On the Bike Nutrition

So, how much to eat?  That is the source of a lot of confusion among triathletes and
after surfing a few sites, I can understand why.  Since, the famous Coyle, Coggan et al.
study of 1986 showing that ingested carbohydrates prolong and maximize exercise
duration, it is common knowledge that you must eat in races longer than about 2.5-3
hours.  I had a hard time finding any sources (on the net, not in research) that
recommended ingesting less than 100 cal per hour.  On the other hand I did read some
disturbing information leaning in the other direction.  Some, seemingly or
self-proclaimed experts, have extensive training diaries on their site and it is routine to
read them consuming 600-800 cal per hour on the bike of an iron distance race.

Let's get down to brass tacks.  Let's consider, for simplicity sake, just carbohydrate
(CHO) catabolism, since that is generally what people ingest during a race.  Yes, we do
burn fat (YEAH!) during prolonged exercise, but we don't need to ingest it---even a very
fit male triathlete (160, 8% body fat) has enough fat calories (over 50,000 !!!) to run
about 10 ironmans.

Optimally, what you would try to do is ingest CHO calories as fast as you can burn them.
We generally burn or oxidize CHO at a rate of 1 to 1.5 grams per minute.  The most
ever recorded in a laboratory study was 2.5 g/min and that was with infusing CHO
directly into the blood via IV fluids and it increased blood glucose concentrations to 10
mmol per liter, twice what is normal.

So let's give you the benefit of the doubt and say that you can oxidize 2 g/min.  This
comes to 120g of CHO per hour.  4 calories per gram of CHO gives you 480 cal per
hour.  So if roughly 500 cal/hour is a stretch then what is 800 cal/hour doing to your
body?  Increasing your risk of GI distress for starters.  But there is something deeper
going on here.  A main limiter of our ability to oxidize CHO is how quickly glycogen can
be released from the liver.  This is a very constant rate of 1 g/min.  So what happens to
that excess CHO that we can't oxidize right now?  It has to be stored back in the liver as
glycogen.  So you may think, "great, I can just use it later, right?"  Well....maybe.  The
problem is that insulin needs to be released to get it stored back into the liver.  This
causes your body the difficulty of simultaneously exerting itself or being pushed to
mobilize substrates, while at the same time responding to a hormone that is pushing it
to store substrates.  Probably not the best thing to do to your metabolic systems when
you have to finish your six hour ride with a 42.2 km run.