At what intensity is fat primarily used as a fuel source during exercise?

Fat is primarily used as a fuel source during low to moderate-intensity exercise due to the body's energy metabolism preferences at different levels of exertion. At lower intensities, the body has sufficient oxygen available to metabolize fat effectively. Fat oxidation is a slower process but yields more energy compared to carbohydrates when oxygen is plentiful, making it the primary fuel source during steady-state activities such as walking, jogging, and light cycling.

As exercise intensity increases, the body shifts toward utilizing carbohydrates for fuel because they can be oxidized more quickly to meet the energy demands of high-intensity efforts. This transition occurs because the immediate energy needs exceed the capacity for fat oxidation, which is limited by the slower breakdown process.

Therefore, during low to moderate-intensity exercise, the balance leans favorably towards fat metabolism, making it the predominant fuel used in these conditions.