Tier 2 — strong

Dietary nitrate supplementation and exercise performance

Jones AM
Sports Medicine 2014 Volume 44, Suppl 1, pages S35--S45

Bibliography

PubMed
PMID 24791915
PubMed Central
PMC4008816
DOI
10.1007/s40279-014-0149-y
Funding
Published in a supplement supported by Gatorade Sports Science Institute (GSSI), a division of PepsiCo, Inc. The author attended a GSSI Expert Panel meeting in April 2012 and received honoraria from GSSI for meeting participation and manuscript writing. The author acknowledges James White Drinks Ltd. and Beet It Ltd. for supplying beverages used in his laboratory studies free of charge.
Competing interests
The author received honoraria from Gatorade Sports Science Institute (a PepsiCo, Inc. division) for meeting participation and preparation of this manuscript. The views expressed do not necessarily reflect the position or policy of PepsiCo, Inc. Beetroot juice used in the author's laboratory studies was supplied free of charge by James White Drinks Ltd. and Beet It Ltd.

Study snapshot

DesignNarrative review
ModelYoung, healthy, physically active human participants across the reviewed literature; discussion largely restricted to normoxia
SampleNot applicable (review synthesising primary studies with sample sizes typically 8-20 per trial)
InterventionDietary nitrate delivered as sodium nitrate (typically 0.1 mmol/kg body mass/day) or nitrate-rich beetroot juice (typically 5-9 mmol nitrate/day) versus placebo, with supplementation periods from acute single-dose (2.5 h pre-exercise) to 15 days
DurationReviewed protocols spanned acute dosing (single dose 2.5 h pre-exercise) through chronic supplementation up to 15 days
EndpointsSteady-state pulmonary oxygen uptake during submaximal exercise; Oxygen cost of walking, running, and cycling; Time to exhaustion during high-intensity constant work-rate exercise; Time-trial performance (4 km, 10 km, 16.1 km, 50 mile); VO2max; Blood pressure; Plasma nitrate and nitrite concentrations

What the study showed, in plain terms

Nitric oxide is a signalling molecule your body uses to widen blood vessels, control blood flow, and regulate how muscles use oxygen. Historically it was thought your body could only make nitric oxide by converting the amino acid L-arginine using enzymes called nitric oxide synthases. That view has changed. Inorganic nitrate — the same nitrate found in green leafy vegetables and beetroot — is now known to be reduced in the body first to nitrite, then to nitric oxide, particularly in low-oxygen conditions like those inside exercising muscle.

This 2014 review from the Exeter group synthesises the first wave of human trials asking a simple question: if you supplement with dietary nitrate, do you perform better? The consistent finding across early studies was that nitrate lowered the oxygen cost of submaximal exercise by roughly 5%, an effect that had long been considered essentially fixed. In some studies, this translated into meaningful improvements in time to exhaustion (up to 15–25%) and modest but real improvements in cycling time-trial performance (roughly 2.7–2.8% over 4 to 16.1 km).

The picture in elite endurance athletes was less consistent. Highly trained athletes, who already have well-developed capillary networks and higher baseline plasma nitrite, appear to respond less reliably to nitrate — at least with the acute or short-term protocols tested at the time this review was written. The mechanisms proposed for nitrate's effects include improved mitochondrial efficiency, altered muscle contractile function, and enhanced blood flow to type II muscle fibres.

Key findings

  • Nitrate supplementation (approximately 5–9 mmol/day, from sodium nitrate or beetroot juice) reduced steady-state pulmonary oxygen uptake during submaximal exercise by roughly 5% — a finding that overturned the long-held assumption that oxygen cost at a given work rate is essentially immutable.
  • Time to exhaustion during high-intensity constant work-rate exercise increased by approximately 15–25% following 4–6 days of beetroot juice supplementation (5.1–6.2 mmol nitrate/day) in recreationally active men.
  • Cycling time-trial performance improved by approximately 2.7–2.8% over 4 km and 16.1 km after acute beetroot juice ingestion in club-level cyclists (mean VO₂max ~56 mL/kg/min); 10 km performance improved after 6 days of concentrated beetroot juice (8 mmol nitrate/day) in trained cyclists.
  • Effects on exercise efficiency were maintained across 15 days of continued supplementation and were still present when normal dietary nitrate intake was not restricted.
  • Elite endurance athletes (VO₂max 60–70 mL/kg/min) showed no consistent ergogenic response to acute or short-term nitrate, with plasma nitrite responses often smaller than in less-trained subjects.
  • Systolic blood pressure fell by roughly 8 mmHg following beetroot juice supplementation across multiple studies reviewed.
  • Proposed mechanisms include improved mitochondrial P/O ratio (reduced proton leak via lowered adenine nucleotide translocase expression), enhanced sarcoplasmic reticulum calcium handling in type II fibres, and preferential increases in blood flow to type II muscle fibres.

What this study can and cannot tell us

This is a narrative review authored by a researcher whose laboratory generated a substantial fraction of the primary studies discussed, and which received both funding from Gatorade Sports Science Institute and free beetroot juice from commercial suppliers. Selection and framing of studies may reflect that vantage point. The review was not conducted using the systematic methods or risk-of-bias assessment that would be expected of a more recent systematic review or meta-analysis.

The evidence base at the time of publication was almost entirely from young, healthy, physically active men. Data on women, older adults, clinical populations, and endurance events longer than approximately 30 minutes remained thin. Ergogenic effects in highly trained athletes were inconsistent across the acute and short-term protocols tested, and the review did not resolve whether longer-term supplementation or higher doses might change that.

Most trials reviewed used small samples (typically 8–20 participants) and short washout periods. Effect sizes for time-to-exhaustion tests substantially exceed those seen in time trials, so the eye-catching "15% improvement" figures do not directly translate to real-world race performance. Long-term safety of chronic high-nitrate vegetable-product intake was acknowledged as unknown; concerns about nitrosamine formation and nitrite salt toxicity remain the main safety considerations flagged.

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