Bonk!

[velogirl]

Here's the science behind the bonk. Glycogen is your body's fuel source during aerobic exercise. Your muscles store a finite amount of glycogen (about 1,400 calories worth). Your liver also stores glycogen (about 700 calories worth), but it's used to fuel your brain. So, let's say you ride for three hours and burn 700 calories an hour and you're replacing 200 calories an hour with a sports drink. It'll look something like this:

Muscle Glycogen = 1,400kcal
hour one = 700kcal (riding) minus 200kcal (drink)
Remaining balance = 900kcal
hour two = 700kcal (riding) minus 200kcal (drink)
Remaining balance = 400kcal
hour three = 700kcal (riding) minus 200kcal (drink)
Remaining balance = -100kcal (yes, negative)

Hmmm......you keep riding so those calories need to come from somewhere but you don't have any muscle glycogen left. So your body draws on your liver glycogen. Eventually, no more glycogen to the brain and you BONK! Yep, you get stupid, confused, and your judgement suffers -- just plain dangerous.

Folks will use the term bonk to describe lots of things when riding -- fatigue, dehydration, too much sun, whatever. But in the truest sense, a bonk is the lack of glycogen to feed your brain. I personally think everyone should do it at least once. There's no better lesson in proper nutrition than to bonk.

[velogirl]

I wasn't explicit about this in my previous post and I know some will wonder, so here are some additional tidbits.

1,400 kcal assumes you have a full tank when you start your ride. That means you need a good solid meal in the morning (assuming a morning ride).

Our bodies are capable of digesting and utilizing about 350 calories/hour. If you ingest more than that you'll get indigestion.

Given the above (350 calories) and assuming that you're burning a higher amount of calories/hour (which you undoubtedly will), cycling is a deficit sport -- you're always burning more calories than you can consume and digest.

The best way to fuel is to ingest small amounts of calories frequently. I recommend drinking or gelling or eating a bite of a bar every 10 - 15 minutes (for rides longer than 60 minutes). Unfortunately, many of us stop every hour or so, ingest all our calories while we're stopped, and then don't eat or drink on the bike until we stop again. Not the best way to fuel. This method will lead to indigestion and also won't allow your body to utilize the full calorie load.