Eumenorrheic Women Require at Least a 6-Hour Fasting Duration Before the Assessment of Fat Oxidation Rates During...

Bowling, LR, Knight, SN, Dillard, CC, and Waldman, HS. Eumenorrheic women require at least a 6-hour fasting duration before the assessment of fat oxidation rates during submaximal exercise. J Strength Cond Res 39(9): e1054-e1058, 2025-Fat oxidation (FATox) rates are commonly used to assess metabolic health, prescribe exercise, and evaluate training adaptations. However, an often-overlooked consideration influencing FATox measurements is the fasting duration before assessment. Currently, 1 study with male subjects examined fasting durations on substrate oxidation responses, but no studies have specifically addressed this variable in women. This study aimed to determine the optimal fasting duration required to accurately assessing FATox rates in women. Eleven women (age: 23 ± 3.6 years; height: 166.9 ± 5.5 cm; mass: 67.6 ± 9.2 kg; bodyfat: 29.0 ± 6.4%; V̇ o2 peak: 31.1 ± 4.5 ml·kg -1 ·minute -1 ) underwent 5 fasting durations (2, 4, 6, 8, 12 hours) after a standardized meal, before completing a graded exercise test to assess FATox rates at various intensities (rest, 25, 35, 45, 55, 65, and 75%) of max wattage ( Wmax ). Using the 12-h fasting duration as a baseline, exercise intensities of 25 through 45% Wmax elicited significantly lower FATox values (∼60%) for the 2- and 4-h fasting durations. Only at 6-h fasting durations and greater were significant differences in FATox values no longer observed (all p > 0.05). Future studies examining FATox rates in women should include at least a 6-h fasting duration or risk making a type II error, because the preceding meal may mask changes in FATox values during low and moderate exercise intensities.