Gabe Mirkin, M.D.
If you are going to exercise vigorously for more than an hour,
you need to eat before you exercise or your muscles and liver will run
out of sugar and you will tire earlier. Your brain gets more than
98 percent of its energy from sugar in your bloodstream. But
there is only enough sugar in your bloodstream to last three
minutes. So you liver has to constantly release sugar from its
cells into your bloodstream. There is only enough sugar in your
liver to last about an hour when you exercise vigorously. Eating
before exercising can help you to exercise longer. If you do not
eat before you exercise for more than an hour, and during
exercise that lasts more than two hours, your liver will probably
run out of sugar. Your blood sugar level can drop, and you will
feel terrible fatigue and tiredness.
Whenever your stomach fills with food, its muscles
contract and require large amounts of blood. When you exercise
vigorously, your heart pumps large amounts of blood to your
skeletal muscles. If your heart is not strong enough to pump
blood to both your stomach and your skeletal muscles, blood is
shunted from your stomach muscles, the muscles lack oxygen,
lactic acid builds up in muscles and they start to hurt. However,
most people can exercise after eating without suffering stomach
cramps because their hearts are strong enough to pump blood to
both their exercising muscles and their stomach muscles.
Another theoretical concern is that eating sugar before
you exercise will cause your blood sugar level to rise and your
pancreas to release insulin, which will cause your blood sugar to
drop too low so you will feel tired. However, the major cause of
tiredness that you feel in your muscles during exercise is lack of
stored sugar in muscles. Taking any extra calories before and
during exercise helps to preserve the sugar that is stored in
muscles and help you to exercise longer. If you are going to
exercise for more than an hour, eat or drink anything you like
before and during your exercise. Most people will not get
stomach cramps while exercising, no matter what or when they
eat.
June 15, 2006