Effects of Progressively Increasing Work Rate Exercise on Body Substrat Utilisation

ABSTRACT
During steady-state of moderate intensity exercise tests, respiratory quotient (RQ) provides an accurate reflection of body substrate utilisation. The body and muscle RQ can be estimated from the increase of CO2 output relative to the increase in O2 uptake. In the present study, we examined the effects of progressively increasing work rate on the substrate utilisation of body and exercising muscles. Eighteen male subjects performed an incremental exercise test. The work protocol started with 20 W cycling at 60 rpm for four minutes as a warm-up period then it was increased 15 W/min by a work rate controller until the limit of tolerance. Ventilatory and pulmonary gas exchange variables were measured using a turbine volume transducer and mass spectrometry and estimated breath-by-breath. During warm-up period, total body RQ was 0.82±0.05, reflecting both carbohydrate and fat oxidation. With increasing work rate RQ increased and at the anaerobic threshold it reached a value of 0.96±0,04, reflecting predominantly (85%) carbohydrate utilisation. At the respiratory compensation point, RQ increased to 1.29±0.19 due to the excess CO2 production from the HCO3- buffering of lactic acidosis. At maximal exercise performance RQ was found to be 1.75±0.42 reflecting CO2 from aerobic and anaerobic metabolism and also CO2 from hyperventilation. Consequently, substrate utilisation for the total body derives proportionally more from carbohydrate than from lipid stores during exercise as work rate increases.