Does fasted cardio really burn more fat?

What is fasted cardio?

You may have heard of fasted cardio, or training on a fasted state for improved body composition. While there is still not enough evidence to conclude that this method does in fact lead to greater fat loss, many trainers and Hollywood A-listers are still recommending this practice, so where did it come from and why are people doing it?

The theory that lower glycogen and insulin levels cause the body to move from carb utilisation to fat mobilisation has been around for quite some time leading some people to believe that training on a low carb state allows the body to “burn” more fat.

Fat oxidation occurs at around 45-65% Vo2max.

Research has shown that fat oxidation only occurs at 45-65% Vo2max, what this means is that our bodies use fat as fuel at a lower intensity and anything above 65% it turns to Carbohydrate oxidation (our bodies using carbs as fuel).

Using fat as fuel for training is not the same as “burning” body fat. The fat you use is dietary fat, not actual fat mass.

So if you’re just plodding along for a long but gentle run then your body is more likely to initiate lipolysis (when fatty acids get separated from their glycerol backbone), but anytime you start to “go ham” and increase intensity (think tabata training) your body flips to carbohydrate oxidation.

There is also evidence that when our bodies use fat as fuel, it over compensates for carboydrate utlisation later in the day, so when we look at fat oxidation its important to consider the process in the course of a 24 hour period as opposed to a single hour or the immediate training window.

So why do some people see results when following the fasted cardio approach. 

Well there could be a few reasons.

First, fasted cardio is recommended after a long period of no food, which athletes tend to do after an over night of fasting followed by a morning of no food. Because of this, athletes tend to have their first meal later in the day, this can lead to less meals to consume and often resulting in an overall calorie deficit in the 24 hour period.

Second, athletes who perform fasted cardio tend to practice this method of exercising during a deficit, which leads to overall weight loss.

Research stats.

One notable study took 20 athletes and compared the results between training on a fasted state vs a fed state whilst in a deficit over 4 weeks and found that both groups had body composition changes due to the overall deficit but the results between the two groups did not differ.

 In other words, being in a deficit is ultimately what changes your body, not the practice of training on a fasted state.

So whats the conclusion?

If you don’t like to eat before training, there is no need to. However if you have been training on a fasted state because you believe it will lead to greater fat loss, there is also no need to. 

Whether you train on a fasted state or a fed state, evidence has shown no difference in body composition.

Research has shown in many studies however, that ensuring that you have enough carbohydrates during peri workouts aids recovery and increases performance.

Resources

Schoenfeld, B.J., Aragon, A.A., Wilborn, C.D. et al. Body composition changes associated with fasted versus non-fasted aerobic exercise. J Int Soc Sports Nutr 11, 54 (2014). https://doi.org/10.1186/s12970-014-0054-7

Margolis, L.M., Wilson, M.A., Whitney, C.C. et al. Initiating aerobic exercise with low glycogen content reduces markers of myogenesis but not mTORC1 signaling. J Int Soc Sports Nutr 18, 56 (2021). https://doi.org/10.1186/s12970-021-00455-z