Effects of Dietary Nitrate Supplementation on Physical Performance and Muscle Strength in Older People—A Systematic Review

Background

Sarcopenia, the loss of muscle strength and mass with age, is a major cause of morbidity for older people. Dietary nitrate supplementation has been proposed as an intervention to improve skeletal muscle function via action as nitric oxide (NO) donors. However, the effect of nitrate supplementation on physical performance and muscle strength in older people is unclear. We aimed to systematically review evidence on whether dietary nitrate supplementation improves markers of muscle strength, muscle mass and physical performance in older people.

Methods

We conducted a systematic review of randomised controlled trials according to a prespecified protocol by two reviewers. We included interventional studies using dietary nitrate supplementation, mean participant age of >60 years, with or without muscle weakness. Outcomes of interest were physical performance, muscle strength and muscle mass. Risk of bias was assessed using a modified version of the Cochrane Risk of Bias tool. Results were grouped by intervention and outcome measures and were described by narrative synthesis.

Results

Twenty-eight studies were included, with a size range of 8–72 participants. Intervention duration ranged from a single dose to 12 weeks. Seven studies were in healthy older people. Most studies had a high or unclear risk of bias; three had a low risk of bias. One-hundred-two outcomes were reported; 67 were related to physical performance, and 35 were related to muscle strength. No included study measured muscle mass. Thirty-three outcomes showed significant improvement, two showed significant worsening and 67 showed no statistically significant difference. Meta-analysis was not possible due to data heterogeneity. Subgroup analyses for different doses of nitrate (above or below 10 mmol nitrate per day), duration of treatment or specific commonly measured outcomes did not indicate any subgroup more likely to show positive results. The proportion of positive outcomes was similar in studies using beetroot extract, nitrate alone or exercise as a co-intervention.

Conclusions

Current evidence is insufficient to decide if dietary nitrate supplementation improves skeletal muscle function in older people. Future studies should be longer, larger and target older people with sarcopenia or frailty.