Ratio of CO₂ produced to O₂ consumed (VCO₂/VO₂), used to estimate the balance of fat versus carbohydrate oxidation.
Use RER when the question is substrate selection — which fuel is being oxidized. Values near 0.70 indicate fat oxidation and near 1.00 indicate carbohydrate; values above 1.00 usually signal hyperventilation or non-metabolic CO₂ rather than fuel. RER (at the mouth/chamber) and RQ (at the tissue) coincide only at steady state, so fix the analysis window and feeding state before comparing groups.
| Primary value | VCO₂ ÷ VO₂ over a defined steady-state window |
|---|---|
| Common units | Unitless ratio, typically 0.70–1.00 |
| Compatible assays | Home-cage metabolic chamber, fasted/fed challenges, graded treadmill exercise |
| Required boundary | Synchronized VO₂ and VCO₂ on the same flow and timeline |
| Do not infer alone | Total energy expenditure, fat mass, or fitness |
Compute RER from VO₂ and VCO₂ sampled on the same timeline over a stable window; avoid transition periods where the gases lag each other. Record feeding state and time of day, because RER tracks the fed–fasted cycle as strongly as any intervention.
RER outside 0.70–1.00 should prompt a check for hyperventilation, acid buffering, leaks, or calibration error rather than a fuel interpretation. A shift in RER reflects substrate balance, not the amount of energy used, and is confounded by diet, feeding state, and exercise intensity.
Store animal ID, feeding state, time of day, ambient temperature, paired VO₂ and VCO₂, sampling interval, steady-state window, RER value, activity state, calibration record, and instrument or software version.
Endpoint pages should cite the method literature behind the scored value and keep high-specificity protocol claims qualified unless the source supports them.
Endpoint articles link to adjacent products, software workflows, and sibling endpoints where the connection is useful and already routable.
