This article is from the Misc Bicycles FAQs, by various authors.
Roger Marquis (email@example.com)
During any workout, blood sugar is the primary determinant of how
well you feel and perform; therefore it is vitally important to eat
during rides. If you burn large numbers of calories and do not replace
them blood sugar will drop and you will feel tired. Most riders
misinterpret this fatigue as lack of endurance but very often it's
simply low blood sugar.
By eating every 15 to 20 minutes after the first 45 minutes of a
ride you can avoid this problem. That means remembering to bring food
on rides and remembering to eat lightly and regularly. Bananas, fig
bars, Clif Bars, fruit juice diluted with water, etc. All these foods
are readily available and easily digested on the bike. Avoid high-fat
foods, they can slow you down, and simple sugars that can cause an
insulin reaction resulting in lower blood sugar levels after the
Eating while working-out does not come naturally to most people.
The physical stress of exercise depresses the brain's hunger center.
Especially during extreme effort hunger will not appear until blood
sugar has dropped dangerously low. So if during a hard ride blood
sugar falls low enough to cause hunger take it as a sign of impending
physical disaster. You may not be able to eat and continue riding and
you may need a prolonged (12+ hours) rest.
Not eating enough on a long ride will usually result in the
"bonk". One time or another every serious cyclist experiences it.
When blood sugar falls so low that further exercise becomes difficult,
that's the bonk. Should that happens find something sweet to eat
_right_away_ and finish the ride via the easiest possible route. Do
not wait to eat and don't ride any further than necessary. It's
possible to become so stressed from bonking that you wake up the next
morning feeling sick. Even if you do not get sick, you probably won't
derive any positive training effect from the workout. Recovery can
take so long that it will actually decrease fitness, even if the
workout had been high quality.
Normal levels of blood sugar (glucose) are also essential for
metabolizing fat. Thus, when blood sugar runs low fat cannot be
metabolized to compensate. This becomes important as fat utilization
rises after the first thirty minutes of exercise. In order to burn fat
you need to ride for at least half an hour - without getting hungry
doing it. It simply is not possible to burn fat by eating less during
a workout. Only by putting in long miles and eating intelligently is
it possible to become more efficient at burning fat. Glucose is also
the only fuel the brain can use. Training in a glucose-poor state
makes it difficult to think clearly and is a clear safety risk.