by Janel Willette and Steve Pyle

Staying hydrated while participating in endurance sports is pivotal to your success. When your body is continuously losing fluid through perspiration (cooling), and breathing (respiration/evaporation), keeping sufficiently hydrated is critical to maintain a steady state of performance. Sprint distance events of an hour or less in cool or rainy conditions are the only exception to the rule.

A physiologist performed a study quantifying the need for hydration in short events raced in moderate temperatures. He found that for events of one hour or less, with temperatures below 20 degrees Centigrade (68F), there was no performance drop-off if the athlete drank nothing during the event. This assumed the athlete was well hydrated prior to the event. He found that the energy used to absorb the fluid was more detrimental than beneficial at sprint distances. Not enough fluid could be lost in that time, at that temperature, to cause a performance drop off.

But, for longer events in warmer weather fluid consumption is absolutely necessary to avoid performance drop-off—or worse. Sweating keeps our body temperature within a safe working range. And all types of metabolic and cellular functions depend on maintaining a certain fluid level within our bodies. Fluid levels determine the viscosity of our blood as well. When we lose too much water our blood becomes thick and cannot be pumped as easily. Our hearts strain to do the necessary work.

As you do aerobic work, your muscles produce a tremendous amount of heat as they generate energy, and your body temperature surges upward. The heat produced by your muscles is then transferred to your blood as it moves through your muscles, and your blood carries the heat to your skin. Meanwhile, your sweat glands have shifted into high gear and soaked your skin with sweat. As sweat evaporates from the surface of your body, your body heat dissipates with it. As a result, you stay cool...to a point.

When you exercise in the heat, some of the water that normally courses through your bloodstream is shunted to the surface of your skin through your sweat glands. As a result, your blood volume begins to drop and your heart strains to pump your blood as its viscosity increases dramatically. The blood vessels leading into your skin constrict to prevent large drops in blood pressure as well. Once the blood flow to your skin diminishes, the blast of heat produced by your hard-working muscles can't be transported to the surface of your skin and your body temperature begins to climb. Simple as it sounds, drinking water will halt this chain reaction, keeping you cool and moving efficiently. The best way to ensure that this occurs is to drink adequately before, during, and after your workouts.

The amount of liquid we can absorb per unit of time can potentially limit our performance in the heat during long efforts. Pure water is absorbed fastest, but we also need calories to maintain a prolonged effort over long distances. Water with a dissolved sugar solution is absorbed into our system more slowly than pure water, so the stronger the solution, the slower it's absorbed.

That's the dilemma. How to get the most possible water into our system over time and still take in some calories as well. You should know that food such as energy bars mix with the water in our stomachs creating a very sweet solution, thus also slowing water absorption. Another consideration is that the more intense the effort, the less we're able to absorb any solution quickly. Clearly, a balance is needed.

Experimentation in training is the key to learning what hydration limitations you'll have to work with in races. Try different sports drinks at different levels of dilution to find what works best for you. Instead of finishing that run and guzzling down fluid, take in sufficient amounts during your workouts. Consistent hydration during training will help your body get used to the process of absorbing fluid during the intensity of competition.

The following tips will reduce the summer heat's potentially negative effect on your running:

• Put no limit on your water intake during the day. In most cases, don't worry about taking in fluids during runs lasting 45 minutes or less.

• During longer workouts, drink ~10-14 ounces of fluid immediately before you head out the door to run and then ~3 to 4 ounces every 10 minutes thereafter. If you get stomach cramps or feel pressure on the left side of your belly initially, slowly increase your hydration to adapt to increasing levels.

• Monitor how you feel. If you have trouble maintaining your regular pace, slow down a bit. And if you become light-headed, slow down and walk for a while. If you experience chills, stop immediately, and find a shady place to sit down. You must allow your body to cool. Even though you're having chills, you're not cold. Your cooling system is about to shut-down and we don't want this to happen.

• Know yourself. Some people have more difficulty dealing with the heat than others do. If you're one of them, it doesn't mean you're a wimp or faint of heart; it simply means that you must train smart. Take it easy during the hot weather, especially if you've had trouble dealing with heat in the past. Once you become heat stressed, you are psychologically susceptible to it for the rest of your life.

• Run at sunrise or sunset. By running in the morning you avoid hot sunlight but usually face more humidity. By running in the afternoon you avoid high humidity but must brave the sun. Experiment to find out what works best for you. Try to avoid training from 11:30 a.m. to 4:00 p.m. when you must battle maximum humidity and the heat.

• Go out of your way to stay as cool as possible. Wear light loose clothing and opt for tree-lined streets or shaded trails.

It's nothing to brag about when you finish a race or workout totally depleted and dehydrated. When your blood gets thick due to dehydration it's impossible for your heart to maintain the same level of work as it would under optimal conditions. If the blood won't flow - you can't go.