Dietary Nitrate and Muscle Power with Aging

Abstract

Reductions in muscle speed and power are highly predictive of disability, institutionalization, and mortality in the elderly. We have recently demonstrated that ingestion of dietary nitrate (NO3-), a source of nitric oxide (NO), increases maximal muscle speed and hence power in healthy younger individuals, in athletes, and especially in patients with heart failure (HF). PURPOSE: The purpose of the present study was to determine whether dietary NO3- improves muscle contractile function in older people, another population (like HF patients) in whom NO production is reduced. METHODS: Six healthy older subjects (5 men, 1 woman; age 73±6 y, height 1.74±0.10 m, mass 82.4±12.1 kg) were studied using a randomized, double-blind, placebo-controlled, crossover design. On one occasion, subjects were tested 2 h after ingesting a concentrated beetroot juice (BRJ) supplement containing 11.2 mmol NO3-. On another, they were tested 2 h after ingesting BRJ depleted of NO3- (placebo). Breath NO was measured periodically, and maximal knee extensor force (torque), speed, and power were assessed using a Biodex 4 isokinetic dynamometer. RESULTS: Dietary NO3- ingestion increased breath NO levels, a marker of whole-body NO bioavailability, from 27±10 to 51±26 ppb (P<0.05). On average, this resulted in an increase in the maximal velocity of knee extension of 10% (i.e., from 9.81±1.38 to 10.75±2.42 rad/s), but this difference only approached statistical significance (i.e., P=0.13). On the other hand, maximal knee extensor power did not differ between the NO3- and placebo trials (i.e., 4.16±1.18 vs. 4.08±1.22 W/kg; P=0.47). This lack of difference, however, seemed to be due to an inadequate dose of NO3- in some subjects, as the relative increase in maximal power was correlated (i.e., r=0.78; P<0.05) with the amount of NO3- ingested per kilogram of body mass. In keeping with this conclusion, maximal power increased (P<0.05) by 6.4±3.9% in the four subjects who ingested >125 mmol/kg of NO3-, but did not improve in the two subjects who ingested less. CONCLUSION: Acute dietary NO3- supplementation appears to improve muscle contractile function in healthy elderly individuals, but only when provided at a dose of >125 mmol/kg of NO3-. The optimal dose of dietary NO3- for improving muscle speed and power in older (or younger) persons remains to be determined.