Which statement about RER is true?

Which statement about RER is true?

• A. Higher RER indicates greater fat oxidation
• B. RER > 1 indicates greater carbohydrate use
• C. RER around 0.85 indicates exclusive fat oxidation
• D. RER does not provide information about substrate use

Answer: (B)

RER shows which substrates are being oxidized by comparing CO2 produced to O2 consumed. Fat metabolism typically gives values around 0.7–0.8, while carbohydrate metabolism pushes toward 1.0. When exercise is very intense, RER can rise above 1.0 because buffering of lactate produces extra CO2, temporarily raising the ratio; this still indicates carbohydrate-dominant metabolism. This is why a value above 1 is associated with greater carbohydrate use. Statements suggesting higher RER means fat oxidation, or that around 0.85 equals exclusive fat use, aren’t accurate, and RER does provide information about substrate use.

RER shows which substrates are being oxidized by comparing CO2 produced to O2 consumed. Fat metabolism typically gives values around 0.7–0.8, while carbohydrate metabolism pushes toward 1.0. When exercise is very intense, RER can rise above 1.0 because buffering of lactate produces extra CO2, temporarily raising the ratio; this still indicates carbohydrate-dominant metabolism. This is why a value above 1 is associated with greater carbohydrate use. Statements suggesting higher RER means fat oxidation, or that around 0.85 equals exclusive fat use, aren’t accurate, and RER does provide information about substrate use.