The downside of mouthwash

On the surface, it’s a puzzling finding: In a study of nearly 1,000 adults in Puerto Rico, those who used mouthwash twice a day or more were about 50 per cent more likely to develop prediabetes or diabetes during the three-year follow-up period. What’s the link between bacteria in your mouth and your metabolic health? The answer, it turns out, could be nitrate, a key component of leafy greens and other vegetables like beets and rhubarb – and the same thing that gives beet juice its endurance-boosting properties.

When you drink beet juice or eat other nitrate-rich foods, “friendly” bacteria in your mouth convert some of this nitrate into nitrite. Elsewhere in your body, nitrite is converted to nitric oxide, which has powerful effects on how your blood vessels function and how your muscles use glucose circulating in the blood. A series of studies over the past decade has shown that nitrate-rich foods like beet juice improve your exercise efficiency, allowing you to spend less energy to maintain the same pace or power output. These foods can also lower blood pressure and help with blood sugar regulation.

The problem, though, is that if you wipe out the friendly bacteria in your mouth, you break that chain of events, because the nitrate doesn’t get converted to nitrite. That’s most likely to happen if you’re using prescription antibacterial mouthwash on a regular basis. But the Puerto Rican study, which was published in the journal Nitric Oxide (that’s how important the molecule is: It has its own journal!), is the first to show that even ordinary over-the-counter mouthwash can have negative effects – although they only showed up in people gargling twice a day. The takeaway message isn’t that you should never use mouthwash; instead, it’s a reminder of the complex relationship we have with our microbiome. Bugs aren’t necessarily good or bad; often they’re both.