It is known that to lose body fat is to improve metabolic health; however diet alone is not effective to achieve this purpose. The combination of the correct diet with exercise is considered the most optimal and effective means. In recent years, due people desiring to find the effective fat loss mechanisms, numerous hypotheses have been formulated on the question:

Is fasting aerobic training an effective strategy to improve fat loss?

The theory is if we implement cardio exercise of mild intensity first thing in the morning (i.e. after an evening fast of 8 hours) it would increase the loss of body fat. This theory is supported by the fact that fasting, with low glycogen and low insulin levels, would increase the rate of oxidation of fats as an energy source. That means utilizing the fatty acids in the blood stream and elevating lipolytic hormones (fat burning hormones). This is not seen in exercise after having breakfast.

It has been found that training after having breakfast, would paralyze the mobilization of fatty acids, because carbohydrates would then be the first energy source used, and therefore a reduction in the rate of lipid oxidation.

There is further evidence, that a fasting cardiovascular training would help activate different metabolic signals that would produce an adaptation to long-term training in favor of improving or increasing the oxidation of fatty acids, such as an improvement in mitochondrial bio genesis (fatty acids are oxidized in the mitochondria). Other possible benefits of fasted training are increased insulin sensitivity and better glucose tolerance. This would be very important in populations with metabolic syndrome, type 2 diabetes, insulin resistance, or in high-level athletes, where their carbohydrate intake is quite high. Fasting cardiovascular training would therefore help them have a better tolerance when consuming carbohydrates.

To add to the conversation, when we are discussing fat loss, what ultimately counts is the global calorie consumption over a long period time, and not just a specific moment after doing a type of fasting exercise. That is to say – are we only concerned with the immediate loss of fat?

A study by Shoenfeld B et al (2014) assessed differences in body composition (fat loss and muscle gain) in a group of healthy women who followed a supervised low-calorie diet. They were divided into 2 groups, one of which had to exercise in the first hour of the morning, and the other group after having breakfast. The training consisted of an hour of mild cardio for 3 times a week, and the diet was a caloric deficit of 500 gms per day according to the needs of the participants. The results showed to have no difference in both groups with respect to body composition, body mass index, fat mass and fat-free mass (lean mass).

Van Proeyen et al, 2016 conducted a study on a group of healthy individuals, using a isocalaric diet, and there were no differences in body composition in both groups at the end of the study, suggesting that the caloric global computation would play an important role beyond fasting per se. The same thing happened in the study with obese people, no group showed significant differences in body composition but both lost fat mass from the lower and abdominal area.

Even so we cannot generalize these results with the dynamics of our human nature. Our body uses different energy sources during the day physiologically, and although at a specific time there is a higher rate of lipid oxidation, this could be compensated with increased use of carbohydrates later in the day if there is an intake thereof. Therefore, lipid oxidation should be considered as a process that occurs over time and not in one hour of exercise a day.


Our conclusion is that caloric balance is still paramount for a decisive body composition.

Thus, for those looking for an improvement in body composition or fat loss, a Gentle cardio exercise will have the same result if you fast or not. If, on the contrary, our goal was to improve at the level of metabolic signaling, including a fasting exercise in our training schedule would be optimal and effective in improving our health parameters.

This is only a general recommendation to guide the population, but any treatment or guideline must be according to individualized preferences and as long as overall calorie balance is controlled over time. Our priority is individualization and is the key to success in the final results.


Sandra Serrano, Physiotherapist, Msc Clinical & Sport Nutrition,  | Dubai Herbal and Treatment Center

Eat Well & Keep moving