Exercising early in the morning can be hard and this can be due to many reasons such as waking up earlier than you’d have liked to or due to the lack of energy that is coming from food in your system. However, it is debatable whether it is a good idea to workout on an empty stomach or if this fasted state is going to cause you adverse effects such as metabolic disbalance.
Let’s take for instance waking up in the morning after 6-12 hour overnight fast. This state will cause your body to somewhat deplete its glycogen and therefore activate the fat burning process by mobilizing the usage of fat as energy to compensate the low glycogen levels. However, as we know when we eat food, our body starts to produce insulin which interferes with the mobilization of body fat.
As the carbohydrates (in the form of glucose) levels are low in the blood stream caused by the overnight fast, more calories from stored fat are burned when exercising to compensate the low glucose levels. Therefore in fasted state less insulin is present which ultimately links to burn more calories that come from stored fats when you do cardio in such fasted state.
Doing cardio in the morning will allow your metabolism to remain high for a period of time after the workout is done and take advantage of the after-burn effect. Of course, you will benefit from doing cardio in the evening, but you will impair the effect of the after burn effect because your metabolic rate drops drastically as soon as you go to sleep. Some researches support this theory, where it has been tested on subjects who burned 1kg of fat faster when exercised in fasted state in the morning, compared to individuals who had few meals throughout the day exercised later.
A study on respiratory gas exchange, caloric expenditure, and carbohydrate/fatty acid metabolism in individuals who exercised after 12 hours of fast has shown 67% of the energy expenditure that came from fat, compared to 50% expenditure achieved when individuals who did the same exercise later in the day or after having food. Also, another study supports the hypothesis that endurance training enhances lipid oxidation in men after a 12 hour overnight fast.
Of course both methods of exercising either in fasted or fed state has their own pros and cons. This highly depends on the individual and their goals. Some other benefits of training on empty stomach can be improved performance and helps avoid stomach upsets. On the other hand there are negatives as well which could be decreased ability to work at higher intensity for longer, lower stamina, and can lead to muscle loss.
As previously mentioned both methods have their benefits and its side effects, this depends all on the individual and its goals. At the end, it is very important to follow your body and find what works and feels best for it.
Read, F. C. M. F. cardio on an empty stomach.
Aceto, Chris. Everything you need to know about fat loss. Club Creavalle, Inc. (1997)
Bergman, BC, Brooks, GA. Respiratory gas-exchange ratios during graded exercise in fed and fasted trained and untrained men. Journal of Applied Physiology. (1999) 86: 2.
Brehm, B.A., and Gutin, B. Recovery energy expenditure for steady state exercise in runners and non-exercisers. Medicine and Science in Sports and Exercise. (1986) 18: 205,