Consensus on Training and Competing in the Heat
Consensus on Training and Competing in the Heat
The development of hyperthermia during exercise in hot ambient conditions is associated with a rise in sweat rate, which can lead to progressive dehydration if fluid losses are not minimised by increasing fluid consumption. Exercise-induced dehydration, leading to a hypohydrated state, is associated with a decrease in plasma volume and an increase in plasma osmolality that are proportional to the reduction in total body water. The increase in the core temperature threshold for vasodilation and sweating at the onset of exercise is closely linked to the ensuing hyperosmolality and hypovolaemia. During exercise, plasma hyperosmolality reduces the sweat rate for any given core temperature and decreases evaporative heat loss. In addition, dehydration decreases cardiac filling and challenges blood pressure regulation. The rate of heat storage and cardiovascular strain is therefore exacerbated, and the capacity to tolerate exercise in the heat is reduced.
Despite decades of studies in this area, the notion that dehydration impairs aerobic performance in sport settings is not universally accepted and there seems to be a two-sided polarised debate. Numerous studies report that dehydration impairs aerobic performance in the condition that if exercise is performed in warm-hot environments and that body water deficits exceed at least ~2% of body mass. On the other hand, some recent studies suggest that dehydration up to 4% body mass does not alter cycling performance under an ecologically valid conditions. However, these results must be interpreted in context; that is, in well-trained male cyclists typically exercising for 60 min in ambient conditions up to 33°C and 60% relative humidity, and starting exercise in a euhydrated state. Nonetheless, some have advanced the idea that the detrimental consequences of dehydration have been overemphasised by sports beverage companies. As such, it has been argued that athletes should drink to thirst. However, many studies (often conducted prior to the creation and marketing of 'sport-drinks') have repeatedly observed that drinking to thirst often results in body water deficits that may exceed 2–3% body mass when sweat rates are high and exercise is performed in warm-hot environments. Ultimately, drinking to thirst may be appropriate in many settings, but not in circumstances where severe dehydration is expected (eg, Ironman triathlon).
In competition settings, hydration is dependent on several factors, including fluid availability and the specificities of the events. For example, while tennis players have regular access to fluids due to the frequency of breaks in a match, other athletes such as marathon runners have less opportunity to rehydrate. There are also differences among competitors. Whereas the fastest marathon runners do not consume large volume of fluids and become dehydrated during the race, some slower runners may conversely overhydrate, with an associated risk of 'water intoxication' (ie, hyponatraemia). The predisposing factors related to developing hyponatraemia during a marathon include substantial weight gain, a racing time above 4 h, female sex and low body-mass index. Consequently, although the recommendations below for competitive athletes explain how to minimise the impairment in performance associated with significant dehydration and body mass loss (ie, ≥2%), recreational athletes involved in prolonged exercise should be cautious not to overhydrate during the exercise.
Resting and well-fed humans are generally well hydrated, and the typical variance in day-to-day total body water fluctuates from 0.2% to 0.7% of body mass. When exposed to heat stress in the days preceding competition, it may, however, be advisable to remind athletes to drink sufficiently and replace electrolyte losses to ensure that euhydration is maintained. Generally, drinking 6 mL of water per kg of body mass during this period every 2–3 h, as well as 2–3 h before training or competition in the heat, is advisable.
There are several methods available to evaluate hydration status, each one having limitations depending on how and when the fluids are lost. The most widely accepted and recommended methods include monitoring body mass changes, measuring plasma osmolality and urine specific gravity. Based on these methods, one is considered euhydrated if daily body mass changes remain <1%, plasma osmolality is <290 mmol/kg and urine specific gravity is <1.020. These techniques can be implemented during intermittent competitions lasting for several days (eg, cycling stage race, tennis/team sports tournament) to monitor hydration status. Establishing baseline body mass is important, as daily variations may occur. It is best achieved by measuring post-void nude body mass in the morning on consecutive days after consuming 1–2 L of fluid the prior evening. Moreover, since exercise, diet and prior drinking influence urine concentration measurements, first morning urine is the preferred assessment time point to evaluate hydration status. If first morning urine cannot be obtained, urine collection should be preceded by several hours of minimal physical activity, fluid consumption and eating.
Sweat rates during exercise in the heat vary dramatically depending on the metabolic rate, environmental conditions and heat acclimatisation status. While values ranging from 1.0 to 1.5 L/h are common for athletes performing vigorous exercise in hot environments, certain individuals can exceed 2.5 L/h. Over the last several decades, mathematical models have been developed to provide sweat loss predictions over a broad range of conditions. While these have proven useful in public health, military, occupational and sports medicine settings, these models require further refinement and individualisation to athletic populations, especially elite athletes.
The main electrolyte lost in sweat is sodium (20–70 mEq/L), and supplementation during exercise is often required for heavy and 'salty' sweaters to maintain plasma sodium balance. Heavy sweaters may also deliberately increase sodium (ie, salt) intake prior to and following hot weather training and competition to maintain sodium balance (eg, 3.0 g of salt added to 0.5 L of a carbohydrate-electrolyte drink). To this effect, the Institute of Medicine has highlighted that public health recommendations regarding sodium ingestion do not apply to individuals who lose large volumes of sodium in sweat, such as athletes training or competing in the heat. A salt intake that would not compensate sweat sodium losses would result in a sodium deficit that might prompt muscle cramping when reaching 20–30% of the exchangeable sodium pool. During exercise lasting longer than 1 h, athletes should therefore aim to consume a solution containing 0.5–0.7 g/L of sodium. In athletes experiencing muscle cramping, it is recommended to increase the sodium supplementation to 1.5 g/L of fluid. Athletes should also aim to include 30–60 g/h of carbohydrates in their hydration regimen for exercise lasting longer than 1 h, and up to 90 g/h for events lasting over 2.5 h. This can be achieved through a combination of fluids and solid foods.
Following training or competing in the heat, rehydration is particularly important to optimise recovery. If fluid deficit needs to be urgently replenished, it is suggested to replace 150% of body mass losses within 1 h following the cessation of exercise, including electrolytes to maintain total body water. From a practical perspective, this may not be achievable for all athletes for various reasons (eg, time, gastrointestinal discomfort). Thus, it is more realistic to replace 100–120% of body mass losses. The preferred method of rehydration is through the consumption of fluids with foods (eg, including salty food).
Given that exercise in the heat increases carbohydrate metabolism, endurance athletes should ensure that not only water and sodium losses are replenished, but carbohydrate stores as well. To ensure the highest rates of muscle glycogen resynthesis, carbohydrates should be consumed during the first hour after exercise. Moreover, a drink containing protein (eg, milk) might allow better restoration of fluid balance after exercise than a standard carbohydrate-electrolyte sport drink. Combining protein (0.2–0.4 g/kg/h) to carbohydrate (0.8 g/kg/h) has also been reported to maximise protein synthesis rates. Therefore, athletes should consider consuming drinks such as chocolate milk, which has a carbohydrate-to-protein ratio of 4:1, as well as sodium, following exercise.
Section 2: Hydration
The development of hyperthermia during exercise in hot ambient conditions is associated with a rise in sweat rate, which can lead to progressive dehydration if fluid losses are not minimised by increasing fluid consumption. Exercise-induced dehydration, leading to a hypohydrated state, is associated with a decrease in plasma volume and an increase in plasma osmolality that are proportional to the reduction in total body water. The increase in the core temperature threshold for vasodilation and sweating at the onset of exercise is closely linked to the ensuing hyperosmolality and hypovolaemia. During exercise, plasma hyperosmolality reduces the sweat rate for any given core temperature and decreases evaporative heat loss. In addition, dehydration decreases cardiac filling and challenges blood pressure regulation. The rate of heat storage and cardiovascular strain is therefore exacerbated, and the capacity to tolerate exercise in the heat is reduced.
Despite decades of studies in this area, the notion that dehydration impairs aerobic performance in sport settings is not universally accepted and there seems to be a two-sided polarised debate. Numerous studies report that dehydration impairs aerobic performance in the condition that if exercise is performed in warm-hot environments and that body water deficits exceed at least ~2% of body mass. On the other hand, some recent studies suggest that dehydration up to 4% body mass does not alter cycling performance under an ecologically valid conditions. However, these results must be interpreted in context; that is, in well-trained male cyclists typically exercising for 60 min in ambient conditions up to 33°C and 60% relative humidity, and starting exercise in a euhydrated state. Nonetheless, some have advanced the idea that the detrimental consequences of dehydration have been overemphasised by sports beverage companies. As such, it has been argued that athletes should drink to thirst. However, many studies (often conducted prior to the creation and marketing of 'sport-drinks') have repeatedly observed that drinking to thirst often results in body water deficits that may exceed 2–3% body mass when sweat rates are high and exercise is performed in warm-hot environments. Ultimately, drinking to thirst may be appropriate in many settings, but not in circumstances where severe dehydration is expected (eg, Ironman triathlon).
In competition settings, hydration is dependent on several factors, including fluid availability and the specificities of the events. For example, while tennis players have regular access to fluids due to the frequency of breaks in a match, other athletes such as marathon runners have less opportunity to rehydrate. There are also differences among competitors. Whereas the fastest marathon runners do not consume large volume of fluids and become dehydrated during the race, some slower runners may conversely overhydrate, with an associated risk of 'water intoxication' (ie, hyponatraemia). The predisposing factors related to developing hyponatraemia during a marathon include substantial weight gain, a racing time above 4 h, female sex and low body-mass index. Consequently, although the recommendations below for competitive athletes explain how to minimise the impairment in performance associated with significant dehydration and body mass loss (ie, ≥2%), recreational athletes involved in prolonged exercise should be cautious not to overhydrate during the exercise.
Pre-exercise Hydration
Resting and well-fed humans are generally well hydrated, and the typical variance in day-to-day total body water fluctuates from 0.2% to 0.7% of body mass. When exposed to heat stress in the days preceding competition, it may, however, be advisable to remind athletes to drink sufficiently and replace electrolyte losses to ensure that euhydration is maintained. Generally, drinking 6 mL of water per kg of body mass during this period every 2–3 h, as well as 2–3 h before training or competition in the heat, is advisable.
There are several methods available to evaluate hydration status, each one having limitations depending on how and when the fluids are lost. The most widely accepted and recommended methods include monitoring body mass changes, measuring plasma osmolality and urine specific gravity. Based on these methods, one is considered euhydrated if daily body mass changes remain <1%, plasma osmolality is <290 mmol/kg and urine specific gravity is <1.020. These techniques can be implemented during intermittent competitions lasting for several days (eg, cycling stage race, tennis/team sports tournament) to monitor hydration status. Establishing baseline body mass is important, as daily variations may occur. It is best achieved by measuring post-void nude body mass in the morning on consecutive days after consuming 1–2 L of fluid the prior evening. Moreover, since exercise, diet and prior drinking influence urine concentration measurements, first morning urine is the preferred assessment time point to evaluate hydration status. If first morning urine cannot be obtained, urine collection should be preceded by several hours of minimal physical activity, fluid consumption and eating.
Exercise Hydration
Sweat rates during exercise in the heat vary dramatically depending on the metabolic rate, environmental conditions and heat acclimatisation status. While values ranging from 1.0 to 1.5 L/h are common for athletes performing vigorous exercise in hot environments, certain individuals can exceed 2.5 L/h. Over the last several decades, mathematical models have been developed to provide sweat loss predictions over a broad range of conditions. While these have proven useful in public health, military, occupational and sports medicine settings, these models require further refinement and individualisation to athletic populations, especially elite athletes.
The main electrolyte lost in sweat is sodium (20–70 mEq/L), and supplementation during exercise is often required for heavy and 'salty' sweaters to maintain plasma sodium balance. Heavy sweaters may also deliberately increase sodium (ie, salt) intake prior to and following hot weather training and competition to maintain sodium balance (eg, 3.0 g of salt added to 0.5 L of a carbohydrate-electrolyte drink). To this effect, the Institute of Medicine has highlighted that public health recommendations regarding sodium ingestion do not apply to individuals who lose large volumes of sodium in sweat, such as athletes training or competing in the heat. A salt intake that would not compensate sweat sodium losses would result in a sodium deficit that might prompt muscle cramping when reaching 20–30% of the exchangeable sodium pool. During exercise lasting longer than 1 h, athletes should therefore aim to consume a solution containing 0.5–0.7 g/L of sodium. In athletes experiencing muscle cramping, it is recommended to increase the sodium supplementation to 1.5 g/L of fluid. Athletes should also aim to include 30–60 g/h of carbohydrates in their hydration regimen for exercise lasting longer than 1 h, and up to 90 g/h for events lasting over 2.5 h. This can be achieved through a combination of fluids and solid foods.
Post-exercise Rehydration
Following training or competing in the heat, rehydration is particularly important to optimise recovery. If fluid deficit needs to be urgently replenished, it is suggested to replace 150% of body mass losses within 1 h following the cessation of exercise, including electrolytes to maintain total body water. From a practical perspective, this may not be achievable for all athletes for various reasons (eg, time, gastrointestinal discomfort). Thus, it is more realistic to replace 100–120% of body mass losses. The preferred method of rehydration is through the consumption of fluids with foods (eg, including salty food).
Given that exercise in the heat increases carbohydrate metabolism, endurance athletes should ensure that not only water and sodium losses are replenished, but carbohydrate stores as well. To ensure the highest rates of muscle glycogen resynthesis, carbohydrates should be consumed during the first hour after exercise. Moreover, a drink containing protein (eg, milk) might allow better restoration of fluid balance after exercise than a standard carbohydrate-electrolyte sport drink. Combining protein (0.2–0.4 g/kg/h) to carbohydrate (0.8 g/kg/h) has also been reported to maximise protein synthesis rates. Therefore, athletes should consider consuming drinks such as chocolate milk, which has a carbohydrate-to-protein ratio of 4:1, as well as sodium, following exercise.
Summary of the Main Recommendations for Hydration
Before training and competition in the heat, athletes should drink 6 mL of fluid per kg of body mass every 2–3 h, in order to start exercise euhydrated.
During intense prolonged exercise in the heat, body water mass losses should be minimised (without increasing body weight) to reduce physiological strain and help to preserve optimal performance.
Athletes training in the heat have higher daily sodium (ie, salt) requirements than the general population. Sodium supplementation might also be required during exercise.
For competitions lasting several days (eg, cycling stage race, tennis/team sports tournament), simple monitoring techniques such as daily morning body mass and urine specific gravity can provide useful insights into the hydration state of the athlete.
Adequately rehydrating after exercise-heat stress by providing plenty of fluids with meals is essential. If aggressive and rapid replenishment is needed, then consuming fluids and electrolytes to offset 100–150% of body mass losses will allow for adequate rehydration.
Recovery hydration regimens should include sodium, carbohydrates and protein.
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