Effect of Menstrual Phase on the Acetate Correction Factor Used in Metabolic Tracer Studies

The acetate correction factor is used to account for retention of carbon label in exchange reactions of the tricarboxylic acid cycle in studies estimating free fatty acid oxidation with carbon-labeled tracers. Previous evidence indicates that substrate utilisation and metabolic rate vary across the menstrual cycle, which may alter the correction factor. We therefore derived the acetate correction factor for each of three menstrual phases (early follicular [EF], late follicular [LF], and midluteal [ML] phase) from the fractional recovery of 13CO2 from a constant infusion of sodium-[1-13C]acetate during 90 min of submaximal exercise (60%[Formula: see text]) in sedentary eumenorrhoeic women. There was no difference in the correction factor between the EF and LF or the LF and ML phases, but the correction factor derived in the ML phase was significantly lower than in the EF phase (p < 0.05). Neither energy expenditure nor whole body substrate utilisation during exercise varied significantly between menstrual phases and therefore cannot explain the observed difference in the correction factor. The lower correction factor in the ML phase, compared to the EF phase, would result in only a small increase of ∼6% in the calculated plasma free fatty acid oxidation rate. Key words: carbon isotopes, women, ovarian hormones, exercise, substrate oxidation