Hydration in Hot Weather: How to Adapt
In hot conditions, especially beyond a 1.5-hour effort, your body’s core temperature can increase dramatically. Your internal cooling system responds appropriately, producing copious sweat; however, unlike your car’s radiator, which recycles its coolant, your sweat evaporates, drips away, and is gone.
Unreplenished fluid loss causes endurance athletes several problems:
Your heart must work harder in order to pump a decreased, but thickened, blood volume.
Fluid depletion inside and outside muscle cells may slow down the metabolic reactions necessary for efficient muscle fuel transport.
Inadequate fluids result in higher cell temperatures, altering metabolic rates for less-than-optimal endurance performance.
At the very least, excess body fluid loss means premature fatigue and decreased performance. If the loss goes unchecked during extended exercise, the potential for dehydration and its serious consequences increases. Once you get into the dehydration range, you’re cooked—literally and figuratively—collecting a DNF and possibly an IV, too. Your basic strategy for staying cool in the summer months should begin with appropriate hydration during exercise.
Be aware of water weight loss during exercise
A good scale (preferably one that measures in less than one-pound increments, such as a balance scale) may well prove to be your most valuable fitness investment. Weigh yourself before and after each outing, carefully noting the time, exertion level, miles, and weather, as well as fluid, fuel, and electrolyte consumption. If you finish weighing the same or more than when you started, you have overhydrated. If you’ve dropped 3% or more, dehydration has occurred. Up to 2% weight loss is safe and reasonable.
For very long events, such as a century bike ride, the average rider will also lose a pound or more in energy stores (glycogen, fat, and muscle tissue) in addition to water, so calculate that into your weight difference.
Once you begin to log your fluid consumption and weight fluctuations, you’ll have the data to accurately calculate your personal needs. Another low-tech hint that will make it easier to track your training data much: make sure you know the capacity of your water bottles and hydration packs.
Rehydrate in the correct amounts
Just like calories and electrolytes, you can’t replenish fluids at the same rate that you deplete them; your body simply won’t absorb as fast as it loses. And since fluids and electrolytes (especially the latter) perform many roles to maintain exercise performance, that will also cause depletion.
Your body will accept and utilize a certain amount from outside)sources, so maintaining fluid intake within a specific range will postpone fatigue and promote peak performance.
Research suggests that while electrolyte needs for individual athletes may vary dramatically, fluid loss remains fairly constant. Based on this research, along with the thousands of athletes we have monitored, we recommend the following:
For most athletes under most conditions: 20–25 oz/hr (approx 590–740 ml/hr)
For lighter weight athletes, or those exercising in cooler temperatures: 16–18 oz/hr (approx 473–532 ml/hr)
For heavier athletes or athletes competing in hotter conditions: up to 28 oz/hr (approx 830 ml/hr.)
To avoid dilutional hyponatremia (over-hydrating), fluid intake should not routinely exceed 28 oz/hr (830 ml/hr). The exceptions are: heavier athletes; athletes exercising at extreme levels (prolonged periods at a high percentage of VO2 max); and athletes competing in severe environmental conditions. (Note that increased fluid intake necessitates an increase in electrolyte replacement as well.)
A typical water bottle contains 20–25 oz of fluid (approx 590–740 ml), so consuming one bottle per hour is an excellent approach.
Adapting to the heat
While there are limits to how much heat the body can withstand during exercise, there are ways to improve your adaptation to heat stress.
Train to get fit in the heat by distance training at an aerobic pace for 14–21 consecutive days. Heat acclimatization and fitness reduce fluid and electrolyte losses by up to 50%.
Train your body to appropriately refuel, rehydrate, and process electrolytes.
Train at a reduced pace to compensate for overheating (lower gears, easy cadence, slower pace).
Slightly increase electrolyte intake; the more fluid lost, the more electrolyte stores are depleted.
Increase fluid volume cautiously; resist drinking above 30 oz per hour.
Keep head, trunk, and quads wet to increase evaporative heat loss.
Far too many athletes try to get a jump on race day needs by over-doing it with fluid, calorie, and salt intake. We are not camels and cannot store water. In the days leading up to an event, the only thing that excess fluid intake will do is overly dilute and flush out your body’s electrolytic mineral content. Neither of these will help your race day performance; actually, they greatly increase the potential for ruining it.
A better approach is to maintain constant daily hydration (half of your body weight in fluid ounces) up to and through race day, regardless of temperatures.
A three-day load with Liquid Endurance will safely and effectively maximize your intercellular water levels, so they can be called upon as needed during your training session or race.
Beat the race-day heat
In addition to proper hydration, adaptation, and appropriate electrolyte consumption, the following can help effectively relieve heat stress if the temperatures soar on race day.
If you’re running, take a one-minute walk, douse yourself with water, and take a good drink. If you’re cycling, coast or easy spin for a minute. The break from heavy exertion allows some internal heat to dissipate. Highly competitive athletes might scoff at walking or coasting, but when it comes to core temperature, nature gives you two choices: cool down or DNF.
Wear the lightest, most evaporation-friendly clothing you can afford. (Cotton isn’t on the list!) Many fibers today provide superior wicking and evaporation that allow your sweat to do the work nature intended. Light-colored clothing is preferable.
Use cold fluids as much as possible as your body absorbs them more rapidly than warm fluids. Use frozen and insulated water bottles and hydration packs.
Know the symptoms of both over-hydration and dehydration. Stop immediately if you feel lightheaded or queasy, or get dry chills. No race or training is worth compromising your health.