Dehydration can have a serious negative effect on
performance. Dehydration occurs through
the loss of water from the body, mainly in the form of sweat that is evaporated
from the skin as the body tries to prevent overheating.
2% body weight lost in sweat can cause impaired performance,
2% of body weight lost from sweat equals roughly 1.4 litres of fluid for a 70kg
male. At 4% the capacity for muscular endurance declines, at 5% heat exhaustion
can set in, at 7% hallucinations can occur and at 10% there is circulatory
collapse and heat stroke.
It is not hard to lose this fluid through sweat during
exercise. You can easily lose 2.5 to 3
of body weight in a game of rugby or football and up to 4kg during a marathon.
It also does not matter what the ambient temperature is. Obviously you will
sweat more on a hot an humid day, but you can still lose a significant amount
of sweat on a cool dry day that can lead to dehydration and impaired
performance.
What’s the
best indicator of dehydration?
• Thirst.
• Concentrated urine.
• Confusion.
• Nausea.
• Headache.
• Fatigue.
• Cramps.
Most people would answer thirst, however this is not a good
indicator of the body’s hydration status. By the time you need a drink, your
body is already dehydrated and performance is already likely to be adversely
affected. It is therefore important to maintain fluid intake by drinking during
exercise and not wait until you feel thirsty.
The colour and volume of urine is a better indicator of
hydration status. Large volumes of clear or pale coloured urine indicates the
body is well hydrated. Small volumes of dark coloured urine indicates that the
body is dehydrated and fluids need to be ingested immediately. There are pee charts that can be used to
monitor hydration status whilst you are urinating.
Sweat volume and electrolyte loss varies from individual to
individual and depending on the ambient temperature. Some individuals can
produce large volumes of sweat during exercise but may be very good at
retaining electrolytes preventing camps and decreased performance, while others
may be “salty sweaters” and lose large amounts of electrolytes leading to
cramping and decreased performance.
There are a number of ways of measuring hydration
status. Specific gravity (SG) measures
the concentration of particles in the urine; the normal range is considered
1.020-1.030g/ml. A high specific gravity (i.e. above 1.030, thus more
concentrated particles in the urine) indicates dehydration. A urine sample can
be collected and measured with a specific gravity device
or pee sticks.
Osmo check measures refractive index, which is directly
correlated to specific gravity and indirectly related to osmolality of the
urine you pass (a high osmolality value means the urine is more concentrated
and the body is dehydrated, whereas a low osmolality means the urine is less
concentrated and the body is hydrated). Values of 400 mOsmols/kg H2O or below
generally indicate hydration, values of 400-800 mOsmols/kg H2O indicate
dehydration and values above 800 mOsmols/kg H2O indicate severe dehydration. A
urine sample can be collected and measured with an osmo check device.
This article was brought to you by Steve Hines who is a Wandsworth Nutritionist. Steve can be contacted via his website.
