Nutritional Advice for Exercising in Hot & Humid Conditions

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Nutritional Advice for Exercising in Hot & Humid Conditions

Key Points

Hot and humid conditions place an increased demand on the thermoregulatory and cardiovascular systems, potentially causing a reduction in performance.
Sweating becomes the principal method of heat loss in a hot environment but becomes difficult in the humidity.
Acclimatization is an important strategy to allow the body to adapt, and subsequently perform in hot and humid conditions.
Dehydration, by as little as 2% body mass, can reduce mental and physical performance. Consequently, it is essential to replace the fluid and electrolytes lost during exercise.
Exercise in the heat is associated with a greater reliance on carbohydrate stores.

Introduction

With a number of events being held in hot, humid conditions and an increasing need for year-round training, nutritional strategies play an important role in helping individuals perform at their best. The two key issues to address when exercising in hot and humid conditions are the maintenance of fluid balance and the availability of carbohydrate as a fuel source.

Environmental Demands

From a physiological standpoint, the most severe stress that an individual can encounter is exercise in the heat. Heat production during exercise can be as high as 15- 20 times that compared to rest. Therefore, the body requires an efficient ability to dissipate heat to maintain a stable core temperature between 36.5 and 37.5 °C.

The body’s natural defence against overheating is sweating (the evaporative loss of heat). Sweating is an efficient method of dissipating heat, however, if the fluid lost as sweat is not replaced, then dehydration occurs and the body’s core temperature will rise anyway. Furthermore, evaporative sweat loss becomes more difficult when the air is humid.

Due to the prolonged nature of their training and events, endurance athletes are most at risk of performance decrements in these conditions.

Physiological Demands of Exercising in The Heat

The physiological response to exercise in the heat is determined in part by the intensity of exercise and the degree of heat stress (environmental conditions). Due to the need to dissipate heat, exercise in hot and humid conditions places two competing demands on the cardiovascular system:

The need to pump blood to the exercising muscles to supply them with energy
The need to pump blood to the periphery (skin) to allow heat loss to occur.

In trying to cope with the two competing demands, sub-maximal heart rate, energy expenditure and ratings of exertion all increase and there is an increased use of carbohydrate for the same relative intensity. Consequently, exercise is harder than in a temperate climate and premature fatigue occurs. In extreme conditions when individuals don’t adapt to the heat and humidity, heat illness can occur. Warning signs include the loss of energy, reduced performance, muscle cramps, light headedness and dark-coloured urine.

Acclimatisation and the maintenance of fluid balance and carbohydrate intake are the three key strategies necessary to cope with exercise in the heat.

Acclimatisation

Acclimatization helps the body to adapt to the demands of the environment. The magnitude of the adaptation that occurs is closely related to the degree of heat stress to which the individual is exposed, whilst the time it takes to adapt is highly individual. The process begins within a few days, with the major changes largely complete in 6-8 days. Full adaptations may take 14 days.

Acclimatisation results in the expansion of plasma volume and a decrease in heart rate, core temperature and muscle glycogen use. Consequently, there is a more effective blood flow distribution, increased sweat rate and more efficient onset of sweating. The loss of sodium in sweat is decreased. The key advice for acclimatisation includes:

Initially reduce the training load, allowing you to adapt to the stresses of the environment
Perform low-intensity, steady-state training in the hotter periods of the day and harder interval/sprint work during the cooler periods
Pay particular attention to hydration and the supply of carbohydrate within the diet.

Hydration

Failure to replace the fluids that are lost as sweat causes dehydration. Dehydration exacerbates the demands of exercising in the heat and the impairments in mental and physical performance. Consequently, the consumption of fluid in the periods immediately before, during and after exercise is essential to the body’s ability to perform in hot and humid conditions.

Much of the hydration advice for exercising in the heat and humidity is similar to that of temperate climates. However, as sweat rates will increase with acclimatisation and the onset of sweating occurs earlier, a greater reliance is placed on beginning exercise hydrated and as far as possible preventing a body mass drop of > 2% occurring during exercise. Key hydration advice includes:

Consume 5-7 millilitres of fluid per kilogram of body mass (ml/kgBM) 4 h before exercise.
If the individual does not produce urine, or the urine is dark or highly concentrated then more fluid should be consumed about 2 h before the event (e.g. 3-5 ml/kgBM).
Consume fluids with sodium and/or small amounts of salted snacks or sodium containing foods to stimulate thirst and retain the consumed fluid.
Drink larger volumes of fluid early in exercise and continue to ingest fluid throughout (little and often: ~ 150 ml every 10-15 min).
After exercise, consume 1.5 litres (L) of fluid for each kg of body mass lost as sweat. Drink the fluid over time as opposed to one large bolus. Isotonic sports drinks are popular options.

Carbohydrate

Exercise in the heat is associated with a greater reliance on muscle glycogen stores but less reliance on ingested carbohydrate. Therefore, it is important to start exercise with full muscle glycogen stores and emphasise the recovery of carbohydrate immediately after exercise. The key carbohydrate advice includes:

Consume 1-4 g/kgBM of carbohydrate (approximately 70-280 g for a 70 kg individual) 3-4 h before exercise. Suitable foods include cereals, bread, rice, pasta and potatoes.
Consume 30-60 g of carbohydrate per hour (g/h) in small feedings every 10-30 min, or as allowed by the event/sport.
Consumed 1.0-1.2 g/kgBM/h of carbohydrate at frequent intervals during the first few hours (0-4 h) of recovery after exercise.
Maintain a high daily carbohydrate intake of 8-10 g/kgBM/day

Be aware that the heat can cause a suppression of the appetite and fluid intake, so chilled, palatable sports drinks and high-carbohydrate snacks such as energy bars or dried fruit can be extremely useful in achieving the required levels.