'Tthere is some truth to that,” says Javier Gonzalez, professor of nutrition and metabolism at the University of Bath. “When we exercise, we always burn a mixture of fuels – mainly carbohydrates and fats. If you fasted all night, you tend to burn a little more fat and fewer carbohydrates than if you ate a breakfast, especially a high-carb breakfast.” But this doesn't mean that working out on an empty stomach is better for losing weight.
“We can only store small amounts of carbohydrates as glycogen in our muscles and liver. Any extra energy – from carbohydrates, fat or protein – ends up being stored as fat in the body. So to lose fat, you need to be in an energy deficit: burning more energy than you consume. Otherwise, it doesn't matter whether you fast or eat – your body balances things out over time,” Gonzalez says.
However, this does not make training on an empty stomach pointless. Research, including Gonzalez's ownsuggests that fasted training may improve metabolic health slightly, particularly how well muscles adapt to exercise and how the body copes with blood sugar spikes that can lead to energy crashes. “Exercising in a fasted state appears to stimulate muscles to clear glucose from the bloodstream,” he says. “So it may improve blood sugar control, especially after meals.”
The effect is most noticeable during endurance exercise such as jogging or cycling. During higher intensity exercise, such as sprint intervals, the body relies heavily on carbohydrates anyway. Overall, the benefits of training on an empty stomach are modest.
“The most important thing is to do the exercise. Whether you do it fasted or fed is just the icing on the cake,” Gonzalez says. If you want to try it, start small – 15 or 20 minutes – and gradually increase the time. “If you feel dizzy or dizzy, that’s your cue to stop and eat something.”
