How nitrate (and beets) work, and why mouthwash is bad

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The performance-boosting effect of beet juice, attributed to the nitrate it contains, has been one of the biggest sports science stories of the past year. A few days ago, Swedish researchers published a new study in the journal Cell Metabolism (press releases here and here) that answers some questions about how nitrate works — and raises some questions about whether mouthwash might actually hurt your performance. Interestingly, this Swedish group has been publishing results on the performance-boosting effects of nitrate since 2007. It’s only when Andrew Jones’s group at the University of Exeter started publishing results in 2009 using beets instead of isolated nitrate that everyone started paying attention.

The study: a randomized, double-blind crossover trial with 14 subjects given either 0.1 mmol/kg of sodium nitrate or a placebo each day for three days. Since their previous studies have shown that this protocol allows subjects to use less oxygen while cycling at a given intensity, this time they looked for the root cause. They isolated mitochondria (the cellular “power plants” that generate ATP to fuel muscle contractions) in the subjects. Sure enough, the mitochondria from the nitrate group used less oxygen to generate a given amount of ATP. Further (rather complicated) experiments suggested that this is because nitrate leads to lower levels of a protein that causes efficiency-sapping proton leakage across the mitochondrial membrane.

So what does it mean? Well, the researchers suggest that it might be nitrate (which, as they note, “has previously been considered merely as an inert end product of NO metabolism or as a potentially toxic constituent in our diet” — think hot dogs) that is responsible for the well-known but poorly understood health benefits of a diet high in vegetables (particularly leafy green ones). And of course, it’s more evidence that nitrate might be the real deal as an ergogenic aid for endurance athletes.

But what really caught my attention was the following offhand remark in one of the press releases:

As an interesting aside, Weitzberg says that the benefits of dietary nitrates suggest that powerful mouthwashes may have a downside. “We need oral bacteria for the first step in nitrate reduction,” he says. “You could block the effects of inorganic nitrate if you use a strong mouthwash or spit [instead of swallowing your saliva]. In our view, strong mouthwashes are not good if you want this system to work.”

Huh? Here’s a brief explanation from the paper itself about how nitrate is used by the body:

Circulating nitrate, normally derived both from endogenous NO [nitric oxide] production and from dietary intake, is actively taken up by the salivary glands, excreted in saliva, and reduced to nitrite by commensal bacteria in the oral cavity.

The statement about mouthwash having a negative effect is just speculation at this point, not backed up by any studies. But it’s interesting…

UPDATE Feb. 9: Thanks to Colby for pointing out that the same Swedish group has actually already done a study looking at the effects of mouthwash, published back in 2008. The gist is as follows: the nitrate we take in through our diet is converted first to nitrite then to nitric oxide, which is what has a positive effect on blood pressure, performance and other parameters. But our cells are unable to convert nitrate to nitrite — to make that conversion, we rely almost exclusively on friendly bacteria in our saliva. To test that hypothesis, the researchers fed nitrate (the equivalent of about 300 g of spinach or beets) to volunteers 15 minutes after they gargled with an antibacterial mouthwash (Corsodyl). Here’s what they saw:

Even though nitrate levels in saliva were similar, the bacteria needed to convert nitrate to nitrite was gone in the mouthwash experiments, so nitrite levels in saliva remained flat. As a result, nitrite levels circulating in the blood (which is what’s shown above) increased far less in the mouthwash than the non-mouthwash case.

Unfortunately, this still doesn’t tell us much about timing and chronic effects. If you mouthwash before bed, will your nitrate-to-nitrite conversion still be impaired at dinner the next day? I really don’t know. I’m certainly going to keep mouthwashing — but it might make me think twice about the gratuitous middle-of-the-day extra gargle!