Validity and reliability of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"><mml:mover accent="true"><mml:mi mathvariant="normal">V</mml:mi><mml:mo>˙</mml:mo></mml:mover></mml:math>O2max determination immediately after a max

Measuring maximal fat oxidation (MFO) and maximal oxygen consumption ( V ˙ 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 V ˙ 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- V ˙ O 2max protocol, followed by the traditional V ˙ 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 V ˙ O 2max test was conducted, starting at the last load and increasing by 15 ​W·min -1 until voluntary exhaustion. The traditional V ˙ O 2max test involved a 15 ​W·min -1 n incremental protocol until voluntary exhaustion, followed by a verification test at 110% of the previous maximum load. No significant differences were observed between MFO- V ˙ O 2max and traditional V ˙ O 2max protocol in V ˙ O 2max values ([2 099.4 ​± ​534.1] ml·min -1 vs. [2 119.1 ​± ​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 MFO- V ˙ 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 V ˙ 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.