Metabolism of Exercise States 100 meter sprint energy source used pre-synthesized ATP creatine phosphate anaerobic glycolysis 100 meters to 1 kilometer energy source used oxidative phosphorylation 1 kilometer to a marathon energy source used glycogen β-oxidation of fatty acids Metabolism of Exercise States Absorptive state immediately after eating insulin in control insulin-sensitive tissues adipocytes myocytes hepatocytes pathways favored by insulin glycogenesis fatty acid synthesis glucose uptake in fat, muscle, liver protein anabolism insulin-resistant tissues brain RBCs Post-absorptive state between meals epinephrine/glucagon in control glucagon-sensitive tissues adipocytes hepatocytes pathways favored by glucagon glycogenolysis fatty acid catabolism protein catabolism gluconeogenesis glucagon-resistant tissues brain RBCs Starvation days 1-3 main source of energy for body catabolism of triglycerides in fat stores main source of energy for brain glucose from gluconeogenic conversion of lactate and alanine > 3 days main source of energy for body catabolism of triglycerides in fat stores main source of energy for brain mostly glucose with some ketone bodies > several weeks main source of energy degradation of muscle and organs after adipose tissue store have been exhausted main source of energy for brain 2/3 ketone bodies 1/3 glucose note: RBCs are always dependent on glucose