Because your body relies on stored carbohydrate, or glycogen, in the liver to keep your glucose-dependent organs functioning during sleep, you will have relatively little carbohydrate available immediately after you wake up. Therefore, your muscles must rely more on fat as a fuel source to conserve carbohydrates. However, this may not contribute to body fat loss if you compensate for your calorie burn later in the day. Additionally, your overall metabolism will slow down if you do not eat soon after waking up, reducing overall calorie burn. Furthermore, fat is a less-efficient energy source than carbohydrate, so if you exercise on an empty stomach, your workout intensity and duration may be compromised.
Metabolism During Sleep
During an overnight fast, your body relies on stored fuels to keep your vital organs functioning. Your liver breaks down glycogen into glucose, which is the only energy source for red blood cells and the primary one for the brain and neural tissues. Enzymes break down fat tissue into fatty acids to provide fuel for most of your other tissues, such as your muscles and kidneys. Your liver can also break these fatty acids down into ketone bodies, which are another fuel source for your muscles and kidneys.
When you wake up and get out of bed, your muscles are more active than when you are asleep; therefore, you are burning slightly more energy. If you do not eat anything, this energy required to move your muscles continues to come from your fat stores, ketone bodies and muscle glycogen. However, if you continue to fast, your body will increase its calorie-burning efficiency to conserve fuel. Although you will be burning a greater proportion of fat if you skip breakfast, your overall calorie burn will be lower than if you were to eat a meal.
Breakfast and Weight Loss
Skipping breakfast may result in burning more fat in the morning, but it likely does not promote long-term weight loss. A 2003 study published in "Journal of the American College of Nutrition" showed that in a population-based group of more than 16,000 individuals, those who had skipped breakfast had a significantly higher body mass index than those who ate breakfast, even though their overall calorie intake was lower than breakfast eaters. Although this does not establish that breakfast-skipping causes obesity, it suggests that it is not effective in promoting overall fat burn.
Exercising on an Empty Stomach
If you exercise after an overnight fast, you will use a high proportion of fat as a fuel source since your carbohydrate stores are low. However, it may not lead to long-term body fat loss if the low muscle glycogen reduces your ability to train at a high intensity. A 2010 study published in "Medicine and Science in Sport and Exercise" showed that the power output during high-intensity interval cycling was significantly lower when cyclists were energy-depleted, even though they burned more fat. Since high-intensity training may increase overall fat and calorie burn for several hours after exercise, you may burn more fat in the long run if you fuel your high-intensity workouts beforehand.