During what state is the body primarily using fat for fuel?

The body primarily uses fat for fuel during the state of rest. At rest, the metabolic processes slow down, and the body relies more on fat oxidation to provide energy. This is because fat is a more efficient fuel source at lower intensities; it takes longer to break down but yields a higher amount of energy per unit of oxygen consumed compared to carbohydrates. During rest, the demand for immediate energy is lower, allowing the body to utilize its fat stores effectively for sustained energy.

In contrast, during intense exercise, the body shifts to using carbohydrates as the primary fuel source since they can be metabolized more quickly to meet the immediate energy needs. During low-intensity exercise, while fat is still a significant fuel source, it's not as predominantly relied upon as during rest. Recovery can involve both carbohydrate and fat metabolism, depending on the intensity and duration of the preceding activity, as well as nutritional intake. Thus, rest is the phase where fat utilization is most significant.