Fasted vs Unfasted Cardio: Which is Better for Fat Loss?

In the fitness world, there has always been a large amount of unfounded advice on which training regime is the most effective for fat loss. One of these claims is that fasted cardio is vastly superior for lipolysis as compared to unfasted cardio. However, the literature on this topic unveils the amount of truth behind this statement. In this sports science article, we shall go through the mechanisms that initially provided support to the notion that fasted cardio is better than unfasted cardio and the further clinical trials that proved that this is, in fact, not the case.

Two major mechanisms have been implicated in bringing about increased fat loss from a cardio session in a fasted state vs an unfasted one. The first involves lowered insulin levels in the fasted state as well as the absence of readily metabolizable carbohydrates. In this article, the fasted state refers to an overnight fast and not any other form of calorie restriction. The mechanism at play here, that contributes to increased fat oxidation, is that the body turns to lipolysis instead of glycolysis. This consists of breaking down triglycerides into free fatty acids and glycerol, which are then transported to the bloodstream and are further taken up by metabolizing cells to use for the process of fat oxidation to produce energy. Coupled with lowered insulin levels, which is known to inhibit the process of lipolysis, the body should, theoretically, be able to use fats as fuel instead of carbohydrates.

The second mechanism that aims to address the claim that fasted cardio fares better at fat loss than unfasted cardio is that the body has a depleted level of glycogen, post an overnight fast, in the muscles causing a shift from glycolysis to lipolysis for immediate energy. However, there is scientific evidence to show that glycogen levels are not depleted over prolonged periods of rest wherein a muscle is not in use, in turn making this mechanism a moot point.

While there have been multiple studies conducted to answer this question, most of them are of poor quality due to the small sample size and other factors like a representation of different groups of the general population and forms of cardio. Therefore, a review article published in 2017, comprehensively analyzed all reliable studies conducted in this area and then drew some conclusions. This included 5 different studies generating a total of 96 individuals that were examined, which comprised of 60 males and 36 females aged between 21-27 years that had a history of track and field or participation in sports activities.

The authors concluded that there was no statistically significant difference between weight loss and fat mass loss in both the fasted and unfasted groups of participants. The change in lean and fat mass of the participants throughout the trials was influenced more by setting a meaningful calorie deficit over a longer period of time and not by exercising in a fasted or unfasted state. Moreover, they also found that consumption of a light meal prior to exercise led to a thermic effect causing more calorie-expenditure post-working out. Thus, it appears that it may very well be more beneficial to perform aerobic exercises in a lightly-fed state.

However, one thing that was amply stressed upon in this review was that all findings are to be taken with a grain of salt. This is because only 5 studies met the criteria for eligibility to be considered in this review, making the sample size far too small to provide conclusive results. Moreover, the differences in other controls like the dietary composition, between studies, make them very difficult to compare and draw conclusions from successfully. There have been other review articles with different eligibility criteria for consideration, that have obtained conflicting conclusions, namely that fasted cardio is superior to unfasted cardio in terms of weight loss and fat mass loss.

Thus, like all things in biology, there is no one answer to the question we have attempted to address in this sports science article. While there is no significant evidence that fasted cardio is better, there is similarly no evidence that it is detrimental to the health of the individual participating in it. This reduces the decision purely to the personal choice of individuals, at least until we have some more clarity in the form of better quality studies with a great increase in sample size that are properly controlled for the range of other factors that may influence their findings.