Respiratory exchange ratio (RER) measures the ratio of carbon dioxide produced to oxygen consumed and is used to assess exercise intensity and substrate use. Which statement is true?

• A. Heart rate divided by VO2 max
• B. Ratio of CO2 produced to O2 consumed and indicates energy expenditure and fuel use
• C. Volume of air inhaled per breath
• D. Blood pH during exercise

Answer: (B)

RER is defined as the ratio of carbon dioxide produced to oxygen consumed, measured from the gases expired. This ratio tells you which fuels the body is burning: fats give a lower RER (around 0.7), carbohydrates push it higher (toward 1.0), and a mix sits in between. As exercise intensity rises, the body shifts toward more carbohydrate oxidation, which raises RER, and during very intense efforts buffering of lactic acid can push RER above 1.0 temporarily. It reflects substrate use and, to some extent, energy expenditure, rather than the actual amount of air you inhale each breath or blood pH. That makes the statement describing RER as the ratio of CO2 produced to O2 consumed and its indication of energy expenditure and fuel use the best fit.