Effect of dietary nitrate on human muscle power: a systematic review and individual participant data meta-analysis

Randomized, double-blind, placebo-controlled, crossover trials; 19 studies; 268 participants (218 men, 50 women); ages varied; populations included healthy adults, older adults, Olympic-level athletes, and patients with heart failure; maximal power measured in non-fatigued state.

Beetroot juice (dietary nitrate) ingestion; nitrate dose per session ranged from 6.4 to 15.9 mmol; most studies used a single acute dose before testing; some used multi-day dosing up to 6 days; forms included BRJ beverages and BRJ powder; one study used potassium nitrate (KNO3) as the nitrate source.

Dietary nitrate supplementation significantly increases maximal muscle power in humans. Pooled effect sizes: fixed effects g = 0.42 (95% CI 0.29-0.56; p = 6.31e-11); random effects g = 0.45 (95% CI 0.30-0.61; p = 1.06e-9); I2 = 22.8% (limited heterogeneity). Acute dosing yielded a larger effect (g = 0.54; 95% CI 0.37-0.71; p = 6.77e-12) than multi-day dosing (g = 0.22; 95% CI 0.01-0.43; p = 0.00363). Age, sex, and test modality did not significantly modify the effect. The average increase corresponds to about a 5% improvement in maximal power, which is likely to have practical athletic and clinical significance. The underlying mechanism remains to be fully established.

Not all studies reported the actual nitrate dose or biomarkers of nitric oxide bioavailability (11/19 and 10/19, respectively). Beetroot juice nitrate content varies between products and over time. Some included studies could not be included due to unavailable data from authors. Populations were diverse (healthy, older adults, heart-failure patients, athletes), which may affect generalizability. Dosing regimens varied (acute versus multi-day) and BRJ formulations; inconsistent biomarker reporting limits mechanistic conclusions.