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.
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