The science behind it
For a long time, nitrate and nitrite were thought of as inert waste products of nitric oxide metabolism, or as unwelcome contaminants in food. That view has changed. Nitric oxide helps regulate blood flow, cell signalling and how tissues respond to low oxygen, and the body can make it from the nitrate in our diet as well as through its own enzymes (Lundberg et al., 2009).
Vegetables are a major source. A single serving of spinach, lettuce or beetroot contains more nitrate than all three of the body’s nitric oxide-producing enzymes generate in a day (Lundberg et al., 2009). Humans lack a specific, efficient enzyme for turning nitrate into nitrite (Chai et al., 2024), and this is where the mouth comes in. After a nitrate-rich meal, about a quarter of the nitrate absorbed into the blood is taken up by the salivary glands and secreted into saliva, where it reaches 10 to 20 times its concentration in plasma (Chai et al., 2024). Bacteria in the mouth then convert it to nitrite, so that saliva ends up with around 1,000 times more nitrite than blood plasma. Swallowed nitrite forms nitric oxide in the acidic stomach, and plasma nitrite rises in a sustained way, which is why the authors describe dietary nitrate as a slow-release “prodrug” of nitrite (Lundberg et al., 2009).
Once in the blood and tissues, nitrite can be converted to nitric oxide through several pathways, particularly when oxygen availability is low. Vitamin C and certain polyphenols can also promote nitrite-derived nitric oxide formation, especially under acidic conditions (Lundberg et al., 2009; Chai et al., 2024). This raises an interesting possibility: the vitamin C in lemon and the polyphenols in berries may complement the dietary nitrate in leafy greens. However, whether combining these ingredients in a smoothie meaningfully increases nitric oxide availability or produces additional health benefits remains untested.
What the evidence tells us
Well supported
The route from dietary nitrate to nitrite and then nitric oxide, and the essential role of oral bacteria in the first step. In healthy volunteers given beetroot juice, blood pressure fell by around 10 mmHg as plasma nitrite peaked. When volunteers spat out their saliva for three hours instead of swallowing it, the effect disappeared, and in rats an antibacterial mouthwash had the same blocking effect (Lundberg et al., 2009). The same holds in people without any extra nitrate in the diet. When 19 healthy adults aged 18 to 45 used an antiseptic chlorhexidine mouthwash twice daily for a week, nitrite production in the mouth fell by 90%, nitrite in the blood fell by 25%, and blood pressure rose by 2 to 3.5 mmHg within a day, with the rise tracking the fall in blood nitrite (Kapil et al., 2013). The authors note that the study was not randomised or placebo controlled, and that it tested only one type of mouthwash.
Promising
Nitrate may reshape the oral microbiome in later life. In a study of older adults, ten days of dietary nitrate supplementation shifted the balance of oral bacteria compared with a placebo. A cluster of bacteria associated with pro-inflammatory metabolism became less abundant, and two nitrate-sensitive clusters showed stable relationships with measures of cardiovascular and cognitive health (Vanhatalo et al., 2021). Other studies have found that nitrate-reducing bacteria such as Rothia and Neisseria become more abundant after nitrate supplementation, and are relatively more abundant in healthy people than in people with high blood pressure (Chai et al., 2024). These are associations from small, short studies, and the researchers present these bacteria as targets for future work rather than as proven benefits.
Plausible pathway
A link to cognitive ageing. Ageing is associated with reduced nitric oxide production, with knock-on effects for cardiovascular and cognitive function (Vanhatalo et al., 2021). With age and illness, the body’s own enzyme route to nitric oxide becomes less effective, which is why the dietary route may matter more in later life (Chai et al., 2024), and nitric oxide regulates both blood vessel tone and signalling between nerve cells (Vanhatalo et al., 2021). It is therefore reasonable to ask whether a nitrate-rich diet supports the brain through better blood flow. That question has not yet been answered by long-term trials measuring cognitive outcomes, so it belongs here rather than higher up.
What we do not know
The studies above used beetroot juice or nitrate supplements, not smoothies. We do not know how much nitrate this smoothie delivers compared with the doses tested, and a single smoothie should not be expected to reproduce the blood pressure effects seen in trials.
Nitrate has also had a mixed reputation. Over the past half century it was linked to a risk of stomach cancer, which led to strict limits on nitrate in food and drinking water. At the 2009 Stockholm meeting summarised by Lundberg et al., one researcher raised concerns about compounds formed where the gullet meets the stomach, and suggested that vitamin C could increase this. Another concluded that the evidence behind the current intake limits is relatively weak, noting that vegetable-rich diets such as the Mediterranean and traditional Japanese diets exceed the World Health Organization’s acceptable daily intake (Lundberg et al., 2009). A more recent review takes a different view of vitamin C, noting that vitamin C, vitamin E and polyphenols can help convert nitrite to nitric oxide and so inhibit the formation of nitrosamines, the compounds behind the cancer concern. The same review concludes that nitrite from diet and oral bacteria is far below risk levels, while still recommending that nitrosamine formation be monitored as nitrate-reducing probiotics are developed (Chai et al., 2024). The question is not fully settled.
The bigger picture
The mouth is the first microbial community a meal meets, and this recipe is a good example of why it matters. The same smoothie then carries fibre from the oats, flaxseed and fruit further down the gut, where the colon’s much larger microbial community ferments it. You can follow that journey in One meal, one axis.
Researchers are now asking whether nitrate-reducing oral bacteria could one day be given as probiotics, alongside a nitrate-rich diet, to help maintain healthy blood pressure (Chai et al., 2024). For now that is a research direction rather than a recommendation, and some bacteria that reduce nitrate can also cause disease, so strain safety would need careful testing.
For my own research on phytochemicals and cognitive ageing, the nitrate story is a useful reminder that plant foods can influence the body through the microbes that process them, not only through what they contain. It is also a reminder of how far the evidence has to travel between a convincing mechanism and a meaningful difference to health in later life.
References
Chai, X., Liu, L., & Chen, F. (2024). Oral nitrate-reducing bacteria as potential probiotics for blood pressure homeostasis. Frontiers in Cardiovascular Medicine, 11, Article 1337281. https://doi.org/10.3389/fcvm.2024.1337281
Kapil, V., Haydar, S. M. A., Pearl, V., Lundberg, J. O., Weitzberg, E., & Ahluwalia, A. (2013). Physiological role for nitrate-reducing oral bacteria in blood pressure control. Free Radical Biology and Medicine, 55, 93–100. https://doi.org/10.1016/j.freeradbiomed.2012.11.013
Lundberg, J. O., Gladwin, M. T., Ahluwalia, A., Benjamin, N., Bryan, N. S., Butler, A., Cabrales, P., Fago, A., Feelisch, M., Ford, P. C., Freeman, B. A., Frenneau, M., Friedman, J., Kelm, M., Kevil, C. G., Kim-Shapiro, D. B., Kozlov, A. V., Lancaster, J. R., Jr., Lefer, D. J., … Weitzberg, E. (2009). Nitrate and nitrite in biology, nutrition and therapeutics. Nature Chemical Biology, 5(12), 865–869. https://doi.org/10.1038/nchembio.260
Vanhatalo, A., L’Heureux, J. E., Kelly, J., Blackwell, J. R., Wylie, L. J., Fulford, J., Winyard, P. G., Williams, D. W., van der Giezen, M., & Jones, A. M. (2021). Network analysis of nitrate-sensitive oral microbiome reveals interactions with cognitive function and cardiovascular health across dietary interventions. Redox Biology, 41, Article 101933. https://doi.org/10.1016/j.redox.2021.101933
