GLOSSARY

Substrate Fate is a material's outcome, such as storage, synthesis or oxidation.

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.

A pool is the amount present; flux is movement through a process per time. A nearly constant pool can coexist with large increases in both entry and exit. One blood concentration does not determine synthesis, use or whole-body turnover. Site, timing and method matter in exercise interpretation. An accumulated intermediate may not mean faster pathway flow, and neither accumulation nor flux alone guarantees better performance.

Turnover involves synthesis and breakdown or entry and exit; net balance is their difference. Protein, fat or glycogen can change little despite active movement in both directions. Substrate cycling can consume energy and contribute regulation, so futile does not necessarily mean functionless. A momentary rate is not equivalent to the net body-composition result over days or weeks, particularly when diet and training change across that period.

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.