Dietary nitrate-induced increases in human muscle power: high versus low responders

Maximal neuromuscular power is an important determinant of athletic performance and also quality of life, independence, and perhaps even mortality in patient populations. We have shown that dietary nitrate (NO₃^- ), a source of nitric oxide (NO), improves muscle power in some, but not all, subjects. The present investigation was designed to identify factors contributing to this interindividual variability. Healthy men (n = 13) and women (n = 7) 22-79 year of age and weighing 52.1-114.9 kg were studied using a randomized, double-blind, placebo-controlled, crossover design. Subjects were tested 2 h after ingesting beetroot juice (BRJ) either containing or devoid of 12.3 ± 0.8 mmol of NO₃^- . Plasma NO₃^- and nitrite (NO₂^- ) were measured as indicators of NO bioavailability and maximal knee extensor speed (V_max ), power (P_max ), and fatigability were determined via isokinetic dynamometry. On average, dietary NO₃^- increased (P < 0.05) P_max by 4.4 ± 8.1%. Individual changes, however, ranged from -9.6 to +26.8%. This interindividual variability was not significantly correlated with age, body mass (inverse of NO₃^- dose per kg), body mass index (surrogate for body composition) or placebo trial V_max or fatigue index (in vivo indicators of muscle fiber type distribution). In contrast, the relative increase in Pmax was significantly correlated (r = 0.60; P < 0.01) with the relative increase in plasma NO₂^- concentration. In multivariable analysis female sex also tended (P = 0.08) to be associated with a greater increase in Pmax. We conclude that the magnitude of the dietary NO₃^- -induced increase in muscle power is dependent upon the magnitude of the resulting increase in plasma NO₂^- and possibly female sex.