Lactate threshold at the same fat-free mass and age is larger in men than women

The lactate threshold (LT) represents the onset of a metabolic acidosis during graded exercise testing (GXT). It is typically measured as an oxygen uptake (VO₂) but then ratio scaled by body mass or VO₂ peak to make comparisons among subjects. Theoretical considerations and empirical evidence suggest that this type of ratio scaling is not valid. A method that allows a dependent variable to be compared between groups at that same value of one or more covariates is analysis of covariance (ANCOVA). Our purpose was to compare the LT, estimated non-invasively by gas exchange (LT_GE), at the same fat-free mass (FFM) and age in 203 sedentary subjects (102 men) aged 20–70 years. Each subject underwent cycle ergometer GXT with LT_GE measured by the V-slope method. In model development, we discovered an interaction term between sex and age. As dimensional analysis predicts a log-linear relationship between LT_GE and FFM, two of the model terms were ln LT_GE and ln FFM. The ANCOVA model was then as follows: dependent variable = ln LT_GE, fixed factor = sex, covariates = ln FFM, age, and sex × age. Sex made a significant contribution to the model (F = 30.7, P < 0.001). At the mean FFM (56.32 kg) and age (44.01 years) of both sexes combined, the LT_GE was 29% larger in males (1,307 ml min⁻¹ versus 1,011 ml min⁻¹). The model’s interaction term resulted in larger differences at younger ages and smaller differences at older ages. We conclude that LT_GE at the same FFM and age is larger in sedentary men compared to sedentary women.