metabolic states

Meaning: Metabolic states describe which fuel pathways dominate at different times after eating, largely set by the insulin to glucagon balance.

In the absorptive (fed) state, blood glucose and insulin are high. Cells take up glucose, the liver builds glycogen and triglycerides, adipose stores fat, and protein synthesis rises.

In the postabsorptive (fasting) state, insulin falls and glucagon rises. The liver first breaks down glycogen, then adds gluconeogenesis from lactate, glycerol, and amino acids, while adipose releases fatty acids.

In prolonged fasting, the liver converts acetyl-CoA to ketone bodies, and the brain uses more of them, which slows muscle protein breakdown.

On the MCAT: A passage gives hormone levels, time since a meal, or an enzyme's activity, and you predict which pathway rises or falls. Glycolysis, glycogen synthase, and fatty acid synthesis are favored when fed; glycogen phosphorylase, gluconeogenesis, and β-oxidation are favored in fasting.

Common confusion: Fatty acids from typical even-chain fats cannot give a net gain of glucose, so gluconeogenesis uses glycerol, lactate, and amino acids. Also, the brain is not limited to glucose: it relies mainly on glucose early in fasting but uses ketone bodies in prolonged fasting.