Scientists have shown that the two biggest challenges faced by runners of all abilities are the loss of fuel and fluid. Whilst the loss of fuel only starts to impact on performance during the latter stages of long training runs and races, losing fluid can quickly lead to dehydration and an early negative impact on performance. If dehydration persists and gets worse, body temperature will rise and the consequences are overheating, and condition called hyperthermia, which could result in death.
All runners need to sweat – the evaporation of sweat is the body’s main defence against a rise in core temperature. But sweating loses body fluid, and experiments have found that once more than 2 percent of body weight has been lost as a result of sweating, physical and mental performance suffer. Closer examination of runners during endurance training and races has found that in hot and humid conditions, sweat rates can exceed 3 liters an hour, with rates of 1-2 liters an hour not uncommon. Since the 2% weight loss threshold is just 1.5kg for a 75kg runner, it is easy to see that dehydration can become a very early threat during long runs, and impact on runners much sooner than the loss of energy. Even on cool days, runners still sweat, and we also lose moisture with every breath as we exhale, so maintaining the correct state of hydration before, during and after long training runs, is essential.
It is hard to believe, but in the early days of marathon racing, alcohol was seen as an ideal means of hydrating, and the 1904 Olympic Champion, Thomas Hicks, was given copious quantities of alcohol during his run. Today our scientific knowledge of hydration has advanced enormously, based on the development of rehydration drinks for individuals suffering from debilitating and dehydrating diseases such as cholera. In 1965, medics working with the University of Florida’s American Football team – the Gators – developed a drink containing carbohydrate and electrolytes to aid the team’s performance (electrolytes support the function of the muscles and nerves, and are lost when we sweat, so their replacement is crucial when sweat rates are high). This drink became the sports drink “Gatorade” and sparked a series of research studies into the effect of similar drinks on endurance performance.
Not surprisingly, research studies found that these drinks were beneficial for long distance runners, improving endurance performance and cognitive function by replacing the fluids and electrolytes lost in sweat.
Scientists discovered that with an optimum blend of carbohydrate (between 4 and 8 grammes of carbohydrate per 100mls) and electrolytes in the form of sodium and potassium, running performance improved and fatigue was delayed. These drinks are quickly absorbed into the bloodstream, providing rapid replacement of energy, fluid and electrolytes. They became known as “isotonic” drinks, and are now widely used, alongside water, in many marathons.
Even with fluid on course during a race, most runners will finish dehydrated to a greater or lesser extent. Weight loss of 4 or 5 kilos at the end of a marathon is not uncommon, much of which will be due to a loss of fluid. Hot and humid conditions will present much more of a challenge to hydration status than cooler days – the heat makes it harder for the body to stay cool, whilst humidity makes it more difficult to sweat efficiently. Sweat that evaporates is most effective in losing heat, but in humid conditions sweat tends to drip rather than evaporate. The body reacts by producing more sweat, hence more fluid is lost and hydration becomes even more important.
Correct and efficient hydration is one of the most important components of success during training and racing, enabling the body to maintain its core temperature and preventing overheating. The science of hydration has made significant advances, and is now an integral part of the advice given to runners before races, who can also benefit from the provision of science-based hydration drinks that are offered on the course during a race.
Author Professor John Brewer, London St Mary's University