Fatigue or Heatstroke?
After you have played a long tennis match on a hot
summer day, you feel weaker and less accurate with your shots.
The fatigue, muscle weakness, tired aching feeling and
decreased coordination that you get in any sport lasting several
hours is caused by low levels of fluids, salt or calories. There are
no early warning signals. By the time you feel hungry, you have
already run low on calories and are ready to crash. By the time
you feel thirsty, you are already severely dehydrated and feel
weak and tired. By the time you are low on salt, you already
have tired, aching or burning muscles; feel weak, tired and dizzy;
and may already have muscle cramps.
The primary limiting factor in sports that require great
endurance is the time it takes for your heart to pump oxygen in
your bloodstream from your lungs into your muscles. A study
from the University of Connecticut (Medicine and Science in
Sports and Exercise, May, 2006) shows that with dehydration,
your heart beats with far less force so it pumps far less blood
with each beat, and is unable to bring as much oxygen to your
muscles.
You can't depend on thirst to tell you when you lack
fluids. Certain brain cells called osmoreceptors tell you when
you are thirty, but only after the salt concentration of your blood
has risen considerably. When you exercise, you sweat. Sweat
contains far more water than salt in comparison to blood. So you
lose far more water than salt during exercise and blood levels of
salt rise. By the time that a your blood salt concentration is high
enough to trip off the osmoreceptors, you are severely
dehydrated and it is too late for you to be able to drink enough
during exercise to catch up with your water deficit. On the other
hand, if you take salt with fluids, then your blood salt levels rise
faster and tell you that you are thirsty earlier.
There are other reasons that you should take salt with
fluids during prolonged exercise. First, it helps prevent muscle
cramps. Remember, during exercise you lose salt and water. If
you are replacing only water, you can eventually take in so much
water that your salt levels drop to cause muscle cramps.
Second, even though salt is a mild diuretic at rest, during
exercise it helps your body to retain water. So when you are
going to exercise for more than a couple hours, particularly in hot
weather, drink small amounts frequently and eat salted foods
such as peanuts. Always stop if you feel sick, have chills,
headache, severe muscle burning or aching, dizziness, or blurred
vision. Seek help if your symptoms do not subside in a few
minutes; you could be headed for heat stroke that can kill you.
Checked 9/29/08