For athletes, dehydration can be disastrous
THE SUMMER months in Ireland are typically associated with warm and occasional hot weather. The long days and odd balmy evening is a time when many individuals engage in regular exercise and other outdoor pastimes.
From a sports performance perspective the issue of training and competition in the heat has become very topical with many major international sporting events now held in extreme climatic conditions. In a month's time Irish Olympic athletes will not only be competing against their international rivals in Beijing, but also have to cope with the soaring temperatures which typically reach in excess of 30 degrees with humidity levels approaching 100 per cent.
Compared to the usual climatic conditions experienced in Ireland, exercise in the heat poses its own unique physiological challenges on the circulatory and thermoregulatory systems of the body which are responsible for regulating internal body (core) temperature.
Exercise in a hot environment will place an additional overload on the body and may result in dehydration, elevated core body temperature, decreased physiological and mental function, exertional heat illness and in very extreme cases death.
The human body is a relatively inefficient biological system in that only about 22 per cent of the chemical energy created by the breakdown of food is converted into mechanical work. The remainder of the energy used during muscular contraction is liberated as heat.
The working muscles, therefore, act like furnaces, creating large amounts of heat which needs to be removed from the body. Typical core body temperature at rest is about 37 degrees and the human body can generally cope with a fall in temperature of about 10 degrees and survive a rise of around 6 degrees.
Elevations in core temperature above 40 degrees will impair normal physiological function and physical performance with increases above 43 degrees being associated with severe thermal stress, heat injury and imminent collapse.
At rest the main mechanisms of heat regulation and dissipation are through the physical processes of convection, conduction, evaporation and radiation, with the latter accounting for about 60 per cent of heat loss. In contrast, during exercise in the heat the body's main method of regulating temperature and therefore avoiding overheating and thermal stress is through the evaporation of sweat which accounts for about 80 per cent of heat loss.
However, in a hot humid environment, typical of that which will be experienced at the Beijing Games, the atmosphere is already saturated with water vapour - therefore sweat tends to drip off the body rather than evaporate. As a result, despite significant fluid loss occurring through sweating, temperature regulation is less effective than in a hot dry environment. This highlights the importance of planning hydration strategies.
In normal conditions daily fluid requirements for an active individual are typically in the region of 3 litres per day. In the heat due to high sweat rates and greater turnover of body water, fluid needs may increase to as much as 10-12 litres per day. In many cases, however, fluid loss through increased sweat output often exceeds habitual daily water intake.
As a result of this fluid imbalance dehydration occurs. This body water deficit in turn will cause a reduced sweating rate, leading to a greater heat storage within the body. The consequence of these processes will be a reduction in performance during prolonged exercise; and the warmer the environment, the greater the likelihood for performance decrements.
The most common illnesses reported for athletes exercising in the heat include heat cramps, heat syncope (fainting), heat exhaustion and in extreme cases heat stroke.
When exercising in the heat, the importance of adequate fluid intake in order to maintain physiological function and physical performance is often underestimated. In hot and humid conditions, for example, water loss can exceed two litres per hour. Sweat rates will vary among individuals even under the same environmental conditions.
During exercise in the heat, it is often difficult to balance the volume of fluid consumed to the volume of water lost through sweating. There are a number of simple methods of monitoring individual hydration status:
• Measure your weight before and after exercise - to ensure you are fully rehydrated use the '150 per cent rule' namely for every kg of weight lost drink 1.5 litres of fluid.
• Monitor urine colour and frequency - ideally your urine should be pale in colour (except for first thing in the morning).
• Remember thirst is an unreliable indicator of fluid requirements, if you are thirsty you are already dehydrated.
• Avoid alcoholic drinks (especially spirits) and caffeine-based drinks, as these will promote increased urine production.
It is also worth remembering that when the weather is hot the sun is usually also very strong. It is therefore essential to use appropriate sun protection such as a high SPF sun block (25 SPF+), a hat and sunglasses.
Even a slight reddening of unprotected skin due to sun exposure has been shown to impair physiological function for up seven days, not to mention increasing the risk of skin cancer, which is the most common form of cancer in Ireland.
According to the National Cancer Registry of Ireland non-melanoma skin cancer forms 29 per cent of all malignant cancers.
Exercise in the heat places additional stresses on the body over and above those normally associated with physical training. It is therefore important to be aware of the key environmental challenges and how they influence physiological function and to prepare accordingly.
• Dr Giles Warrington is a sports and exercise physiologist and lecturer in the School of Health and Human Performance at Dublin City University