The debate starts on the topic of which body fuel is, and should be, used during exercise. Most commentators acknowledge that in low intensity exercise (eg walking, hiking and jogging), fat is metabolised for energy and the body respires aerobically. Fat is used for energy during aerobic respiration as oxygen is required to metabolise it. Aerobic respiration takes longer as oxygen needs to be circulated to the muscles for the process to be complete.
During high intensity exercise (eg running and sprinting), the body respires anaerobically – without oxygen – as energy is required faster than oxygen can be circulated to the muscles. Anaerobic respiration involves the partial breakdown of glucose (called glycolysis) producing a short burst of energy for a few minutes. Lactic acid is also produced during glycolysis and begins to take effect within minutes causing cramp and muscle fatigue. Stored carbohydrate – glycogen stored in the muscles – is metabolised during anaerobic respiration.
Glycogen is thought to provide fuel for approximately 2 hours of medium to high intensity exercise. On a personal level I would disagree with that. I would start to notice decreasing energy levels after around 1 hour 15 or 1 hour 30. After this time period (which is likely different for each individual) a new source of fuel is needed – either eating or drinking simple carbohydrates (sugar/glucose) or lowering the intensity level of your exercise so fat can be metabolised. So when competing in a sprint triathlon, running or cycling race, or a race that will last longer than 1hr 30 to 2 hrs (approximately) then simple carbohydrates – such as sports drinks are required.
Out of all body energy sources available – fat, carbohydrate and protein (which in extreme circumstances such as starvation can be converted to energy), fat has the highest energy content. Fat contains 9 kcals (food calories) per gram, carbohydrates and protein each contain 4 kcals per gram. So it makes sense to metabolise fat where possible but this may constrain you from exercising at a medium to high intensity.
This raises the question of whether the body can metabolise fat during medium to high intensity exercise.
The key to improving the body’s ability to metabolise fat at medium to high intensity exercise is to increase your lactate threshold. The lactate threshold is the maximum steady state effort that can be maintained without lactate continually increasing. Exercising above this level is where lactate production in the blood is faster than lactate clearance. After a few minutes cramps and muscle fatigue set in. Increasing your lactate threshold is done by exercising at an intensity that is periodically above your lactate threshold through interval training or fartlek (these entail training at periodic bursts of high intensity followed by lower intensity exercise). This forces the body to become more efficient at lactate clearance. While your body still will not be able to metabolise fat above your lactate threshold, it will be able to metabolise fat at a higher intensity of exercise and so use fat as a fuel for exercise more frequently.
Before beginning medium to high intensity exercise, or increasing the intensity of your exercise, we would recommend you seek professional advice.