Measuring maximal fat oxidation (MFO) and maximal oxygen consumption $(\dot{\mathrm{V}}{\mathrm{O}}_{2max})$ typically requires two separate days, which can be time-consuming. This study aimed to assess the validity and reliability of a single-day test for measuring MFO and $\dot{\mathrm{V}}{\mathrm{O}}_{2max}$ in individuals with pre-diabetes/type 2 diabetes (T2DM).

Twenty adults with pre-diabetes/T2DM (10 women; [53.1 ​± ​8.8] years) participated in this cross-sectional study. Participants underwent a single-day MFO- $\dot{\mathrm{V}}{\mathrm{O}}_{2max}$ protocol, followed by the traditional $\dot{\mathrm{V}}{\mathrm{O}}_{2max}$ test on a cycle ergometer 4–7 days later, with both assessments performed using indirect calorimetry. The single-day test commenced with the MFO test, involving 15 ​W per 3 ​min increments until the respiratory exchange rate reached 1.0. After a 5-minute (min) rest, the $\dot{\mathrm{V}}{\mathrm{O}}_{2max}$ test was conducted, starting at the last load and increasing by 15 ​W·min^-1 until voluntary exhaustion. The traditional $\dot{\mathrm{V}}{\mathrm{O}}_{2max}$ test involved a 15 ​W·min^-1n incremental protocol until voluntary exhaustion, followed by a verification test at 110% of the previous maximum load.

No significant differences were observed between $\mathrm{M}\mathrm{F}\mathrm{O}-\dot{\mathrm{V}}{\mathrm{O}}_{2max}$ and traditional $\dot{\mathrm{V}}{\mathrm{O}}_{2max}$ protocol in $\dot{\mathrm{V}}{\mathrm{O}}_{2max}$ values ([2099.4 ​± ​534.1] ml·min^-1 vs. [2119.1 ​± ​548.3] ml·min^-1, p ​= ​0.715) or heart rate ([163.2 ​± ​18.2] beat/min vs. [164.5 ​± ​16.1] beat/min, p ​= ​0.485). The $\mathrm{M}\mathrm{F}\mathrm{O}-\dot{\mathrm{V}}{\mathrm{O}}_{2max}$ protocol test demonstrated excellent reliability (intraclass correlation coefficient ​= ​0.957), limits of agreement ranging from 300.06 ​ml·min^-1 to −306.77 ​ml·min^-1, and the standard error of measurement was 181.67 ​ml·min^-1.

Therefore, the single-day test is a valid and reliable method for assessing both MFO and $\dot{\mathrm{V}}{\mathrm{O}}_{2max}$ in men and women with pre-diabetes/T2DM. With a mean total duration of (34 ​± ​8) min, this test enhances clinical applicability by significantly reducing the time and cost associated with traditional assessments.