Last year, I took a sabbatical from my entrepreneurial life in California and became a college professor and assistant cross country and track coach at Georgia Southern University. It would be an understatement to say that running in southern Georgia in the summer is a challenge. The summers are downright sticky, as stepping outside your air conditioned house feels like walking into a steam room. Most people running in the Georgia summer wish they could handle the heat as well as Apollo, the Greek god of the Sun, who was known for bringing heat and light to the Earth. But even Apollo would have a hard time running in the Georgia heat, as it presents a number of thermal and cardiovascular challenges. Many places in the country also experience harsh summer conditions, so knowing how to handle it will protect your athletes.
Physiology of Environmental Heat and Dehydration
You run outside on a sunny, hot, humid day. The crimson, mid-August sun hangs overhead against the azure sky like the blade of a guillotine. A couple of miles into your run, your body temperature, already on the rise from muscle contraction, increases even more. Since your primary mechanism of cooling your body is through the evaporation of sweat from the skin’s surface, your sweat rate increases. As a result, you lose body water and begin to become dehydrated. Despite the occasional compliment you get in the gym about your well-defined muscles, water, not muscle, is the major component of your body. So, when you lose water, there are consequences. A major consequence of dehydration is an increase in core body temperature during exercise, with body temperature rising 0.15 to 0.2 degrees Celsius for every one percent of body weight lost due to sweating.
Water is vital for many chemical reactions that occur inside your cells, including the production of energy for muscle contraction. Therefore, dehydration influences your athletes’ workouts. Indeed, exercise performance declines with only a 2 to 3 percent loss of body weight due to fluid loss. Since the effects of heat and dehydration on physiological function summate to have a greater effect than either one alone, being dehydrated when running in the heat causes performance to decline even more, and can even be a recipe for disaster, with the risk of heat-related illnesses rising dramatically. The problem, as you discover about three miles into your run, is that running in the heat makes it very difficult to prevent dehydration, since your sweat rate exceeds your ability to ingest and absorb fluid while running. While mild to moderate exercise typically results in sweat losses of 0.8 to 1.4 liters per hour, high environmental temperature combined with intense exercise can increase sweat rate to 1.4 to 2 liters per hour. However, your gastrointestinal system can absorb only about 0.8 to 1.2 liters of fluid per hour. Thus, heat stress and dehydration often occur together.
Humidity presents an even greater challenge. When it’s humid, the air is already saturated with water, limiting the amount of sweat evaporating from your skin. As a result, the ability to dissipate heat is minimized and core body temperature rises rapidly, leading to hyperthermia. In extreme cases, hyperthermia can lead to heat exhaustion and heat stroke. Heat exhaustion, the most common heat illness, is the inability to continue exercise in the heat. Heat stroke, which is a medical emergency, occurs when body temperature rises to a level that causes damage to the body’s tissues (>103-104 degrees F). In an attempt to prevent body temperature from rising to dangerous levels while running, your central nervous system orchestrates a complex response in which blood vessels supplying your inner organs constrict, while blood vessels supplying your skin dilate, causing blood to be diverted away from inner organs and directed outward to the skin to increase cooling through the convection of air over the skin’s surface. It may seem somewhat counterintuitive that as your core body temperature rises while you run in the heat, skin temperature decreases as a result of convective cooling. More blood being directed to the skin means less blood (and therefore less oxygen) going to the active muscles, causing running pace to decrease and the perception of effort to increase. When your body has a choice between maintaining the exercise intensity and cooling itself so you don’t overheat and die, it’s going to choose the latter. So, on this hot, humid day, your running pace slows and you feel fatigued. You notice a sprinkler on a neighbor’s lawn and run past it, hoping to cool yourself, but you quickly realize that spraying water on your body, while refreshing, is not effective for decreasing body temperature. To decrease body temperature, you need to ingest the fluid. Since you don’t want your neighbors to see you trying to drink from their sprinklers, you forego drinking any fluid and continue running.
As if trying to prevent you from overheating on your run weren’t enough, accompanying the increase in thermal strain when running in the heat is a greater cardiovascular strain. Profuse sweating to increase evaporative cooling causes a loss of plasma volume from the blood, and total blood volume decreases. When blood volume decreases, stroke volume (the volume of blood pumped by the heart with each beat) decreases. A decreased stroke volume means oxygen flow to your muscles is then compromised, and the running pace decreases. To compensate for the decreased stroke volume, your heart must work harder to pump blood, and heart rate drifts upward in an attempt to maintain cardiac output (the volume of blood pumped by the heart each minute) and blood pressure. This rise in heart rate during prolonged exercise without an increase in intensity is called cardiac drift. Heart rate rises 3 to 5 beats per minute for every one percent of body weight loss from dehydration.
Due to both the thermal and cardiovascular strain of running in the heat, the ability to run declines linearly with an increase in environmental temperature. While most research has examined the effect of dehydration on prolonged cardiovascular exercise, resistance exercise performance has also been shown to decrease when dehydrated, however, it seems to take a greater amount of dehydration (at least a 5 percent loss of body weight) to see strength decrements.
After you complete your run fully exhausted, dehydrated, and a little lightheaded, your T-shirt drenched with sweat, you walk into your air conditioned house and ask yourself, “How can I prevent this from happening?”
Recommendations for Exercising in the Heat
The two most important things you can do to prepare for summer outdoor training sessions are hydrate and acclimatize.
Hydrate
Because of the decrease in running performance and the potential health danger of dehydration, there has been plenty of research (and an onslaught of sports drinks) on strategies to overcome, or at least blunt, the effects of dehydration. Beginning the workout fully hydrated or even “hyper-hydrating” before a workout can delay dehydration when running, maintain running performance, and decrease the risk for heat-related illnesses. Pre-exercise fluid intake enhances the ability to control body temperature and increases plasma volume to maintain cardiac output. You should drink fluids before you exercise in the heat so you begin every workout fully hydrated, and you should continue to drink during workouts longer than one hour. For specific recommendations on how much and which ingredients to drink, see “What Should Runners Drink?” below.
A good indicator of your hydration level is the color of your urine. The lighter the urine color, the better the level of hydration. Your urine should look like lemonade rather than apple juice.
Acclimatize
Chronically exposing oneself to a hot and humid environment simulates adaptations that lesson the stress. Cardiovascular adaptations to exercising in the heat (e.g., decreased heart rate, increased plasma volume) are nearly complete within 3 to 6 days, while core body temperature and electrolyte concentration changes take 9 to 10 days. Full acclimatization is complete after two weeks, as the increased sweating response catches up to the other adaptations. Therefore, you should take two weeks of slowly introducing yourself to the heat to be fully acclimatized and prepared for prolonged training sessions. When preparing for intermittent exercise (e.g., interval workouts, resistance training), however, you may not need as long to acclimatize. Research has found that just four 30- to 45-minute sessions of intermittent exercise in the heat was enough to cause acclimatization and resulted in an improvement in intermittent running capacity. Furthermore, subjects who went through the acclimatization protocol had a lower core body temperature and an increase in thermal comfort during exercise compared to subjects who did not acclimatize. While exercising in the heat will always present a stress, acclimatization has a moderate prophylactic effect, minimizing the stress and reducing the risk of heat-related illnesses. For specific recommendations about how to acclimatize to the heat, see “How Should You Acclimatize to the Heat?” below.
Other Strategies for Running in the Heat
If you have a choice of when to run, the best time is the early morning, when the temperature is lower. Not only is it cooler and thus safer to run in the morning than in the afternoon or evening, you may also get a better workout. Research has shown that endurance exercise capacity in the heat is significantly greater in the morning than in the evening, and is accompanied by a lower initial core body and skin temperature. If you must run during the hotter part of the day, try to maximize time in the shade, wear sunscreen, and wear loose-fitting, moisture-wicking, light-colored clothes that reflect the sunlight.
Next time you run in the heat, make sure you follow these guidelines. If you adequately hydrate and acclimatize, not only will you crush your workouts and eliminate your risk for heat illness, you may even challenge Apollo’s heat.
| What Should Runners Drink? |
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FLUID (water or sports drink) SODIUM (contained in sports drink or table salt mixed in) (necessary only if running >60 minutes or if sodium deficient) GLYCEROL (structural backbone of triglycerides; available at specialty nutrition stores like GNC or online) creates osmotic gradient in circulation that causes fluid retention, which facilitates hyper-hydration, protects against dehydration, and maintains body temperature |
| How Should You Acclimatize to the Heat? full acclimatization takes 2 weeks – Increase core temperature by being outside in heat – Sweat 400-600 ml/hr – Outside temp >85 degrees F (30 degrees C) – Daily exercise – Continuous, daily exposure: – hot & dry: 100 min/day – hot & humid: >100 min/day |