Final answer:
Fatty acids are the ideal source of energy for prolonged, low-intensity activities as they provide sustainable energy due to the body's abundant fat reserves utilized during aerobic metabolism.
Step-by-step explanation:
The source of energy ideal for prolonged, low-intensity activity is primarily fatty acids. During such activities, the body utilizes aerobic metabolism, which is more efficient in producing large amounts of ATP - the energy currency of the cells - compared to anaerobic metabolism. As the low-intensity exercise continues, the body taps into the nearly unlimited fat reserves stored mostly in adipose tissue, making fatty acids the predominant fuel source.
Alongside fatty acids, the body also uses a small amount of glucose for energy. However, glucose is limited in comparison as glycogen stores can be depleted, leading to fatigue. In contrast to fatty acids and glucose, proteins contribute minimally to energy production during these activities, with amino acids being utilized more during states of dietary insufficiency or prolonged endurance exercises.