In a few of my previous posts I have touched on how the body powers itself, where energy derived from, and the difference between Aerobic and Anaerobic metabolism.

Aerobic, very simply stated means that in order for the body to produce energy, Oxygen (O2) is required in the process.

So in order for those Carbohydrates to provide us with energy, the cell requires a certain amount of O2 to be present in the chemical reaction for energy to be produced.

Carbohydrates are the energy source (Substrate) most used by the body during physical activity, while Fats fill between 30%-80% of the bodies energy demands throughout the day.

It is interesting to note that an average, adult male has about 90 000 - 110 000 Kcal of Fat energy available for use at any given time, while he has only about 2 000 Kcal of Carbohydrate energy available.

Fats are by far the superior source of energy, prioviding 460 ATP molecules, versus a meager 36 ATP molecules from Carbohydrates.

Translated: Fats are more than ten times more energy dense than Carbohydrates.

Because most of our training and competing is predominantly Aerobic in nature, it stands to reason that we should look to better developing our Fat burning (Lipolytic) energy pathways, and thus conserve the limited (2 000 Kcal) amount of Carbohydrates we have for those instances when we require Anearobic bursts, or more powerfull accellerations.

In order for us to be able to use Fats as an energy source, there are many factors that need to be just right:
  1. Fat burns in the flame of Carbohydrates: There needs to be Carbohydrate energy avaialble to get the energy stored within the Fat out. (Somthing for you to think about if you are planning to go on a Zero Carbohydrate diet).
  2. The metabolism of Fats is in part regulated by the concentration of Lactic Acid in the body, meaning, if the Intensityof the activity gets to high, Lactic Acid production will increase, because of the increased use of Anaerobic energy production, and the ability to use Fats for energy gets reduced, and ultimately shut down. (Read this Post for more on Why You need Base Training).
By keeping the Intensity of exercise on the lower side of the Aerobicscale, the body has been shown to:
  • increase its use of Fats as an energy source
  • become more effective in the acquisition of energy from fats, and thus use less Carbohydrates.
  • store fat droplets in the muscle itself so fats are readily available for energy metabolism.
As an endurance athlete, the benefits are pretty obvious:
  • You will be able to tap into an almost limitless reservoir of energy (100 000 Kcal worth)
  • You'll have sufficient stored Carbohydrate energy (Glycogen), for when you need to crank things up a little more in training and racing.
  • Carrying less deadweight, because your reduced Fat Mass, can only help your running and cycling (maybe not your swimming buoyency though).
I have not forgottent about the Anaerobic Metabloism, and next week I will look at the energy systems responsible for Speed and Power.

James Greenwood is a competitive tri and multisport athlete currently training for Ironman Canada 2009. A level 1 Triathlon Coach, he holds a post graduate degree in Exercise Science, and is a Certified Strength and Conditioning Specialist through the NSCA. James is also currently the resident health and fitness programs expert at, and has starred in a number of multisport specific fitness videos.
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