Throughout steady state sub-maximal exercise (running) the predominant energy system employed is the aerobic system. Although all energy systems are simultaneously partially active to some extent during exercise however varying intensities and duration can fluctuate the body’s fuel selection resources. During steady state exercise (continuous steady pace) carbohydrates and fats from muscle glycogen, blood glucose, and lipids respectively are the primary substrates utilised by the metabolic system. As exercise duration increases, fat oxidation becomes more prevalent in providing energy. Conversely, an increase in intensity will result in muscle glycogen becoming the dominant energy source.
In plain terms, to burn more fat the duration has to be reasonably long with the intensity capped at 60% VO2 max for the desired fat burn effect. If the intensity of the exercise is increased beyond 60% VO2 max the primary fuel selection then turns to carbohydrates and less fat will be burned. For best fat burning results, exercise should be done fasted to maximise fat burning as there would be limited amounts of carbohydrates to burn. An optimum time period for this would be first thing in the morning when the body is almost glycogen depleted, at this time an ideal fat burning exercise would be a fasted run (without breakfast or drink) which should be performed at a reasonably low intensity for a minimum of 20 minutes and maximum of 60 minutes. Anything over 60 minutes muscle tissue may start to burn as well as fat especially when the body becomes tired. If you are exercising fasted, only perform steady state exercise such as running or cycling, do not perform any resistance type training with weights as this has been shown to rapidly breakdown muscle tissue unless carbohydrates have not been consumed beforehand.
Kevin Poole: MSc, BSc, Liverpool Personal Trainer