My answer is YES. And NO.
Nothing is simple when it comes to the physiology and biochemistry of the human body, especially when you throw in exercise on top of normal daily function. Research does in fact actually show that when you do cardio fasted in the morning you burn more fat. A recent study from the UK published in the British Journal of Nutrition found that when subjects were fasted when they did morning cardio they burned 20% more fat than when they had a meal before the cardio. Several earlier studies show similar results.
So yes, you do burn more fat when you do fasted cardio. The reason for 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. However, it also moves toward breaking down more muscle protein and converting the amino acids into glucose for fuel. So 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. That’s not a huge problem as long as you consume a fast-digesting protein like whey (along with some casein protein, of course) after the cardio to replenish the lost aminos and rebuild back the muscle protein that was broken down. You can see why this is not ideal if gaining muscle mass is your primary goal.
One problem with the aforementioned studies looking at just how much fat is burned during exercise when a person is fasted or fed is the fact that focusing only on how many calories and how much fat are burned during exercise is a flawed approach. The real benefit of exercise, particularly HIIT cardio and weight lifting, is the fact that it burns more calories and fat after the workout is over – for the rest of the day while you’re doing nothing. This is where the benefits of exercise really come from in regards to fat loss. For example, studies show that HIIT workouts that take far less than 50% of the time as slow and steady cardio workouts lead to as much as twice the fat loss as long steady-state cardio workout. So even though the slow and steady cardio may burn more total calories and fat during the actual workouts, the HIIT programs led to greater fat loss. This is because HIIT workouts burn more calories and fat the rest of the day, which adds up to more calories and fat than you can burn during a workout.
So for most people, your best bet is to not worry about doing cardio fasted first thing in the morning. If doing cardio at this time is best for your schedule, then by all means do it then. But I would advise that, depending on your diet, you have at least a protein shake after the cardio workout. And if your goal is mass gaining, then I highly recommend you have a protein shake and some carbs, such as fruit, before the workout as well. If you’re trying to limit carb intake, you may want to avoid the carbs until after the workout is over. But again, that depends more on your diet.
With all that being said, I personally do a bit of fasted cardio myself and do recommend it ONLY for certain people at certain times. Anecdotally, I’ve found that fasted cardio can work well for men with body fat that is 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 holds on for dear life. Over the years I have found that once they have dropped the majority of the subcutaneous fat on the rest of the body, fasted cardio does seem to work well to rid that last bit of fat. Although there is 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 that stored fat so that it can be burned away for good. But if you’re a male at about 8% body fat or more, or a female at about 16% or more, then fasted cardio is likely not going to make much of a difference in your fat loss efforts. Just do cardio, preferably some form of HIIT, consume a clean diet, and the fat will come off.
Gonzalez, J. T., et al. Breakfast and exercise contingently affect postprandial metabolism and energy balance in physically active males. Br J Nutr. 23:1-12, 2013.
Deighton, K., et al. Appetite, energy intake and resting metabolic responses to 60 min treadmill running performed in a fasted versus a postprandial state. Appetite. 2012 Jun;58(3):946-54.
Hansen, K., et al.. The effects of exercise on the storage and oxidation of dietary fat. Sports Med 35: 363-373, 2005.