I made it my goal today to try and find out whether exercising in fasted or fed state was better for fat oxidation.
This has been an ongoing debate for a long time. A brief background so that the subsequent rationale is clear: when we exercise, we need a fuel source. That fuel source usually comes in the form of carbohydrates, which in turn come from glycogen stored in the liver and muscle.
When we deplete our glycogen stores, the body must then look to other fuel sources, namely protein and fat. Protein is metabolised to give amino acids, and fat to give fatty acids and glycerol. These cannot be used as an energy source by themselves, but they can be fed into gluconeogenesis (making glucose) to provide fuel.
So the rationale goes: if you exercise in a state where you have little to no available carbohydrate, you will then start oxidising fat to use as energy, thus exercising in the fasted state provides a more efficient way to “burn fat” and lose weight.
It seems a reasonable enough thought-process. But is it true?
There are staunch opponents on either side, as I found out, and equally there are studies which support both sides. However most of these studies are rather short term and look at acute fat oxidation, and there are relatively few studies that look at long term change in body composition. Furthermore there are a number of confounding factors (e.g. individuals level of fitness influencing use of energy sources) that make this question very difficult to answer clearly.
The link above provides a very good assessment of the available evidence on the subject. Ultimately the conclusion is that while there has been a substantial body of work on the subject, there is no conclusive evidence to show that exercise in the fasted state significantly affects fat loss in the long term.
The author ends therefore by suggesting that if fat loss is your goal, to find a type of cardio that is comfortable, works for you and you can maintain on a regular basis. Not bad advice.