From late spring to early autumn, hot weather is the norm across many parts of the U.S. This is also the competition period for many endurance athletes, and the combination of heat and greater training and racing intensities requires special strategies to combat dehydration and overheating.
During exercise in a hot environment, evaporation of sweat is the primary means the body uses to cool itself. The body also increases blood flow to the skin for greater heat loss, although this diminishes the blood supply that provides oxygen to working muscles. Consequently, record-breaking endurance performances are usually set in cooler conditions.
During intense exercise in very hot and humid conditions, evaporative cooling through sweating may become insufficient, particularly in longer events or if hydration is inadequate. This scenario can lead to a number of problems due to dehydration, hyperthermia, or the combined effects of the two. Strategies for dealing with the heat become more than just a performance issue under these conditions - they become a health issue.
Acclimatization is the physiologic and psychological adjustment to a new environment. In terms of heat acclimatization, this can be something as dramatic as moving from a cool, dry climate to a hot, humid one, or simply adjusting from spring to summer.
Some of the physiologic adaptations during heat acclimatization include: reduced heart rate, core temperature, and utilization of muscle glycogen, as well as increased blood flow to the skin, plasma volume, and work time until exhaustion. Well-conditioned athletes have a higher heat tolerance than sedentary people, as regular exercise creates "internal heat stress" and thus pre-acclimatizes athletes to some degree.
Heat acclimatization usually takes 10 to 14 days, although 75% of the adaptations are believed to occur within the first five days. Exercise sessions during the acclimatization process should be shorter and less intense, gradually building up to normal by the end of the two week period.
Dehydration is an ever present threat in hot environments. Maintaining hydration levels during exercise in the heat reduces dehydration, core temperature, and cardiovascular strain and increases exercise performance. Unfortunately, science cannot provide a hard and fast rule for optimal fluid intake in hot weather, due to individual differences in sweat rates. What is known is that 0.8 to 1.0 liters per hour seems to be the upper limit for fluid absorption, while sweat loss of greater than 3 liters per hour has been observed in the laboratory. Thus there may be times when some degree of dehydration is unavoidable.
It is also known that carbohydrate-electrolyte sports drinks are well tolerated during exercise in the heat and improve endurance. One recent study found that high water intake during one hour cycling in the heat improved performance by 6.5% compared with lower intake. Adding 79 g of carbohydrate to the high-volume regimen improved performance another 6.3%. However, hypertonic solutions (greater than 12% carbohydrate) such as fruit juices and soft drinks may cause gastrointestinal distress and impair exercise performance in the heat.
Hyperhydration Before Exercise
Drinking excess water or preferably a fluid replacement drink containing electrolytes, before hot-weather exercise is another recommended strategy. Current guidelines indicate you should drink 500 ml (17 oz.) of fluid 2 hours before competition, the rationale being that gastric emptying is enhanced when the stomach is relatively full. As chronic mild dehydration is not uncommon in some athletes, hyperdration ensures you don't begin a competition already dehydrated.
Several studies have shown that sodium replacement maximizes rehydration. In fact, sodium is more important for fluid restoration after exercise than during exercise. In a recent study, subjects achieved greater postexercise fluid retention with a high electrolyte fluid replacement drink than with water or a more dilute sports drink. Optimal postexercise rehydration requires high fluid volume replacement (more than 150% of water weight lost) and high fluid sodium content. Plain water is a poor choice for postexercise fluid replacement as it reduces thirst drive and has little or no sodium.
Summary of Hot Weather Training and Racing Strategies:
- Acclimatize to the heat.
- Maintain proper hydration during training and racing with a carbohydrate-electrolyte sports drink.
- Hyperhydrate before exercise.
- Drink a postexercise recovery drink that contains more sodium than your "during" energy drink.