VO(2) kinetics of mild exercise are altered by RER

We propose that variations in fat and carbohydrate (CHO) oxidation by working muscle alter O(2) uptake (VO(2)) kinetics. This hypothesis provides two predictions: 1) the kinetics should comprise two exponential components, one fast and the other slow, and 2) their contribution should change with variations in fat and CHO oxidation, as predicted by steady-state respiratory exchange ratio (RER). The purpose of this study was to test these predictions by evaluating the VO(2) kinetic model: VO(2)(t) = alpha(R) + alpha(F)(1 - exp[(t - TD)/-tau(F)]) + alpha(C)(1 - exp[(t - TD)/-tau(C)]) for short-term, mild leg cycling in 38 women and 44 men, where VO(2)(t) describes the time course, alpha(R) is resting VO(2), t is time after onset of exercise, TD is time delay, alpha(F) and tau(F) are asymptote and time constant, respectively, for the fast (fat) oxidative term, and alpha(C) and tau(C) are the corresponding parameters for the slow (CHO) oxidative term. We found that 1) this biexponential model accurately described the VO(2) kinetics over a wide range of RERs, 2) the contribution of the fast (alpha(F), fat) component was inversely related to RER, whereas the slow (alpha(C), CHO) component was positively related to RER, and 3) this assignment of the fast and slow terms accurately predicted steady-state respiratory quotient and CO(2) output. Therefore, the kinetic model can quantify the dynamics of fat and CHO oxidation over the first 5-10 min of mild exercise in young adult men and women.