ENERGY SOURCES

When you turn the ignition key in your car, some source of fuel (gas, diesel, or
fermented organic manure) must be delivered to the engine for it to fire. Even
though the spark to ignite it may be there, if the fuel tank is empty, the engine
will not fire. Muscles burn a substance called adenosine triphosphate (ATP) and,
like a car engine, must have an ongoing supply available to continue doing work.
Whenever you place a demand on the muscles to perform a task, such as turning
the pedals, the energy systems must work continuously to convert energy sources
into ATP to keep up with that demand.

The fuel necessary for activated muscles to contract is delivered through one
of the body’s two energy systems—aerobic and anaerobic. Energy stored in the
body is converted to fuel and delivered to the working muscles through one or
both of these energy systems. The energy sources that supply these energy systems are carbohydrates, fats, and, to some extent, proteins.

When you are riding, the preferred source of fuel changes as your intensity
increases. At slow, easy paces, the body can burn mostly fat to produce enough
energy to support muscle contractions. As you ride faster and harder, your muscles
will start craving carbohydrates to keep up with the demands of a higher effort
level. Carbohydrates ignite and burn much like the matches and kindling that you
use to start a fire. Fats are more like the logs on the fire that burn slowly and can
produce a long-lasting fuel source. Your body has a limited supply of matches to
burn, so you need to learn to use them sparingly and replenish them frequently.

CARBOHYDRATES AS FUEL

Carbohydrates are a powerful but limited energy source that can be quickly
converted to fuel for muscle contractions. Carbohydrates are either stored in the
body within the muscles and liver or are moving around within the blood. Those stored
in the liver and muscles take on a slightly different configuration called glycogen.
Glycogen stored in the muscles is available for rapid conversion to fuel but can be
depleted quickly once you step on the gas. Muscles can develop the ability to store
larger quantities of glycogen and to spare, or conserve, those carbohydrates.
The way you train plays a major role in how your muscles work and use carbohydrates.

One of the main objectives of Base Building for Cyclists is to teach you how to train your muscles to store more carbohydrates but use less of that stored energy to perform a given task. This will make you a more fuel-efficient athlete. Efficiency is a major factor in athletic performance. One clear advantage of using fewer carbohydrates to accomplish a given effort is that you have more of this high-power energy available for when you need it most. Conserving fuel means that you can ride longer or harder when you need to. Your muscles will demand carbohydrate energy when you’re chasing a breakaway, battling a headwind, climbing a tough hill, or outsprinting an angry pit bull. These are all good reasons to ration your limited supply of carbohydrates! The small amount of carbohydrates found circulating in the blood take a form called glucose. The amount of glucose in your blood is also referred to as your blood sugar level. If your blood sugar levels run low, the body will release glycogen stored in the liver to maintain appropriate glucose levels. The liver can store only a few hundred calories of carbohydrate, which will quickly become depleted, so it is best to maintain healthy blood glucose levels through ingestion and digestion of a carbohydrate source such as a sports drink or gel and water while cycling for extended or intense periods of time. The liver will also release stored glycogen into the bloodstream throughout the night while you’re sleeping to maintain blood sugar levels. This means that your liver will be low or depleted of glycogen in the morning and will need to be refilled if you want to have your carbohydrate stores fully loaded to support that day’s activities. The process of ingesting, digesting, and storing carbohydrates within the liver can take three to four hours; so if you have an important event that will demand a lot of carbohydrate energy, you’ll want to eat a meal several hours before the start of your event. Liquid fuel sources tend to digest faster and may be a better choice for prerace meals.