Dietary nitrate supplementation enhances muscle contractile efficiency during knee-extensor exercise in humans

Abstract

The purpose of this study was to elucidate the mechanistic bases for the reported reduction in the O(2) cost of exercise following short-term dietary nitrate (NO(3)(-)) supplementation. In a randomized, double-blind, crossover study, seven men (aged 19-38 yr) consumed 500 ml/day of either nitrate-rich beet root juice (BR, 5.1 mmol of NO(3)(-)/day) or placebo (PL, with negligible nitrate content) for 6 consecutive days, and completed a series of low-intensity and high-intensity "step" exercise tests on the last 3 days for the determination of the muscle metabolic (using (31)P-MRS) and pulmonary oxygen uptake (Vo(2)) responses to exercise. On days 4-6, BR resulted in a significant increase in plasma [nitrite] (mean +/- SE, PL 231 +/- 76 vs. BR 547 +/- 55 nM; P < 0.05). During low-intensity exercise, BR attenuated the reduction in muscle phosphocreatine concentration ([PCr]; PL 8.1 +/- 1.2 vs. BR 5.2 +/- 0.8 mM; P < 0.05) and the increase in Vo(2) (PL 484 +/- 41 vs. BR 362 +/- 30 ml/min; P < 0.05). During high-intensity exercise, BR reduced the amplitudes of the [PCr] (PL 3.9 +/- 1.1 vs. BR 1.6 +/- 0.7 mM; P < 0.05) and Vo(2) (PL 209 +/- 30 vs. BR 100 +/- 26 ml/min; P < 0.05) slow components and improved time to exhaustion (PL 586 +/- 80 vs. BR 734 +/- 109 s; P < 0.01). The total ATP turnover rate was estimated to be less for both low-intensity (PL 296 +/- 58 vs. BR 192 +/- 38 microM/s; P < 0.05) and high-intensity (PL 607 +/- 65 vs. BR 436 +/- 43 microM/s; P < 0.05) exercise. Thus the reduced O(2) cost of exercise following dietary NO(3)(-) supplementation appears to be due to a reduced ATP cost of muscle force production. The reduced muscle metabolic perturbation with NO(3)(-) supplementation allowed high-intensity exercise to be tolerated for a greater period of time.

Dietary nitrate supplementation enhances high-intensity running performance in moderate normobaric hypoxia, independent of aerobic fitness

“…These findings are in line with most previous investigations [4,5,22,37], and similar to the results of a recent meta-analysis [6]. The physiological mechanisms underlying the decreased V O2 subsequent to nitrate supplementation have not been fully elucidated, but may be accounted for by: a) an improvement in the efficiency of mitochondrial respiration [35] and/or b) enhanced efficiency of muscle force production [7].…”

“…Tissue oxygenation in the vastus lateralis was also elevated following nitrate supplementation in that study. These findings are supportive of the notion that nitrate supplementation may reduce muscle O2 consumption during exercise in hypoxia, presumably via the aforementioned mitochondrial [35] and/or muscle [7] alterations that also manifest as a reduced V O2. Interestingly, Arnold et al [38] found that runners identified as 'responders' to nitrate supplementation during a 10,000 m TT in moderate hypoxia (FIO2 15.4 %) typically experienced a lower SaO2 during the placebo TT than 'non-responders' (82 vs 84 %).…”

“…Bailey et al [7] reported reduced intramuscular PCr, ADP, and Pi perturbations and lower V O2 during leg extension exercise following nitrate-rich beetroot juice supplementation (5.1 mmol·d -1 x 6 days), reflecting a lower ATP turnover for a given work rate. It was suggested that the decreased ATP turnover occurring after nitrate supplementation might be related to a reduced ATP requirement for actin-myosin cross-bridge cycling and/or calcium handling, given previous reports that NO slows actin-myosin cross-bridge cycling [59,60] and inhibits calcium-ATPase activity [61,62].…”

“…These include, but are not restricted to: lower blood pressure [1][2][3], reduced O2 consumption (V O2) during steady-state exercise [4][5][6], attenuated muscle metabolic perturbations [7,8], and enhanced muscle force production [9,10]. These effects are not directly attributable to the nitrate anion, which is relatively inert [11].…”

“…This suggests that nitrate supplementation may be more likely to enhance NO generation and therefore improve performance in hypoxia relative to normoxia. Moreover, an improvement in mitochondrial [35] and/ or muscle contractile efficiency [7] subsequent to nitrate supplementation which reduces V O2 is likely to be especially advantageous when O2 availability is low. This is supported by the findings of a recent study, in which nitrate supplementation improved tolerance to severe intensity cycling in hypoxia (FIO2 13.1 %) but not normoxia [36].…”