Mia likes to run marathons. When she is running a marathon, how does her body get the fuel it needs?

Answer 1

Several minutes into her marathon, Mia's breathing will become faster and harder. Her heart will begin to beat more frequently and forcefully. These changes in pulmonary and cardiovascular function help to deliver needed oxygen to her muscles. With sufficient oxygen now available, her muscle cells are better able to use aerobic metabolic pathways to produce ATP. Aerobic metabolic pathways are important in that substantial amounts of ATP can be generated over an extended period of time—as is needed for running a marathon.
Aerobic pathways, which can be thought of as long-term energy systems, use glucose, fatty acids, and—to a lesser extent—amino acids to generate ATP. These energy sources are catabolized to produce NADH + H+ and FADH2, which in turn are used to generate ATP via the electron transport chain. Although these pathways are relatively slow in terms of the rate of ATP production, the energy yield is rich. Fatty acids used for energy during physical activity are derived primarily from triglycerides stored in adipose tissue. Intramuscular triglycerides also help fuel Mia's running. Aerobic pathways also use stored muscle glycogen and glucose circulating in the blood.
During prolonged activity such as running a marathon, the combined efforts of gluconeogenesis and glycogenolysis help provide sustained glucose to active muscles. However, once glycogen stores are exhausted, glycogenolysis can no longer contribute to the glucose pool. During this time, gluconeogenesis alone may not be able to provide glucose at the rate needed by skeletal muscles either. When this happens, Mia may lose her stamina or "hit the wall," a term used to describe the feeling of profound fatigue that can occur during an athletic event.
If Mia consumes sports drinks, energy bars, or energy gels during exercise, these energy sources can help provide an additional source of glucose.