Fluid replacement

If you’re exercising for an hour or less in moderate temperatures, taking in water may actually hamper performance

Fluid replacement during exercise has long been seen as important for endurance performance, especially when the exercise is to be performed in the heat. That view still holds good when exercise of longer than an hour is undertaken. However, new evidence from researchers at the University of Cape Town Medical School in South Africa suggests that for an intense exercise period of one hour or less in moderate temperatures (eg, 20 deg C, medium humidity), the ingestion of water during exercise is of no benefit and may even reduce performance capacity. This may contradict the practices of many athletes and coaches currently involved in high-level performance

In the Cape Town study, eight endurance-trained cyclists used their own bikes which were mounted on a cycle ergometer. First, they performed a test which involved cycling as far as possible in one hour. The exercise intensity was found to be about 85% of peak oxygen consumption, and the amount of fluid lost as sweat was found to be approximately 1.7 litres each. At both seven and 14 days later, the cyclists repeated the one-hour performance, while randomly ingesting either no fluid or approximately 1.7 litres of artificially sweetened, coloured water

What the scientists found was that less distance was covered in the trial when fluid was allowed than when no fluid was taken. The distances covered were 43.1km (no fluid) and 42.3 (fluid). The reduced intensity of exercise was found to correspond to a reduced heart rate during the fluid trial (166 beats per minute was reduced to 157 bpm). In addition, a subjective measure of abdominal fullness not surprisingly showed evidence of a ‘bloated’ sensation during the fluid trial

Why did it happen?
A possible explanation for these findings is that drinking interfered with the subjects’ ability to concentrate on cycling at the very high intensity required (85% of peak oxygen consumption). Also, a large amount of water may have remained in the intestines due to reduced absorption into the body at such very high exercise intensities. This would have caused the bloated sensation and adversely affected concentration

As an alternative to pure water, a carbohydrate-electrolyte solution may have improved both absorption from the intestines and the utilisation of water inside the body space. The last point refers to the fact that fluids with a moderate carbohydrate-electrolyte concentration have a greater tendency to remain in the circulation (blood volume) than pure water, which tends to move to the less useful intercellular space. Functions such as temperature control and regulation rely heavily on the maintenance of blood volume. (Any carbohydrate ingested during exercise of less than one hour is unlikely to be used to produce energy.)

Thus, given the findings of this research study, those athletes who compete in events of little more than an hour’s duration may have something to learn. Namely: taking in large amounts of fluid during a competitive race of one hour or less is not necessary and is possibly detrimental

This is especially so if the competition takes place in typical British or (now that we’re in the European Union) north-west European weather. However, despite the fact that absorption of water is restricted during very high-intensity exercise, SMALL amounts of fluid (weak carbohydrate-electrolyte solutions) are unlikely to inhibit performance significantly and are probably still beneficial. Experimenting during training can help individual athletes find a technique that suits them personally

Nevertheless, in spite of these findings, fluid ingestion remains of paramount importance for more lengthy exercise periods, especially those of greater than two hours in the heat and high humidity. In such cases, the amount of fluid lost as sweat is sufficient to make impaired performance a certainty, and dehydration and hyperthermia distinct possibilities. Finally, pre-exercise (up to two hours before) and post-exercise fluid intake also remain priorities for all exercise types and durations when optimum performance and recovery are required

(‘Water ingestion does not improve 1h cycling performance in moderate ambient temperatures’, European Journal of Applied Physiology, 1995, Vol. 71, pp. 153-160.)
Alun Williams

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