Last Word on Viewpoint: Use aerobic energy expenditure instead of oxygen uptake to quantify exercise intensity and predict endurance performance.

We thank all of the authors who commented on our Viewpoint (see Ref. 1), and we wish to clarify a few points. We are aware that numerous scientists have properly calculated Ėaero in units of energy for low-to-moderate aerobic exercise intensities. Others have expressed exercise economy and maximum aerobic capacity in units of energy, but did so by assuming a constant energetic equivalent for oxygen across aerobic exercise intensities. The novelty of our Viewpoint was to urge scientists to express exercise economy and maximum aerobic capacity properly. Scientists can properly calculate Ėaero max via expired gas analysis using V̇O2max and the known energy yield of oxidized carbohydrates (6); carbohydrates are the sole oxidized substrate at maximal aerobic intensity (2). We contend that using %Ėaero max rather than %V̇O2max elicits nontrivial differences in endurance performance predictions. For a hypothetical runner (V̇O2max: 70 ml O2·kg · min , Ėaero max: 25.6 W/kg, and a lactate threshold at 80% of V̇O2max), using Joyner’s model (3) and Ėaero we calculate that increasing RER by 0.01 (e.g., 0.97 to 0.98) predicts an 18 s faster marathon time. Often a small difference in race time has a large effect on the race’s outcome. For example, the 2016 women’s Olympic marathon silver medalist would have won an Olympic gold medal if she ran 18 s faster. During longer endurance events, e.g., ultramarathon running, RER values can be as low as 0.82 (5), and thus the predictions based on % V̇O2max and %Ėaero max are more substantial. We agree with the many commenters who pointed out that using physiological thresholds can be valuable when quantifying exercise intensity and predicting endurance performance. As such, in our Viewpoint (and in the previous paragraph) we predicted marathon race times using a runner’s maximum aerobic capacity, running economy, and lactate threshold. Distinctly, we proposed that in many situations scientists should use Ėaero submax and Ėaero max rather than V̇O2submax and V̇O2max, regardless of whether they are used in combination with a physiological threshold or not. There are many equations that convert V̇O2 and V̇CO2 to Ėaero. But the various equations can yield Ėaero that differ by up to 5.2% using the same V̇O2 and V̇CO2 values (4). Accordingly, scientists should use the conversion equation that has the most appropriate biochemical/physical assumptions to their research question. In our Viewpoint, we utilized an equation that we developed from Perronet and Massicotte (6), because it has the fewest “fudge factors” and corrects for prior mistakes (see Ref. 4). We agree that all conversions from V̇O2 and V̇CO2 to Ėaero are limited by their assumptions and predictive errors, but using V̇O2 alone risks even greater errors. Finally, we recognize that the RER does not always reflect the RQ, and caution should be taken when using the RER in situations where nonmetabolically produced CO2 contributes significantly to the RER, such as at exercise intensities above the lactate threshold or in hypoxia. We thank the Journal of Applied Physiology for hosting this energetic discussion.