As a Coach and Trainer of athletes of all levels, it amazes me how little understanding exists amongst athletes about the nature of how the body produces the energy that drives it.

The body gets its energy from the food we eat every day, and from this food, our body is able to extract the energy it requires to keep us alive and for physical activity requirements.

The body can derive energy from Carbohydrates, Fats and Proteins, but is most efficient in metabolizing Carbohydrates, through a process know as Glycolysis.

If the body does not require the energy derived from food immediately, it stores it either as Glycogen (the storage form of Carbohydrates) in the muscles and the liver, or as fat.

Fats are an unbelievably rich source of energy, however, the process of Lipolysis (the process of getting the energy from the fat molecule), is not the bodies first choice for energy metabolism (Glycolysis is).

The main problem with Glycolysis, is that the stores of Glycogen in the body are very limited, and can only provide around 60 minutes of energy, before the stores are depleted, and if you are an Endurance athlete, you will experience a Bonk first hand.
Bonking is a phenomenon that occurs when the bodies stores of Carbohydrate energy are depleted to the point that the body will start to look to Fats and Proteins as its primary energy provider - not the most effective or efficient system of energy production.

There is also a lot of uncertainty around the terms Aerobic and Anaerobic amongst athletes in all sports. The meanings of these words are very important in your understanding of how the body produces energy.

Aerobic means in the presence of the Oxygen, which means that the chemical reactions that produce energy need Oxygen in the process.

Anaerobic means that the energy is in an a form that is immediately usable.

The Aerobic pathway utilizes Fats and Carbohydrates as its primary energy sources, while the Anaerobic pathway uses Carbohydrates.

I often get asked about the relationship that exists between Fats and Carbohydrates. It is a simple one:

As the intensity of exercise increases, the bodies reliance on Fats decreases, and its utilization of Carbohydrates increases, until it is relying almost 100% on Carbohydrates.

This is not ideal because as you know, these Carbohydrate energy sources are limited, and it is very difficult (if not impossible), to replace the energy that has been spent, Calorie for Calorie.

This is why, as Endurance athletes, a large percentage of your training is done at in the Aerobic training zones. The logic behind this approach is to attempt to reduce the bodies reliance on Carbohydrates, and at the same time, improve its ability to use Fats as its primary energy source.

This will leave your Glycogen stores available for when you require activity at a higher intensity (Anaerobic efforts), and help you avoid the dreaded Bonk.

The bottom line is that in order for the body to perform at an optimal level, the right mix of fuels need to be acquired i.e. the correct balance between Carbohydrates and Fats.

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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