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Nutritional needs for exercise in the heat

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Abstract

Although hot conditions are not typically conducive to optimal sports performance, nutritional strategies play an important role in assisting an athlete to perform as well as possible in a hot environment. A key issue is the prevention of hypohydration during an exercise session. Fluid intake strategies should be undertaken in a cyclical sequence: hydrate well prior to the workout, drink as much as is comfortable and practical during the session, and rehydrate aggressively afterwards in preparation for future exercise bouts. There is some interest in hyperhydration strategies, such as hyperhydration with glycerol, to prepare the athlete for a situation where there is little opportunity for fluid intake to match large sweat losses. Recovery of significant fluid losses after exercise is assisted by the simultaneous replacement of electrolyte losses. Carbohydrate (CHO) requirements for exercise are increased in the heat, due to a shift in substrate utilization towards CHO oxidation. Daily food patterns should focus on replacing glycogen stores after exercise, and competition strategies should include activities to enhance CHO availability, such as CHO loading for endurance events, pre-event CHO intake, and intake of sports drinks in events lasting longer than 60 min. Although CHO ingestion may not enhance the performance of all events undertaken in hot weather, there are no disadvantages to the consumption of beverages containing 4–8% CHO and electrolytes. In fact, the palatability of these drinks may enhance the voluntary intake of fluid. Although there is some evidence of increased protein catabolism and cellular damage due to production of oxygen radicals during exercise in the heat, there is insufficient evidence to make specific dietary recommendations to account for these issues.

Introduction

Since many important competitive events in recent times have been conducted in hot weather, and most sports require year-round conditioning, it is now common for athletes to undertake considerable periods of training and competition in the heat. Although these conditions are not typically conducive to optimal performance, particularly for events involving prolonged aerobic or intermittent high-intensity exercise, nutritional strategies play an important role in assisting the athlete to perform as well as possible. Exercising in the heat creates special nutritional needs, which must be balanced against its lesser-known effects on appetite and voluntary intake of fluid and food. This paper will highlight the major issues in nutrition for sports performance in the heat, summarizing some practical recommendations for achieving nutrition goals. Strategies will be considered for chronic periods of exercise in the heat (i.e. training needs) as well as acute strategies for optimizing performance (i.e. competition strategies).

Section snippets

Summary of metabolism during exercise in the heat

The most notable effect of exercise in a hot climate is to increase fluid losses. In a hot environment, the chief method of dissipating body heat produced by the exercising muscles or absorbed from the environment is via the evaporation of sweat. In such conditions, sweat losses during sustained submaximal exercise may be as high as 2–3 l/h. In fact, Alberto Salazar is noted to have lost an average of 3.7 l/h of sweat during the 1984 Olympic Marathon in Los Angeles, held in a hot and humid

Fluid strategies for exercise in the heat

The maintenance of fluid balance is a key issue in the health and performance of the athlete. To provide an understanding of fluid intake strategies for exercise it is helpful to have a clear understanding of definitions related to fluid balance. An athlete with a normal total body water (TBW) is said to be in a state of ‘euhydration’, while having a TBW below normal is defined as being in a state of ‘hypohydration’ and is achieved by the process of ‘dehydration’. Having a greater than normal

Fuel requirements

The availability of CHO as a substrate for skeletal muscle contraction and the central nervous system is a critical factor in the performance of prolonged (>90 min) submaximal or intermittent high-intensity exercise, and plays a permissive role in the performance of brief high-intensity work (see Hargreaves, 1999). Since total body CHO stores are limited, they are often substantially less than the fuel requirements of the training and competition sessions undertaken by many athletes. We might

Fluid intake during exercise: water or sports drink?

Fluid intake during exercise can overcome or reduce many of the problems associated with sweat-induced hypohydration, and its importance as a nutritional strategy for sport is greater during exercise in hot conditions where sweat losses and fluid deficits are greater. Indeed it provides an additional benefit of reducing muscle glycogen utilization, probably by attenuating the rise in both circulating epinephrine concentrations and muscle temperature (Hargreaves et al., 1996). Since fluid needs

Other nutritional issues

There is preliminary evidence that exercise in the heat increases protein oxidation and cellular damage due to generation of free oxygen radicals. Insufficient research has been undertaken to determine whether this translates into additional requirements for protein, anti-oxidant vitamins and minerals or other micronutrients. There may be some advantages to increasing protein intake and supplementing with anti-oxidant vitamins in the early days of exposure to a hot climate, however, these

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    This paper was presented at the International Conference on Physiological and Cognitive Performance in Extreme Environments, Canberra, Australia, March 2000.

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