Exercising in the fasted state (ie, going training in the morning without consuming breakfast beforehand) is widely used as a fat loss tool.
But does fasted cardio actually work for fat loss?
There is a theoretical basis behind the fat burning advantage of fasted exercise and this is predicated upon increasing lipid oxidation during the training. When we sleep, we burn most of our glycogen stores in our body due to recovery processes during sleep and normal bodily functions. Therefore, it would be plausible that with no glycogen stores (carbohydrate stores), our bodies have to tap into our fat stores for energy whilst training. This ignores the dynamic nature of the human body which throughout the day continually adjusts its use of substrates for fuel (be it carbohydrates, fat or even proteins).
Any potential increases in fat oxidation from a fasted exercise state might be neutralized by an increase in the thermic effect of exercise from eating pre-exercise. In other words, our bodies would burn the same amount of calories (coming from fat or carbohydrates) regardless of training fasted or not.
To finalise this, whether we train in a fasted state or not will not affect fat burning in the long run. Our bodies adapt and change over the course of the day to various situations. If we want to lose weight, we must simply be in a calorie deficit throughout the day.
The take home Is that it doesn’t matter.
When you exercise without food, ie, early morning. We don’t have carbohydrate stores to use as energy (they have been used up whilst we sleep for repair). So the body uses fat reserves during that fasted session.
But afterwards, the body burns around 90% carbohydrates for the remainder of the day.
If you eat before training, you use up more carbohydrate stores as the primary fuel source. But for the remainder of the day, you burn more fat as the carbs have been used for fuel during the exercise!