The going belief in the fitness world has long been that working out on an empty stomach results in increased fat loss. In fact, a 2013 study published in the British Journal of Nutrition concluded that working out on an empty stomach, or fasted, burned nearly 20 per cent more fat than doing so after breakfast.
But nutritionists warn that this might not be the best course of action. The logic behind increased fat burn is that when hungry, the body has depleted glycogen stores — stored up carbohydrates that give us energy — and so it needs to use fat as fuel. It sounds like a great way to get fit faster, but it could actually be counterproductive.
“If someone hasn’t eaten in an extended period of time — and we talk about fasted exercise as taking place first thing in the morning, so your last meal would have been dinner the night before — your glycogen is depleted and your body will use fat as fuel, but it could also use muscle,” says Kirstin Schell, a registered holistic nutritionist. “And since muscle fuels our metabolism, it’s actually not very helpful.”
Studies have shown that doing a regular exercise routine at your normal intensity would not cause you to tap into muscle mass for added energy, but if your goal is to increase your intensity (and if you want to achieve results, you’ll need to do this), then you will use valuable muscle to fuel your workout.
Schell says another downside to exercising fasted is that you’ll simply have less energy to push yourself harder. And while a study published in the Journal of Science and Medicine in Sport showed that the effects of fasted exercise are different on men than on women — they’re more beneficial for men — both groups showed decreased performance when they worked out on an empty stomach.
Plus, you run the risk of experiencing the natural effects of physical exertion without sufficient nutritional fuel: lightheadedness.
Experts also point out that since the body doesn’t like to be starved, your metabolism will respond to the increase in fat burn by slowing down. And the next time you eat, it will inherently store more fat to compensate for the loss.
Instead of hitting the treadmill or the running trail hungry, go for a breakfast that amounts to 200 to 300 calories, and includes a balance of protein, carbohydrates and fat.
“You want something that’s easily digestible but provides you with enough energy,” Schell says. “Try a banana with nut butter or a small smoothie with Greek yogurt, a piece of fruit and some protein powder. Overall, it’s pretty small in terms of volume but energy dense so you won’t crash.”
If you’re working out later in the day, Schell says there’s less risk of being in a fasted state, even if you ate at noon and are hitting the gym at 6 p.m., but you could supplement with a snack-sized meal about an hour before to top out your energy stores.