GLOSSARY

Muscle Substrate Metabolism covers fuel entry, storage and conversion in muscle.

For example, when comparing an easy session with hard training, record preceding food, duration, intensity, sampling time and recovery conditions together. A changed number or an activated pathway alone does not justify changing the diet; practical decisions must remain connected to the training goal, individual tolerance and actual performance trend.

Tissues do not contribute equally to fuel use: active muscle, heart, brain, kidney and adipose tissue have different demands and capacities. The brain uses more ketones during fasting adaptation, but the body's glucose requirement does not disappear. The kidney can also contribute to glucose production and nitrogen disposal, so findings from one muscle should not be generalized to the whole body.

Muscle both consumes fuel and renews its stores and proteins; uptake does not mean that all material is immediately oxidized. Adipose tissue is also an active participant in storage, release and signaling rather than an inert reservoir. For an athlete, changes in mass and performance must be assessed with food, training and recovery time rather than inferred from a momentary increase in one pathway.

Availability, delivery in blood, cellular uptake and final use are related but distinct questions. Partitioning between tissues changes with blood flow, transporters, energy demand and feeding state. Abundance in blood does not prove rapid cellular oxidation: a tissue may store the material, convert it to another molecule or have limited uptake. These distinctions prevent delivery from being mistaken for actual metabolic flux.