The results of this study indicate that dietary nitrate supplementation in individuals with moderate COPD a) does not significantly increase submaximal exercise endurance as measured by ESWT distance, time or dyspnea score and b) decreases resting systolic blood pressure with c) compensatory increases in resting heart rate.
Unfortunately given logistical (funding) constraints, it was impractical to measure serum NO3
− and NO2
− levels: ideally, these would have been measured at baseline and post supplementation, with additional analyses carried out to determine whether improvements in exercise endurance were associated with increased NO3
− and/or NO2
−, and whether medications or comorbidities interacted with this effect. We observed a decrement in blood pressure in the active arm, suggestive of successful nitrate supplementation, however, quantitative determination of NO3
− and/or NO2
− may have provided definite confirmation. Confirmation of return of NO3
− and NO2
− to baseline values after the four day washout period may also have been useful.
Our trial also suffered from a high attrition rate: from 35 subjects, only 19 completed the trial. This is significantly higher than in other trials of dietary nitrate supplementation in individuals with COPD, in which withdrawals were limited to a maximum of 1 per trial [
]. The attrition rate in our study is reflective of the burden of illness in individuals with COPD, with multiple drop-outs due to underlying illness or intercurrent illness. 10 of our patients withdrew due to medical illness (predominantly COPD exacerbations), and in the safety phase, one withdrew with symptomatic postural hypotension induced by BR, confirming the reported [
] effect of dietary nitrates on blood pressure. These withdrawals may reflect the tertiary nature of our center, with a high proportion of more unwell patients.
The dose and timing of nitrate used in our study differed to that used by other investigators who have examined its effect on exercise endurance in COPD. Kerley et al. and Berry et al., both of whom found positive effects, used acute doses of 12.9 mmol and 7.58 mmol respectively [
]. However, Shepherd et al. used 6.77 mmol twice a day for 2.5 days and found no difference [
]. We used 4.8 mmol twice a day for three days with a further dose on the morning of walk test and found no significant difference. Although a trial in healthy cyclists used a wash-in period and demonstrated benefit (three days 0.1 mmol nitrate/kg/day) [
], it is possible that physiological alterations in aging, or in COPD render wash-in periods less useful, or alternatively, that the dose we used was too low.
The effect of increased adiposity has been proposed as a modifying factor in the response to dietary nitrate, by means that are unknown [
]. The BMI (kg/m
) of our study population (29.1 ± 6.5) was similar to that in Berry et al. (29.2 ± 5.5) who found similar an increase in exercise performance. Kerley et al. had a slightly lighter population and found an increase in exercise endurance with a BMI of 27.3 ± 6.4, whereas Shepherd et al., whose patients were heavier than ours, and found no difference with a BMI of 30.8 ± 3.2. The influence of adiposity on nitrate supplementation’s effect on exercise supplementation remains yet to be defined.
In healthy older individuals, short term dietary nitrate supplementation at 9.6 mmol/day reduced resting blood pressure but there was no effect on walk time, an effect broadly similar to our findings [
]. Interestingly, although decreases in the oxygen cost of submaximal exercise have been relatively consistently found in younger individuals [
], the oxygen cost of submaximal exercise was not decreased in those healthy older individuals, for reasons that are unclear. Additionally, decreases in the oxygen cost of exercise were not found in the studies of nitrate supplementation of individuals with COPD, suggesting that if exercise endurance is increased in COPD, the mechanisms may be different to that in younger individuals [
We found a heterogeneous response to dietary nitrate in our study population, consistent with Berry et al. who, despite an overall positive effect, found that two of their 11 participants had a decrease in exercise time. This variation in response was also noted by Kerley et al., and its mechanism remains unclear. It may therefore be that individuals with COPD differ in their response to dietary nitrate and that our study population may have been comprised of a greater proportion of non-responders.
Skeletal muscle in individuals with COPD may affected by myriad factors, which include deconditioning, hypoxia, hypercapnia, systemic inflammation, malnutrition, and drug therapy. Multiple abnormalities result, including redox imbalance, autophagy induction, mitochondrial dysfunction, a protein catabolic state with reduced anabolism and structural abnormalities [
]. Intriguingly, it also appears that epigenetic modification may play a role in muscle phenotype and performance in COPD [
]. Additionally, exercise limitation may result from not only skeletal muscle factors, but also lung and haemodynamic factors [
]. These complex, interwoven factors and their varying extents in any given individual with COPD may make prediction of response to dietary nitrate supplementation difficult, and furthermore may render it challenging to precisely identify a mechanism for exercise enhancement, if it exists.
An important question arising from this study is whether the observed difference in endurance distance (11%) and time to fatigue (6%) is clinically important. A change in distance of 60-115 m has been reported as a minimally clinically important difference in the ESWT in the context of a pharmacological trial [
]. In our study, the observed incremental improvement in ESWT was only 79 m whilst the standard deviation of the observed values for in the baseline ESWT was 583 m. These values represent a much reduced effect size of approximately 14% compared with our initial estimate of 55%. Were the sample size larger and the standard deviation reduced, the change in distance may have been clinically significant. An important question is therefore whether the study was under-powered.
Study sample size estimates prior to this study were based on estimates of effect sizes derived from the published evidence. A meta-analysis of the effect of nitrate supplementation on exercise performance in healthy individuals has demonstrated that there is a significant moderate beneficial effect upon exercise performance as measured by time to exhaustion (effect size = 0.79 (95% CI, 0.23-1.35) [
]). Estimates of the effect size derived from studies of the incremental effect of pulmonary rehabilitation program upon exercise distance and time in the endurance shuttle walk test ESWT yielded an effect size of 1.6 [
]. This was deemed to be a clinically important incremental improvement, and was therefore used as a guide for the choice of effect size for the power calculation for this study. We used the published ESWT data to inform us of the anticipated incremental improvement and the standard deviation for baseline performance.
The sample size we chose reflected favorably with the sample sizes chosen for recently reported studies of dietary nitrate supplementation in COPD, in which sample size ranged from 11–15 [
]. Our trial, with a final sample size of 19, did not demonstrate an effect of dietary nitrate supplementation in COPD, concordant with the results of Shepard et al. (n = 13) [
]. This contrasts with the results of Berry et al. and Kerley et al., who found a positive effect on exercise performance for dietary nitrate supplementation in COPD, and whose sample sizes were 15 and 11 respectively [