Fasted Cardio

Fasted cardio is any aerobic exercise that is performed in a fasted state. Aerobic means “heart-pumping”, like running, rowing, cycling, swimming, or CrossFit metcon. In this case the exercise is performed after not eating for a number of hours.

The logic behind fasted cardio is the idea that if you don’t have any glucose in your system from a recent meal, then your body is forced to rely on other sources of energy stored in the body, i.e. fat. In theory, this makes sense, however in practice, there are more factors that determine how and where that energy is sourced.

Let’s get into the science.

Sources of Energy In The Body

There are different ways that the body stores energy, and how it actually uses that energy. The body runs on glucose — sugar — which is what carbohydrates and proteins are ultimately metabolized into through digestion. Glucose is the simple sugar found in the blood after digestion that the body runs on.

The first choice when the body needs energy is glycogen. Glycogen is a form of sugar, made up of a number of linked glucose molecules that are stored in the muscles and liver.

When there is no glycogen available, the second choice of the body is to metabolize fat as energy.

The third option is actual proteins. The body can break down muscles as an energy source.

Energy Production In Fasted State

After about 10-12 hours your body has gone through the majority of its glycogen stores (extracted from muscles, the liver, etc) so you are in a fasted state. The idea is that if we perform cardio in this state, then we burn fat as the main energy source, running the body on fat metabolism, in order to rapidly burn fat. At least that is the idea.

However, the body might also choose to run on protein, burning muscle mass. In the end, how our body uses energy at a given moment is a complex process. At any time there are a number of factors that determine how our body uses available energy source, such as hormone secretion, gene activation, and enzyme activity (to name a few).

Research is scant, and the research that has been done has been inconclusive, with mixed results. A 2017 study did not find the theorized changes in body composition. [1]

Resources

1. Fasted cardio is an attempt to burn stored fat. UCLA Health. <>. Accessed 6 Jan 2021.