NUTRIENTS FOR THE ATHLETE: FLUID
Of all the physiological factors that can cause
early fatigue during exercise, dehydration is arguably the most important. Dehydration is
a common occurrence, even during exercise in the cold. Dehydration decreases performance,
impairs cardiovascular function that can impair physical performance, and pose serious
health problems.
Athletes can lose a large amount of fluid when
exercising. An athlete can become dehydrated in as little as 30 minutes.
Dehydration occurs when fluid (sweat) loss exceeds
1% of body weight (800 ml in the case of an 80-kg male). Work capacity and temperature
control can be impaired with a loss of as little as 2% of body weight and can cause a
5-10% drop in performance. A 1/10th-temperature change can decrease performance.
During any form of exercise, working muscles
produces heat, and body temperature rises. Getting rid of this heat requires fluid
evaporation from the skin (sweating) which cools the body. When fluid evaporates from the
skin the body therefore losses valuable water (as high as 1-2 quarts/hour) during heavy
exercise. When the athlete fails to ingest enough fluid and dehydration becomes
sufficiently severe, sweating decreases in an attempt to conserve body water. As a result,
blood thickens, heart rate increases and body temperature rises and you get fatigue,
headache, nausea, chills, stomach discomfort and increasing the chance of heat cramps,
exhaustion, or stroke.
Recent research illustrates that maintaining normal
or near hydration during exercise maintains cardiovascular and thermoregulatory responses
and improves exercise performance. It is always the athlete’s best interest to
prevent unnecessary increases in core temp that will ultimately lead to premature fatigue.
As you exercise, be alert for these conditions.
They'll increase your loss of fluid through sweat, which could make your body become
dehydrated faster.
- Temperature
: The higher the temperature, the
greater your sweat losses.
- Intensity
: The harder you work out, the greater
your sweat losses.
- Body size
: The larger the athlete, the greater
the sweat losses. Males generally sweat more than females.
- Duration
: The longer the workout, the greater
your fluid losses.
- Fitness
: Well-trained athletes sweat more. And
they start sweating at a lower body temperature. Why? The function of sweating is to cool
the body. The well-trained athlete cools his or her body more efficiently than an
untrained person.
Most active people have experienced dehydration and
probably haven’t been aware of it. Exercise blunts the thirst mechanism and cannot
rely on thirst as an indicator of dehydration. Providing adequate fluid intake before,
during and after exercise can prevent dehydration. Fluid replacement practices should be
designed to match fluid intake and sweat loss. This approach will require sports
competitors to ingest more fluid, often more that they are accustomed to drinking.
The rate of fluid replacement should match rate of
sweating (at least 80%)
Each pound of weight loss corresponds to 450 ml (15
oz) of dehydration (8 oz = 1 cup)
Plain, cool water (5° to 10°C;
40° to 50°F) is an effective fluid replacement, and it is the most readily available and
least costly alternative. Drink plain water before or during activities lasting 60 minutes
or less.
Electrolytes (sodium, potassium, chloride, and
magnesium) are lost in sweat, but the loss of water is considerably greater. However, in
sport or training that requires more than 1 hour of continuous effort, a
carbohydrate-electrolyte solution should be provided to improve performance.
Guidelines for proper hydration:
- Learn about dangers/disadvantages of dehydration
- Become aware of sweat losses (weigh before and after
exercise)
- Drink to a plan
- Drink early
- Make fluid available
- Practice during training
