Local oxygen consumption in cycling :the effect of chronic nitrate supplementation on muscle oxygen consumption (mVO2) during low and high intensity constant-load cycling
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Introduction: Nitrate supplementation administered from beetroot juice has been studied more extensively in recent years due to its suggested effect on exercise efficiency by reducing the O2-cost during submaximal cycling endurance exercise. Mainly amongst trained recreational individuals, with the focus primarily on pulmonary oxygen consumption (pVO2), although physiology suggests peripheral adaptations. The purpose of this study was to use near-infrared spectroscopy (NIRS) to investigate the effect of chronic nitrate (NO3-) supplementation on muscle oxygen consumption (mVO2) during low and high intensity constant-load cycling. To examine this, we used vastus lateralis and tibialis anterior. Methods: 9 healthy recreational active men performed two constant-load cycling tests pre and post (6 days) of placebo or nitrate supplementation, each of 7 min duration, at 50% of the work rate found to elicit blood lactate levels of 4 mmol∙l -1 during incremental exercise (OBLA), and at 70% maximal aerobic power (MAP). Pulmonary gas exchange (pVO2), heart rate and NIRS measurements of the muscles vastus lateralis (VL) and tibialis anterior (TA) were obtained continuously through both tests, while blood pressure, blood lactate and RPE was measured at specific time intervals. Results: The main findings of the present study were that chronic NO3- supplementation, quite surprisingly, did not significantly affect pVO2 and blood pressure at 50% OBLA and 70% MAP in recreational active subjects. Hence, since we could not provoke an effect on pVO2, as was expected, the corresponding absent of effect on mVO2 in the VL muscle and in the TA muscle seems rather reasonable. Although NO3- treatment showed trends towards lower O2 cost compared to PL in the VL muscle at 50% OBLA and 70% MAP. Discussion: The protocol is comparable with others that did find an effect on pVO2 (and to some extent on mVO2), so is dose and population. Results suggests that nitrate might not be as efficient as an ergogenic aid as previously depicted. The rational for the absence of effect of nitrate on mVO2 during both low and high intensity cycling in the present study is rather unclear, despite the corresponding lack of effect in pVO2 . Factors such as intensities and fiber type characteristics might be of importance. Conclusion: Chronic NO3- supplementation did not affect mVO2 in the VL muscle or in the TA muscle, pVO2 or blood pressure at low (50% OBLA) and high (70% MAP) intensity cycling in recreational active subjects.