V(over-dot)O-2 kinetics of mild exercise are altered by RER

We propose that variations in fat and carbohydrate (CHO) oxidation by worki ng muscle alter O-2 uptake ((V) over dot O-2) kinetics. This hypothesis pro vides two predictions: 1) the kinetics should comprise two exponential comp onents, one fast and the other slow, and 2) their contribution should chang e with variations in fat and CHO oxidation, as predicted by steady-state re spiratory exchange ratio (RER). The purpose of this study was to test these predictions by evaluating the (V) over dot O-2 kinetic model: (V) over dot O-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 (V) over dot O-2(t) describes the time course, alpha(R) is resting ( V) over dot O-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 paramet ers for the slow (CHO) oxidative term. We found that 1) this biexponential model accurately described the (V) over dot O-2 kinetics over a wide range of RERs, 2) the contribution of the fast (alpha(F), fat) component was inve rsely related to RER, whereas the slow (alpha(C), CHO) component was positi vely related to RER, and 3) this assignment of the fast and slow terms accu rately predicted steady-state respiratory quotient and CO2 output. Therefor e, the kinetic model can quantify the dynamics of fat and CHO oxidation ove r the first 5-10 min of mild exercise in young adult men and women.