 15-20 minutes during exercise. This approach is based on the premise that thirst is a poor indicator of the body’s need for fluids, since an adult has already lost 1-2% of his water weight by the time thirst is triggered, and the body’s ability to regulate temperature has already been compromised. To stay on top of hydration, athletes have been encouraged to drink at regular intervals, regardless of satiation.

The new

In recent years there has been a backlash against the standard hydration regimen, partly driven by press-worthy incidences of hyponatremia – the dilution of blood-sodium levels often caused by over-hydrating with plain water. In one well-known case, a 28-year old participant in the 2002 Boston Marathon actually died from this condition. As the sport has grown to attract more non-elite level athletes, the problem has grown (due to the longer time periods and the greater duration of fluid intake) and race directors have taken measures to prevent its occurrence, such as providing more electrolyte drinks in addition to water on the course. The International Marathon Medical Directors Association was also sufficiently concerned that it revised its hydration guidelines, recommending thirst-based fluid replacement instead of a calculated number of ounces per minute.

In 2003, the debate intensified and prominent publications such as the British Medical Journal openly questioned the standard “stay ahead of your thirst” approach, arguing that thirst was indeed an adequate indicator for hydration. The American College of Sports Medicine (ACSM) countered, standing firmly behind its 1996 position statement on hydration. The debate lives on.

The here and now

With all of this back and forth, active adults are left in the middle, not quite sure whether to pack a bladder full of fluids or to just stop at water fountains along their route. There is a solution. There are facts that both sides agree on and sensible strategies can form the basis of a practical hydration approach for everyone.

Since the primary factor driving dehydration during exercise is the loss of body fluids through sweat, it stands to reason that the amount of fluid intake should have some correlation to the amount lost. This is exactly the approach endorsed by the USA Track & Field Association (USATF) and others. By determining your individual sweat rate, you can calculate how much fluid should be replaced during a given exercise bout. And as the ACSM cautions, you should be careful to not drink beyond that amount in order to avoid over-hydration. Sweat rate should be used along with your own thirst to regulate an appropriate amount of fluid intake.

To determine your sweat rate, weigh yourself immediately before your workout. Exercise for one hour at moderate intensity, keeping track of how much fluid you consume during that time. Do not urinate during the hour unless you choose to measure the amount of urine. At the end of the hour, dry off any sweat, and weigh yourself again. Subtract your post-workout weight from your pre-workout weight, convert to ounces (1 pound = 16 ounces) and then add the number of ounces you drank in between. The total is the number of ounces you lost due to sweat and gives you an estimate for how much fluid you should be drinking during an average hour-long workout. It’s important to repeat this test on a regular basis since your sweat rate changes with the environment and the type and intensity of your workout. It’s also important to note that if you actually gain weight during the workout, you are drinking too much, and you should re-assess your hydration plan.

Once you know approximately how much fluid you’ll need to replace, it’s important that you ensure the availability of adequate amounts throughout your workout. Probably the most common reason for athletes not drinking enough is that they just don’t have anything to drink with them. Whether you pack water on your back, stash bottles along your route, or plan to take water stops at regular intervals, it is crucial that you have access to fluid when you need it. Plain water is sufficient for exercise bouts up to an hour in duration, but for longer efforts you should use a sports drink for some of your fluid intake. Sports drinks include both carbohydrate and electrolytes to help maintain appropriate blood sugar and sodium levels throughout the workout.

So before you hit the road or head for the gym next time, make sure you have a plan for your hydration needs. You’ll have a better workout and will be able to keep on doing what you love. Grandma will be proud of you, too.

Ashley Rosilier is co-owner of Run Gear Run in San Antonio, Texas, and has a Masters in Engineering from the University of Texas at Austin. She is a certified coach with USA Triathlon and Road Runners Clubs of America and coaches with national programs from USA Fit and Heart Zones Team Danskin Training. She is the proud mother of four children and enjoys running, swimming, and cycling. For more information on training programs visit www.RGRTraining.com and check out www.RunSanAntonio.com for local event listings.