The Longevity exercise physiology team at EdgecliffMarrickville, Randwick, Lindfield and Drummoyne take another look at fuel selection and how it is affected by intensity.

In part 1 of this blog series, we covered exercise duration and fuel selection in 0-5 second, 5-30 second and 30-60 second efforts. This is the amount of time it takes for your aerobic (oxygen-relying) system to kick in and take over as the primary source of energy. By the time 3 minutes of steady-state exercise has passed, you are completely relying on your aerobic system and this is often when you feel a “second wind”. The vast majority aerobic energy will come from either carbohydrates or fat. In this blog, we will address when you would use each fuel source, based on your exercise intensity.


As humans, the most efficient way for us to store energy (when excess energy is consumed) is in adipose tissue (body fat). This energy source is very inefficient but it will last a long time, making it perfect to sustain our relatively low energy requirements at rest through a process called lipolysis. Fat will provide nearly all of your energy required when you are relaxing or sleeping and your primary energy requirements come from thinking, breathing and your heart beating. This body fat can be found anywhere in your body, including fat within your muscles.

Low intensity

During a light walk or a slow jog (depending on your fitness level) fat will not be able to completely keep up with your energy requirements, so carbohydrate sources will begin to kick in. This can be either local stores within the muscle (glycogen) or sugar circulating in your blood (glucose). At low intensities, it is reasonable to expect 85% of your energy to come from fat and 15% of your energy to come from carbohydrates.

Moderate intensity

As you would expect, increasing intensity requires even more contribution from a faster carbohydrate source as fat sources struggle to keep up. Depending on your fitness level, moderate intensity could be a fast walk, walking uphill or going for a jog. At this intensity, it is reasonable to expect 50% contribution from carbohydrate sources and 50% contribution from fat sources. This is often referred to as the “fat burning zone” as you will burn the highest quantity of fat per minute when exercising at this intensity.

High intensity

The rate of energy expenditure during high intensity activity means that the only fuel source able to sustain this effort will be carbohydrates. Essentially, this will be the case if you are told to travel as far as you can in a fixed period of time longer than 1-2 minutes.

Although this is always true, there are very complex interactions long term with adaptations to exercise and reasons that training in the “fat burning zone” is futile in efforts to lose weight and high intensity training may be more indicated for you.


In order to find out how to achieve your goals and what intensities you should be training at you should consult an Accredited Exercise Physiologist at Longevity EdgecliffMarrickville, Randwick, Lindfield and Drummoyne.

Give us a call on 1300 964 002 to book an initial consultation.