Conventional clean-eating wisdom has it that nitrates and nitrites are bad—really bad. Both are food preservatives added to processed meats like bacon that have been linked to gastrointestinal cancer and heart disease. The Environmental Working Group (EWG) even listed them on its "Dirty Dozen" list of dangerous food additives last year. That's why so many natural meat products proudly display "nitrate-free" and "nitrite-free" claims on packaging.

But now, two new animal studies from the U.K. show that a diet rich in nitrates can actually improve cardiovascular health by thinning blood and widening blood vessels, lessening the risk for clots and stroke. Recent human trials have also shown that meals heavy in nitrates can lower blood pressure and improve athletic performance.

Wait, what? So are nitrates and nitrites good or bad?

To answer that question, you first need to understand how they're different. Nitrates and nitrites are both naturally occurring chemical compounds found in soil, water, plants, and even our own bodies. One of the most common forms is a natural salt called sodium nitrate, which is exceptionally good at preserving meats and has been used for this purpose for generations. But, near the turn of the 20th century, meat producers made an important discovery: When sodium nitrate interacts with bacteria in meat, it converts to sodium nitrite. Today, just about every manufacturer skips that bacterial conversion and just adds synthetic sodium nitrite directly to cured meats.

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In some ways, that's a good thing: Nitrites keep the fat in meat from going rancid while inhibiting the growth of dangerous bacteria like listeria and botulinum. But in the 1970s, researchers discovered that when meat containing sodium nitrite is heated above 266°F, it creates nitrosamines, or compounds that are carcinogenic to animals. That triggered the USDA to limit the amount of nitrites that may be added to cured meats and to require that all products containing nitrites include vitamin C, which prevents the formation of nitrosamines. Still, in 2010, WHO listed ingested nitrates and nitrites as probable human carcinogens.

But here's the shocker: For all the fuss that's been made about nitrates, nitrites, and nitrosamines, cured meats only account for a minuscule 6% of our dietary nitrate intake. Around 80% of our nitrate consumption comes from veggies, according to the CDC (about 21% is accounted for by drinking water). Celery, leafy greens, beets, parsley, leeks, endive, cabbage, and fennel are the most potent sources, but you'll get some nitrates from almost any plant you eat.

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Why are plants so high in nitrates? They pick up from soil, nitrogen-based fertilizer, water, and nitrogen in the atmosphere. When we eat nitrates in plants, bacteria in our mouth convert them to—you guessed it—nitrites. Nitrites are then absorbed and stored in our cells until they're turned into nitric oxide, a compound that's proven to relax blood vessels and increase blood flow.

Nitrites are so essential, in fact, that our bodies even create it on their own. "For every kilogram of body weight you carry, your body naturally produces about a milligram of nitrite," says Jeff Sindelar, PhD, associate professor at University of Wisconsin-Madison department of animal sciences.

The bottom line? Some researchers have found associations (but not direct causal links) between high nitrite consumption and certain cancer risks. "There is some epidemiological evidence suggesting that, when added to foods, these compounds may be associated with esophageal or stomach cancer," says Johanna Congleton, PhD, EWG senior scientist. "Those studies are the ones that give us cause for concern."

But others have called for the re-classification of dietary nitrates as a nutrient, not a potential carcinogen. "Worrying about consuming these products at high enough levels where they might be considered toxic is really an insignificant concern," says Sindelar. (That's not a carte blanche to gnaw on salami logs every day. There is well-documented consensus that high consumption of processed meats can up your risk for heart disease, cancers, and death—no matter what preservatives have been added to them.)

There's no simple answer. Whether nitrates and nitrites are a boon or a blight may depend heavily on what foods they come from and the particular human ingesting them. One thing is for sure: Nitrates' and nitrites' reputation as avoid-at-all-cost, cancer-causing nasties does not appear to be entirely accurate.

We'll be waiting for more news with a spinach salad—and, okay, maybe a slice or two of bacon.

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