Table 2 |.

Terminology

Related literature terms	Meaning	Distinction between the terms
[U-13C] metabolite; [1-13C] metabolite; M + 1 metabolite	Notations to convey the labelled form of a metabolite	[U-13C]: a metabolite uniformly labelled with 13C in all carbon positions; [1-13C]: has 13C in the 1 position, as determined by International Union of Pure and Applied Chemistry nomenclature: M + 1: has a single heavy-isotope-labelled atom in any one position
Circulatory turnover flux (Fcirc); Endogenous production rate; Rate of appearance (Ra); Rate of disappearance (Rd);	The rate at which a circulating metabolite is produced or consumed by the whole body, as measured by dilution of infused isotope tracer	Ra = turnover = whole-body production of circulating metabolite; Endogenous production rate = Ra; Rd = Ra + infusion rate; Fcirc = Ra for a minimally perturbative infusion
$F_{\text{circ}}^{\text{intact}}$, $F_{\text{circ}}^{\text{avg}}$	Circulatory turnover flux, measured by either intact tracer or average labelling remaining in the tracer	$F_{\text{circ}}^{\text{intact}}$: rate of any transformations that alter the labelling pattern of the infused metabolite; $F_{\text{circ}}^{\text{avg}}$: discounts fluxes that recycle labelled tracer atoms (radioactive tracer infusions measure $F_{\text{circ}}^{\text{avg}}$)
Enrichment Labelling Lintact or Lavg); Atom percent excess; Specific activity	Fractional labelling of a metabolite in the blood or tissue resulting from providing isotope-labelled tracer	Atom percent excess = Lavg: the fraction of a metabolite made up of heavy-isotope atoms from the tracer; Lintact: the fraction of a metabolite with the same number of heavy atoms as the original infused tracer; Enrichment and labelling are more general terms; Specific activity refers to radioactivity
Tracee; Unlabelled metabolite; Endogenous metabolite	Metabolite endogenously present in the blood and tissues, including naturally occurring isotope forms	Tracee: unlabelled circulating form of the metabolite that is infused as the tracer; Unlabelled and endogenous metabolite are more general terms
Normalized labelling; Fractional contribution	Enrichment of the downstream metabolite divided by enrichment of the precursor (for example, the tissue labelling of a metabolite divided by the circulating labelling of infused tracer nutrient)	Normalized labelling: metabolite enrichment divided by infused tracer circulating enrichment; Fractional contribution: fraction of a metabolite produced from an upstream source
Secondary tracer; Direct contribution; Indirect contribution	Infused tracers can label downstream metabolites either directly or via other circulating metabolites	Secondary tracer: other circulating metabolites labelled by the tracer; Direct contribution: contribution of the infused tracer to downstream metabolite without passing through other circulating metabolites; Indirect contribution: contribution of the tracer to the downstream metabolite via a secondary tracer