Nitrates can be either helpful or harmful to your health, depending upon the amount and which biochemical pathway they follow.
Nitrates (NO3) become nitrites (NO2), and these can then get further broken down into either nitric oxide (NO), which is beneficial to the cardiovascular system especially, or into nitrosamines, which are carcinogenic.
So what makes the difference?
Nitrates in Vegetables
Nitrates are found in the soil, so the vast majority of dietary nitrates come from plants, especially vegetables—which, from a macronutrient standpoint, are primarily carbohydrates as opposed to fat or protein. Veggies are of course the healthiest form of carbohydrates, and the nitrates in veggies tend to turn into nitric oxide: the body’s natural vasodilator, or anti-hypertensive. Leafy greens are perhaps best known for this, as well as beets. Other good sources include citrus, cacao, and watermelon.
Why do the nitrates in fruits and veggies tend to go down the nitric oxide pathway, rather than the nitrosamine pathway? One possibility is because fruits and veggies are also rich in antioxidants such as Vitamin C, bioflavonoids, and polyphenols, which block formation of nitrosamines.
Nitrates in Meat
Nitrates are not naturally found in meat; they are added as preservatives in processed meat products, in order to bind oxygen and keep the meat from turning brown. As I wrote about here, processed meat such as bacon, hot dogs, sausage, and lunch meat has been extensively linked with bowel cancers. This is because these nitrates, while fewer in number compared to those found in fruits and veggies, tend to form nitrosamines rather than nitric oxide.
Why? As mentioned above, they lack the antioxidants found in fruits and veggies. But there’s more to it than just that.
The vast majority of the nitrates in fruits and veggies get absorbed in the small intestine, probably because these are mostly carbohydrate, and most soluble carbs are absorbed by the time they get to the small intestine. This means these nitrates bypass the nitrosamine-forming fermentation process that happens in the colon. The nitrates added to meat, on the other hand, make it to the colon—perhaps because meat is mostly protein and fat.
Also, any iron found in animal meat will be in the heme form, which tends to oxidize in the presence of nitrates (just like it does in the meat while it’s still in the package), leading to formation of nitrosamines. By contrast, iron in fruits and veggies is comparatively quite low, and it is not in heme form.
Excessive levels of nitrates can themselves be toxic, as well. As mentioned above, nitrates oxidize hemoglobin so that oxygen cannot bind to it: a condition called methemaglobinemia. Since the amount of nitrates added to preserve meat is relatively small, nitrate toxicity occurs from either highly concentrated nitrates in plants, exposure to commercial fertilizer, or most commonly, contaminated water.
As mentioned above, nitrogen is essentially one of the macronutrients for plants. Plants metabolize it into nitrites, which are used as building blocks for amino acids, which then form proteins. (While plants are mostly carbohydrate, they do contain some protein also.) High nitrate levels build up in plants if the conversion pathway to nitrites gets down-regulated, which can happen in conditions less optimal for its growth, such as drought or reduced sunlight. But since there’s still nitrogen in the soil, as long as the plant is alive, it continues to soak up macronutrients, building up nitrate levels without burning them off (much like a human might store empty calories he’s not using).
Even still, it’s unlikely a human will consume enough of these plants to cause a nitrate problem (few of us really eat that many leafy greens in a sitting). But babies are more at risk for this than adults, as they are less able to cope with a high nitrate load. This is one reason why weaning is typically delayed at least until after four months of age, as many of the foods prepared as baby foods tend to be naturally higher in nitrates.
A more common source of nitrate toxicity comes from contamination of well water with high nitrogen fertilizer, usually in rural communities, or water contaminated with animal or human waste, which can occur if sewage is septic rather than municipal. In cities, drinking water is tested for nitrates, so this should not be a problem there, in theory.
Like many other substances in the body, nitrates are kept in a delicate balance: we need just enough, not too much. If you drink from well water, or have a septic tank, make sure your water has been checked for nitrate levels.
But the source matters, too. Organic fruits and veggies are more likely to be high in micronutrients, and probably less likely to be too high in nitrates, depending upon growing conditions. And fruits and veggies are the source you want. Read your ingredient lists and avoid any processed meat products containing nitrates or nitrites.