Pathways don’t run identically everywhere. Compare each tissue’s role, then open the pathways that support it.
Coordinates blood-fuel balance
Uses incoming glucose, replenishes glycogen, and can convert surplus carbon into fatty acids.
Releases glucose early from glycogen, then makes glucose from lactate, glycerol, and amino acids; fatty-acid oxidation supplies energy.
Uses fuel for contraction and stores local glycogen
Insulin-sensitive GLUT4 uptake supports glucose use and glycogen storage.
Uses local glycogen during activity and can oxidize fatty acids; it does not release free glucose from glycogen because it lacks glucose-6-phosphatase.
Needs a steady supply of usable fuel
Glucose is a major fuel for the brain under ordinary fed conditions.
The brain continues using glucose during shorter fasts and can increase ketone use during prolonged fasting; it does not directly use long-chain fatty acids as a major fuel.
Carries oxygen and relies on glucose
Uses glycolysis for ATP and the pentose phosphate pathway for NADPH.
Still relies on glucose: mature red blood cells lack mitochondria and cannot use fatty-acid oxidation or oxidative phosphorylation.
Stores and releases energy-rich fat
Insulin promotes glucose uptake and storage of fatty acids in triglycerides.
Lipolysis releases fatty acids for other tissues and glycerol that can support hepatic gluconeogenesis.
Balances blood chemistry and contributes to fuel balance
Uses oxidative metabolism to support filtration, transport, and other demanding work.
The kidney can contribute to gluconeogenesis, especially as fasting continues; its role changes with duration and acid-base state.
These short views describe common human physiology. Nutritional state, exercise, development, disease, and cell type all affect fuel use.