If burning fat and shedding weight is your primary exercise goal, Viljoen says doing a moderate-intensity session on an empty stomach could help you burn a bit of extra fat during the session.
“However, there's a big misconception about how big and worthwhile this effect really is. Firstly, the amount of body fat you can burn during exercise is actually quite limited - and while you do burn fat it is in small quantities. The real benefits of exercise for weight loss comes more from how well it helps to raise your body's metabolic rate and to build or maintain muscle mass. Both of these factors help with fat-burning throughout a 24-hour day.
“Secondly, skipping breakfast and exercising on low energy stores makes the exercise feel harder than it really is, so you're not able to push hard for long enough. Nor does it raise your metabolic rate as much as you might have been able to if you had had a light meal before your session.”
So if weight-loss is your goal, Viljoen advises rather keeping to a pattern of wholesome regular meals, at least three times a day, and says you should practice portion control so you don’t end up putting back double the kilojoules you just burned.
Overall, Viljoen recommends you have a balanced, healthy diet the majority of the time as a bad diet will cancel out most of your efforts at the gym and what you eat before or after your session won’t make much difference then.