One of the “tricks” bodybuilders, fitness models and other physique athletes commonly use to maximize fat loss is to do their cardio first thing in the morning on an empty stomach. For those following an intermittent fasting (IF) eating plan, this “fasted cardio” would take place at some point outside of the feeding window.

The theory behind fasted cardio is that, since you don’t have any food in your system, the body will turn to body fat for energy that much quicker—more or less immediately. The result (again, in theory) would be that a 30-minute cardio session would be essentially burning body fat for energy for the entire 30 minutes.

Sounds great. But does fasted cardio really work that way? Will hopping on the treadmill or elliptical first thing in the morning, before breakfast, really have you burning more fat for energy than if you did it after work or on your lunch break?

The truth is: Yes, but also no.

The first thing to remember is that nothing is so black and white when it comes to your body’s physiology, especially when you factor in both formal exercise and normal daily functions.

Research does, in fact, suggest that fasted cardio in the morning may burn more fat as an energy source. A recent study from the UK published in the British Journal of Nutrition found that when subjects were in a fasted state during their morning cardio, they burned more fat than when they had a meal before the cardio.

So yes, you may burn more fat for energy with fasted cardio. The physiological reason I use to explain this is that, after you've been fasting overnight while you sleep, your body conserves the precious carb stores it has because glucose (what blood sugar is composed of) is the primary fuel for the nervous system. During the night, your body moves toward burning more fat for energy. However, it also moves toward breaking down more muscle protein and converting the amino acids into glucose for fuel.

When you do cardio in the morning before eating anything, your body is forced to burn more fat for fuel. But it's also burning up more muscle protein for fuel. That's not a huge problem, provided you consume a fast-digesting protein like whey (along with some casein protein) after the cardio to replenish the amino acids you lost and rebuild the muscle protein that was broken down. Even then, if gaining muscle mass is your primary goal, fasted cardio probably isn’t your best option.

All that said, I personally do a bit of fasted cardio myself, though I recommend it only for certain people at certain times. Anecdotally, I've found that fasted cardio can work well for men with body fat levels in the low single digits (somewhere around 5%-6% body fat) or females with body fat in the low teens (somewhere around 13%-14% body fat) who have one stubborn area of unwanted body fat.

For example, many males, especially older males, tend to hold fat on the lower back and obliques; many females tend to hold fat on the hips and thighs. And no matter how hard they train and diet, this fat often holds on for dear life. From my experiences and observations, I’ve found that fasted cardio can work well to get rid that last bit of fat.

Although there’s no direct data to look at, it may be that when a person is so low in body fat and only have fat on their stubborn areas, exercising in a fasted state may be the spark that those resistant fat cells need to release stored fat. But if you're a male at about 8% body fat or more, or a female at about 16% or more, fasted cardio is likely not going to make much of a difference in your fat loss efforts.

Just do cardio—I would suggest some form of HIIT—and consume a clean diet. You can expect to get leaner without having to train on an empty stomach!