Ask 20 different people what they eat before they exercise, and you’ll get 20 different answers: That’s because the best thing to put into your body before a workout is widely considered – by humans and many experts – a matter of personal preference.
One writer, for instance, says she runs 4.5 miles a day on an empty stomach, often pushing herself through vicious hunger pangs, only to reward herself with a calorie-bomb of a doughnut and a requisite coffee at the finish line.
Known as fasting cardio, or aerobic exercise performed on an empty stomach about 12 hours after your last meal, some athletes think this strategy will turn their bodies into a more efficient fat-burning machine. The theory: Your body’s go-to source of fuel is carbs. If you don’t provide it with that fuel, but still force it to move, it has no choice but to turn to plan B: Burning body fat to fuel the muscles and every other physiological system.
And while research suggests the body burns less fat when there are carbs in your system, there’s no proof that exercising when your stomach’s empty can actually change your body composition over time, mostly because studies tend to focus on the increase in fat burn during exercise, completely ignoring the body’s response throughout the other 23-plus hours of the day. So while that writer who runs through her hunger may burn a bit more fat than a woman who runs off the calories she ate at breakfast, fasting may not change one’s body composition much at all – especially if fasting works up an appetite that triggers poor eating decisions throughout the day.
Even if you eat like a kale-loving saint after fasting cardio, you may not see any visual improvements in the way your body looks. It’s unclear where the extra fat you may burn during fasting cardio comes from.
Diabetics and anyone with hypoglycemia or other health issues should probably forget about fasting, or at least consult a doctor before forgoing breakfast; Fasting before intense activity can make some people feel faint, and it can feed into disordered eating habits, in certain circumstances.