C[omp]ute

From: andrew cooke <andrew@...>

Date: Sat, 9 Dec 2017 15:40:35 -0300

"Fuel" rather than "Nutrition" because I am thinking mainly about the
calories.  The following thoughts were prompted by a paper I read
online which I can no longer find - repeating the search leads me to
other, more complex studies.  So not only do I not have a reference,
but what follows may be oversimplified.


Glycogen v Fat

When exercising aerobically we burn fuel from three main sources:

 * Fat reserves, which are virtually unlimited (for most of us), but
   slow to access.  Fat reserves are stored throughout the body.

 * Glycogen reserves, which are limited but easier to access.
   Glycogen reserves are local to the muscle being used.

 * External sources (ie eating).

At low exertion levels the body preferentially burns fat.  But as the
power output increases this cannot be accessed quickly enough and the
proportion of glycogen increases.

Since the amount of glycogen is limited this explains "bonking" or
"the wall" - the unpleasant experience of glycogen reserves being
depleted.  As you would expect, this can be postponed by:

 * Lowering power output (and so reducing both absolute and relative
   burn rates for glycogen)

 * Increasing glycogen reserves.  Either via training or carb-loading.

 * Eating while exercising.  An external source of fuel will displace
   the need for glycogen / fat burning, but is limited in volume (you
   simply can't eat that much while exercising) and delayed (it takes
   time to digest what you eat).

Given all the above it is clear that the need to eat while doing
endurance exercise will depend on both the size of glycogen reserves
and the power levels required - both of which can vary strongly from
person to person.


Glycemic Complexity

One factor briefly mentioned above is digestion time.  High glycemic
index carbs are those that are easy to digest and provide a large
amount of energy for a short time, while low index carbs are slow to
digest, providing a lower, more extended boost.

Curiously, the index doesn't match the difference between simple and
complex sugars: white bread (starch) apparently has a higher index
than sugar (sucrose).

This is important for me because, it turns out, the time I stop riding
to eat is quite significant in how well I perform (over longer
segments on Strava).  So I guess I should look eating porridge before
a ride...


Andrew