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 (LTGE), 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 LTGE 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 LTGE and FFM, two of the model terms were ln LTGE and ln FFM. The ANCOVA model was then as follows: dependent variable = ln LTGE, 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 LTGE 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 LTGE at the same FFM and age is larger in sedentary men compared to sedentary women.