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Spinach at St. Lawrence Market in Toronto on Aug. 22, 2020.Glenn Lowson

It all started with marathoners and their obsessive quest for an edge.

Back in 2009, a research team at Britain’s University of Exeter published the seemingly improbable results of a study exploring the endurance-boosting effects of beet juice. Six days of daily drinks enabled volunteers to last 16 per cent longer in a cycling test – an effect, the lead researcher marvelled, that “cannot be achieved by any other known means, including training.”

The idea was swiftly embraced by marathoners leading up to the 2012 Olympics and spread rapidly to other endurance sports. Researchers, too, rushed to investigate beet juice’s mysterious powers, launching hundreds of studies and soon finding that the drink might improve health, as well as athletic performance. They eventually identified nitrate as the key active ingredient. And that, as a newly published review paper in the journal Medicine & Science in Sports & Exercise recounts, is when things got complicated.

Long maligned as a possible carcinogen in processed meats, nitrate is actually a crucial and underappreciated element in famously healthy foods such as spinach and arugula, according to the new review, whose authors include University of Exeter physiologist Andrew Jones, who also led the original 2009 study. But harnessing the benefits of nitrate is far from straightforward.

The real goal of eating nitrate-rich foods is to increase levels of nitric oxide, a key signalling molecule that helps regulate all sorts of bodily functions, most notably the properties of your blood vessels. For example, it enables your vessels to dilate and contract smoothly, keeping your blood pressure in a healthy range. It also helps regulate blood sugar and optimizes muscle contraction, which is why athletes benefit.

When you drink beet juice or eat leafy greens, the nitrate content is absorbed from your stomach into your saliva, in which form it returns to your mouth. Helpful bacteria in your mouth then convert the concentrated nitrate from your saliva into an intermediate form called nitrite, which you then swallow again. This nitrite is then converted into nitric oxide and circulated throughout the body.

After a decade of research, it’s now clear that this somewhat circuitous pathway really works. Drinking a dose of nitrate-rich beet juice produces an immediate and measureable drop in blood pressure. And you can get the same result by simply adding about 250 grams of nitrate-rich vegetables such as arugula and beet, containing a total of 400 milligrams of nitrate, to your lunch, according to a Dutch study published last month.

But there are caveats. If you wipe out those crucial oral bacteria by regularly using strong antibacterial mouthwash, you won’t be able to convert the nitrate to nitrite, blocking the health benefits you’d otherwise see. In fact, one 2018 study found that twice-a-day mouthwash users were roughly 50 per cent more likely to develop prediabetes or diabetes than non-users over a three-year-period.

That doesn’t mean you should never clean your mouth, says University of Exeter physiologist Anni Vanhatalo, a co-author of the new review paper who researches the role of oral bacteria on health and performance.

“Occasional tongue scraping and brushing teeth regularly could be thought of as ‘gardening,’” she says. “A little bit of regular pruning and weeding will maintain the healthy diversity of the oral ecosystem and prevent a small handful of bacteria behaving like invasive weeds and taking over the entire garden.”

And the question of possible negative effects still lingers.

“For nitrate, it’s important to distinguish between sources,” says Mary Ward, a senior investigator at the U.S. National Cancer Institute who studies the health effects of nitrate.

The hypothetical cancer risk associated with nitrate stems from its potential transformation in the body into molecules called N-nitroso compounds, Ward says. For this to occur, the nitrate has to be ingested with other molecules called amines and amides, which are found in red meat. That’s why processed meats preserved with nitrates are considered by some to be risky.

Conversely, the presence of antioxidants such as vitamin C hinders the conversion of nitrate to N-nitroso compounds. For that reason, eating lots of nitrate-rich (and amine- and amide-free) vegetables, which also contain lots of antioxidants, shouldn’t be a problem. Still, Ward points out, there’s no data on the effects of, say, mega-dosing beet juice over a prolonged period of time. As always, moderation is probably a good idea.

For marathoners seeking an edge in their next race, that sort of occasional beet juice binge is nothing to worry about. For those of us more interested in the possible blood pressure and blood sugar benefits, the recent Dutch study suggests that we’re better off doing what nutrition experts have been telling us all along: attack those leafy greens.

Alex Hutchinson is the author of Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Follow him on Twitter @sweatscience.

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