Beetroot Juice Reshapes the Mouth Microbiome, But Leaves the Gut Untouched

Nitric oxide is one of the body's most important vascular signals, keeping blood vessels relaxed and blood pressure in check, yet the body cannot make it alone. It depends on bacteria in the mouth to convert dietary nitrate into nitrite, a precursor that tissues then reduce to nitric oxide. A randomized, double-blind, placebo-controlled crossover trial published in Molecular Nutrition & Food Research now shows that four weeks of beetroot juice consumption measurably shifts the oral microbiome of older adults with treated hypertension toward exactly the nitrate-reducing species the body needs, while leaving the intestinal microbiome essentially untouched.

Neisseria rises, Veillonella falls in oral bacteria after nitrate

The most striking result is in the numbers. After four weeks on nitrate-rich beetroot juice, participants showed a statistically significant increase in the relative abundance of the genus Neisseria in oral samples, alongside a significant decrease in Veillonella (PERMANOVA p < 0.001). An increase in Rothia, another nitrate-reducing genus, was also recorded. Before the intervention, Neisseria and Veillonella were already the most abundant genera in participants' mouths, so the shift represents a meaningful rebalancing of a community that was already present, not the introduction of entirely foreign microbes. Nitrate-depleted placebo beetroot juice, consumed during the crossover phase, did not produce the same effect, confirming that inorganic nitrate was the active driver.

The direction of change matters. Neisseria species are among the most efficient nitrate reducers in the human oral cavity; more of them means a more capable pipeline for converting dietary nitrate into nitrite and, ultimately, nitric oxide. Veillonella, by contrast, is less central to that pathway, so its relative decline reinforces the idea that nitrate loading selectively favors the bacteria best equipped to metabolize it.

Why this matters for blood pressure and cardiovascular risk

Hypertension affects hundreds of millions of people globally, and the population studied here, older adults aged 56 to 71 already on antihypertensive medication, represents a group for whom additional, non-pharmacological support is clinically valuable. The oral nitrate-to-nitrite-to-nitric-oxide pathway is a well-recognized contributor to vascular tone, and research on dietary strategies for cardiovascular health has increasingly focused on how food components interact with physiology at the microbial level rather than in isolation.

By promoting an enriched community of nitrate-reducing oral bacteria, regular beetroot juice consumption could amplify the cardiovascular benefit of a nitrate-rich diet beyond what the nitrate molecule alone delivers. The study's authors note that these oral microbiome modulations are associated with improved cardiovascular health outcomes, a connection that points toward the mouth, not just the gut, as a meaningful site of diet-microbiome-cardiovascular interaction. For patients already managing hypertension with medication, even modest additive effects on nitric oxide availability could translate to clinically relevant improvements in blood vessel function over time.

How a crossover design strengthened the evidence

Fifteen participants completed the randomized, double-blind, placebo-controlled crossover trial. Each person consumed either nitrate-rich or nitrate-depleted beetroot juice for four weeks, then crossed to the other condition, meaning every participant served as their own control. This design eliminates the individual-level variability in baseline microbiome composition that would otherwise obscure intervention effects in a small sample, making it a methodologically sound choice for a pilot study of this kind.

The team analyzed oral and intestinal microbiome compositions from both phases and paired those data with measurements of blood and salivary nitrate and nitrite levels, oxidative stress markers, inflammation biomarkers, and blood pressure. This multivariate approach allowed the researchers to assess whether microbiome shifts tracked with systemic physiological changes, not just with each other. Animal models have historically been a first step in understanding how dietary compounds alter microbial communities; this human trial adds direct evidence in the population most likely to benefit from dietary nitrate strategies.

Why the gut microbiome did not change

The absence of intestinal microbiome changes deserves as much attention as the oral findings. The gut harbors far more microbial biomass and diversity than the mouth, but it appears less responsive to a four-week nitrate intervention, at least at the doses and durations tested here. One likely explanation is anatomical: dietary nitrate is largely absorbed in the small intestine and recycled into saliva, where oral bacteria reduce it. By the time residual nitrate reaches the colon, concentrations may be too low to meaningfully shift a community adapted to processing a wide range of fermentable substrates.

This selectivity is not a limitation so much as a clarification. It suggests the oral microbiome is the primary biological interface for dietary nitrate, and that interventions targeting nitric oxide production via food should be evaluated with oral, not just fecal, sampling. Studies that measure only gut microbiota composition may be missing the key site of action.

Open questions about dose, duration, and long-term stability

With 15 participants and a four-week window, this trial is explicitly a pilot. It cannot determine the minimum effective dose of dietary nitrate, the timeline over which oral microbiome changes become stable, or whether the shifts persist after beetroot juice consumption stops. The participants were a specific subgroup, older adults with treated hypertension, so the findings may not generalize to younger or normotensive individuals whose baseline oral microbiomes and cardiovascular risk profiles differ.

The correlation analyses linking microbiome changes to blood pressure and biomarkers of oxidative stress and inflammation were not reported here in detail, leaving open the question of whether the oral bacterial shifts actually translated into measurable physiological benefit in this cohort or simply represent a biological intermediate. Future trials with larger samples and longer follow-up will need to connect the microbial dots to clinical endpoints.

Beetroot juice as a targeted oral microbiome intervention

The finding reframes beetroot juice not just as a source of dietary nitrate, but as a tool for selectively enriching the nitrate-reducing bacteria that make dietary nitrate cardiovascularly useful in the first place. That feedback loop, nitrate feeding the bacteria that process nitrate, points toward a practical dietary strategy that works with the body's own microbial infrastructure. The study was funded by the Austrian Science Fund (KLI 858) and involved researchers from the University of Vienna and Edith Cowan University's Nutrition and Health Innovation Research Institute, among other institutions.

For clinicians and researchers working on non-drug approaches to hypertension management, these results suggest that mouth bacteria deserve a seat at the table alongside gut microbiota when evaluating how diet shapes cardiovascular physiology.