What provides the majority of energy during very-low-intensity exercise (25% VO2 Max)?

During very-low-intensity exercise, such as at 25% of VO2 Max, the primary source of energy is provided by plasma fatty acids. At this low level of exertion, the body primarily relies on fat oxidation for energy, as the demand for energy is not high enough to require the rapid availability of carbohydrates. Plasma fatty acids from adipose tissue and the bloodstream are mobilized and utilized by muscles for sustained energy during light physical activities.

At low exercise intensities, the metabolic pathways favor the utilization of fat over carbohydrates since they provide a larger reservoir of energy and can be oxidized more readily when glycogen stores are not heavily taxed. As exercise intensity increases, the reliance usually shifts towards carbohydrates, primarily due to their quicker availability for energy production. Therefore, at this low intensity, it is plasma fatty acids that predominately fuel the exercise rather than carbohydrates, muscle glycogen, or protein sources.