The tongue bacteria that put leafy-green nitrate to work were more common in people without prediabetes

Nitrate is the compound that makes spinach and other leafy greens interesting to physiologists. You swallow it with your vegetables, it is absorbed, and then something odd happens: the salivary glands pull nitrate back out of the blood and pour it into saliva. Bacteria living in the mouth turn that nitrate into nitrite, and some of the nitrite goes on to become nitric oxide, a molecule the body uses to relax blood vessels. That first step is done by the bacteria, not by us.

A study published online in Cell Reports Medicine on 7 October 2026 by researchers at the National University of Singapore asked whether that bacterial step has anything to do with early trouble with blood sugar. They took samples from the top surface of the tongue of 472 people in a population-based cohort and used metagenomic sequencing, which reads the genes of every microbe present, to see which bacteria were there and what they were equipped to do. Eleven types of bacteria and nine microbial pathways differed between people with and without prediabetes.

Two findings stood out. Rothia mucilaginosa was the only microbe linked with a lower prevalence of prediabetes, and nitrate reduction was the pathway most strongly linked with it. People without prediabetes also had more nitrate and nitrite in their saliva, which fits the idea that more of the nitrate they took in was available to be used. When the team grew a Rothia strain isolated from the study in the lab, it could take nitrate down two routes in low-oxygen conditions: one towards nitric oxide, and one towards ammonium, which doesn't make nitric oxide. Adding these microbial features to the usual clinical risk factors improved the researchers' estimate of who would progress to prediabetes.

None of this shows that the bacteria protect anyone. It comes from one group of people, the main comparison is between those who had prediabetes and those who didn't, and people with healthier blood sugar may differ in diet, habits or gum health in ways that also shape their mouth bacteria. Several authors declare commercial interests, listed in the paper. The authors go as far as suggesting that tongue cleaning may affect these microbial functions, which is an idea to test, not advice. What the study does is add weight to earlier work showing that wiping out mouth bacteria has measurable effects elsewhere in the body, and it gives a reason to treat the mouth as part of how food becomes health.