Personal Trainers Forum

[Personaltrainer]

Dehydration is progressive depletion of body fluids caused when
fluid losses, for whatever reason, exceed fluid intake. This leads
to hypohydration (or dehydration). Athletes may arrive at the start
of their event already hypohydrated - for example, after a long,
hot journey to the event, or inadequate rehydration in between
successive events, or 'making weight' strategies in, for instance,
boxing, light-weight rowing and horseracing. Possible reasons for
insufficient fluid intake during an event include:

1. poor understanding of fluid requirements. Because sweat rates
vary so widely and are probably greater than most athletes
appreciate, a worthwhile plan is to record fluid intake during
training or events (also noting the weather conditions) and
weighing, without clothes, before and after, to assess how well
fluid intake matches losses.

2. limited opportunity to drink, or availability of drinks.
Orienteers and fell runners common carry little or no drink,
relying instead on streams. For events lasting an hour or more, if
the availability of streams cannot be guaranteed, consumption of
500 ml of an electrolyte, or CHO-electrolyte, drink 10 minutes
before the start will benefit performance in later stages.

3. poor drinking strategies. As I've said, exercise suppresses
thirst, and the event itself can distract the athlete from
drinking. Subsequent hypohydration impairs gastric emptying and can
cause gastric upset, nausea or vomiting, which in turn limit
further ability to drink.

4. poor tolerance of drinks during competition. The nausea that
many athletes blame on sports drinks could be caused by dehydration
itself, or too concentrated a drink. Dehydration can be mistaken
for carbohydrate depletion. Drinks that contain more than 10% CHO
impair gastric emptying (and therefore rate of fluid provision) and
stimulate the secretion of body fluids into the intestine to dilute
the drink before it can be absorbed, temporarily worsening
dehydration.

High CHO drinks can themselves cause nausea, more so in a
dehydrated athlete. Carbonated drinks are likely to cause bloating
or discomfort, and fructose (fruit sugar) in high concentrations
can cause gastric upset and/or diarrhoea. Although fructose is
included in some sports drinks, it is inefficient as a sole source
of CHO because it is absorbed slowly and requires conversion to
glucose by the liver before it is available for energy.

5. inability to match excessive sweat rates. Exercise intensities
exceeding 70% VO2max will progressively suppress gastric emptying.
The highest reported rate of gastric emptying is 2400ml/hour, in
resting subjects. During exercise gastric emptying rarely exceeds
1-1.2 litres/hour, unless a large volume is maintained in the
stomach. Runners in particular experience discomfort with high
volumes in the stomach, and in race situations it is unlikely that
athletes could drink more than two litres/hour, even if they could
tolerate it. Some degree of dehydration is therefore inevitable
when rates of fluid loss exceed sustainable drinking capacity.

In practice, competing endurance runners and canoeists generally
drink about 500 ml/hour, and dehydrate at a rate of 500-1000
ml/hour. Even triathletes, who as a group are relatively
knowledgeable about nutrition and hydration, have been shown to
lose an average of 1.7% and 3.7% of bodyweight for a 3-hour and a
12-hour event respectively. Whereas cyclists are known to tolerate
1.2 litres/hour of a 6% CHO-electrolyte drink, runners are more
likely to experience discomfort, and may believe that time lost
through slowing down to drink may not be recovered. The volume that
most athletes choose to drink during exercise replaces less than
50% of their losses. With this in mind, effective post-event
hydration is vital in between consecutive heats or events spanning
several days.