The idea behind fasted cardio is that your body is more likely to use its own fat stores for fuel instead of the food you just ate. Fasted cardio can indeed work, because it helps you to be more efficient at using fat for fuel. Carbohydrates are the most available fuel source for your body, but only a small amount of them can be stored in the liver and skeletal muscles in the form of glycogen. On the other hand, fats are longer term energy sources stored in muscle, liver, and fat cells. The process of breaking down fat takes much longer than the process of breaking down carbohydrates. Even in a fasted state, it can take up to one hour to deplete the glycogen (carbohydrate) stores and begin using the stored fats (lipolysis). However, hormones, diet, and exercise intensity can all make it possible to increase the use of carbohydrate stores and quicken the shift to using fats as energy sources.
After not eating for 10 hours, your insulin levels are very low. This is important because insulin inhibits the breakdown of fats. Thus, your body is better able to release and transport fatty acids to be oxidized. Your blood sugar is low in the morning, and there is not enough glucose to sustain your workout. This forces your body to begin using stored fats for energy. In addition, since insulin is low, human growth hormone is higher. Growth hormone increases the rate of fat burning and is produced at higher rates while you sleep and are in the fasted state.
The timing of your carbohydrate intake can also be critical. Avoid high glycemic carbohydrates later in the day. The GI or glycemic index indicates how quickly the blood glucose levels will rise upon consumption. Refined, white flour foods such as crackers, rice, noodles, cereals, and pasta are generally higher on the glycemic index. Eating high GI foods may slow down the rate that you deplete carbohydrates, making it harder for your body to begin breaking down fats. Try to make the last meal of the day a high protein food and a low GI carbohydrate, such as vegetables.
The intensity of exercise is another possible factor. As a person’s RER (respiratory exchange ratio) increases, the breakdown of carbohydrates increases and the breakdown of lipids or fat stores decreases. Thus, longer steady states of exercise could be better for fasted morning sessions. However, highly conditioned, regular exercisers are more likely to deplete glycogen and start using fat sooner than beginner exercisers.
In summary, fasted cardio can be effective in burning more fat if you follow a low glycemic diet, limit your carb intake later in the day, perform moderate intensity aerobic exercise for a sustained period of time, and perform cardiovascular exercise on a regular basis.
Here is an example of one of my recent fasted cardio sessions. I did utilize a few intervals during this session but kept the intensity at a moderate level and ended with a sustained period of lower intensity exercise. Please note that incline walking lunges are an advanced exercise, and may not be suitable for everyone. You should slowly build up strength for shorter segments (such as 3 to 5 minutes) before attempting longer periods of this exercise.
5 minutes warm up walking at a level 4
1 minute jog at a level 6
1 minute run at a level 7 to 8
1 minute walk at a level 4.5
Repeat above 3 intervals 5 times
25 minutes of incline walking lunges: incline of 15 and a speed of 1.5
5 minutes cool down walking at a level 4 (no incline)
Bahadori, B. et at. “A mini-fast with exercise” protocol for fat loss. Medical Hypotheses, 2009: 73(4), 619-622.
Hansen, K., Shriver, T., & Schoeller, D. The effects of exercise on the storage and oxidation of dietary fats. Sports Medicine, 2005: 35, 363-373.
Greenwood, T., PH.D Running on Empty: Can You Burn More Fat on An Empty Stomach? Fitness Rx for Women, December 5, 2013
Schoenfield, B. Does cardio after an overnight fast maximize fat loss? Strength & Conditioning Journal, 2011: 33(1), 23-25.