Not eating before a workout burns more body fat and we lose weight faster.
We often hear about the “fat burning zone” in fitness so it seems logical that if we don’t eat before a workout and stay in the fat burning zone, we will only use body fat to fuel our workouts. This is a good example of how a kernel of truth gets overblown and becomes nutritional dogma.
Fasted cardio does not burn more fat or cause people to lose more body weight any more than doing cardio in the fed state. Losing weight and/or body fat is all about a total hypo-caloric deficit.
A small study published in the International Journal of Sports Nutrition studied the effects of fasted cardio versus fed cardio in a group of 20 healthy females (1). All study participants were given a calorie goal that would induce weight loss. The only difference was that some of the participants did fasted cardio while the others had a meal. This study showed there were no differences in body weight or body fat changes between the two groups.
The key nutrients providing energy during exercise are carbohydrates (in the form of glycogen) and fat. At all times the body is using a “fuel mix” that includes both carbohydrates and fat. However, when exercise intensity goes up, the need to use carbohydrates goes up because it can produce energy more quickly (2).
To ensure we burn mostly body fat and not muscle, it’s important to keep a supply of glycogen in our muscles that we can use for fuel instead of protein.
Over time, training with a lower caloric intake may limit our ability to keep up with a fitness routine, lower our performance, and can cause a loss of muscle mass. Since muscle mass burns more energy at rest than fat, we want to preserve as much muscle as possible when trying to lose weight (3).
The Bottom Line
Overall energy balance (energy in versus energy out) will determine weight gain, loss, or maintenance.
Running, walking, and cycling are all forms of aerobic activity that will burn additional calories. If motivation levels are higher in the morning before eating breakfast then maybe fasted cardio can work for you. If you have more time and energy in the afternoon to get your cardio in, then that’s also great! Whether we are in the fasted state or not, it is the overall calorie deficit that will cause weight loss and body composition changes.