Question: I’ve heard that nitrates in food are dangerous and that I should avoid them. What are nitrates, why are they a problem, and what foods contain them?
Answer: Nitrates are salts that contain the nitrate ion, which is a negatively charged particle made up of the elements nitrogen and oxygen. The most common nitrate salt is sodium nitrate. Another class of compounds, called nitrites, are closely related to the nitrates. Like nitrates, nitrites are salts. They contain the nitrite ion, which — like nitrate — is negatively charged and made up of the elements nitrogen and oxygen. The suffix -ite or -ate indicates a lesser or greater quantity of oxygen. For example, NO2? is nitrite, while NO3? is nitrate. Nitrate and nitrite are commonly added to foods as preservatives, because they help to prevent bacterial growth. Specifically, nitrate and nitrite inhibit the growth of Clostridium Botulinum, the bacterium which causes botulism. It is also gives cured meats that nice pink color and depth of flavor.
The reason nitrates are a problem in food is that the human body can convert them into nitrites. Nitrites (whether formed from nitrate or ingested) can then react with a class of compounds called secondary amines. Amino acids — the building blocks of protein — are secondary amines. Because protein is a major structural and functional component of every body cell, there is no shortage of secondary amines in the body. When nitrites react with secondary amines, the resulting compounds are called nitrosamines. These are carcinogenic — they cause cancer — in animals , and strong evidence suggests they’re carcinogenic in humans as well. While the Environmental Protection Agency (EPA) doesn’t regulate the amount of nitrate and nitrite in food, they do make recommendations regarding consumption. The EPA suggests limiting nitrate and nitrite consumption to 1.6 mg/kg/day and 0.1 mg/kg/day respectively.
Nitrates and nitrites are also associated with methemoglobinemia, or “blue baby syndrome.” Because of their small size, infants under 6 months of age are most susceptible to this condition, which occurs when nitrites react with hemoglobin. Hemoglobin is the blood protein that carries oxygen to the tissues, and methemoglobin, the product of the reaction with nitrite, isn’t an efficient oxygen carrier. Generally speaking, infants who get blue baby syndrome have been exposed to excessive amounts of nitrate and nitrite in drinking water, which is most commonly the result of manure runoff getting into the water supply. Municipalities track nitrate and nitrite in the water and will issue “blue baby warnings” if levels get too high for safety.
While nitrates and nitrites are added to certain foods — especially bacon, ham, and lunch meat — as preservatives, there are natural sources of these compounds as well. Many vegetables, for instance, are nitrate-rich, particularly lettuce, spinach, beets and celery [2,3]. However, vegetables contain primarily nitrates as opposed to nitrites. While the human body can convert the former into the latter (nitrites being responsible for the disturbing chemistry), the conversion is very inefficient in adults. As such, vegetables are not a major source of nitrites. There is absolutely no reason to limit vegetable consumption out of concern for nitrates and nitrites. The most significant source of nitrites in the average diet is processed or cured meat. Reducing cured meat consumption to approximately three meals a week while maintaining a normal vegetable consumption is a very reasonable strategy for managing nitrate and nitrite intake.
Pregnant women may be advised to avoid cured meat altogether; this is partly due to the nitrates and nitrites, and is partly due to the fact that lunch meat can be contaminated with bacteria that are particularly problematic during pregnancy. Because nitrates and nitrites can cross the placenta into a fetus [4,5], it’s safest to avoid foods with added nitrates or nitrites during pregnancy. Again, however, there is no evidence to support avoiding vegetables, because of the inefficient conversion of nitrate to nitrite and because of the many health benefits associated with high vegetable consumption.
- Swann et al. Nitrosamine-induced carcinogenesis. The alklylation of nucleic acids of the rat by N-methyl-N-nitrosourea, dimethylnitrosamine, dimethyl sulphate and methyl methanesulphonate. Biochem J. 1968 Nov;110(1):39-47.
- van Velzen et al. The oral bioavailability of nitrate from nitrate-rich vegetables in humans. Toxicol Lett. 2008 Oct 1;181(3):177-81. Epub 2008 Aug 3.
- White, J. Relative significance of dietary sources of nitrate and nitrite. J Agric Food Chem. 1975 Sep-Oct;23(5):886-91 .
- Althoff et al. Transplacental effects of nitrosamines in Syrian hamsters: I. Dibutylnitrosamine and nitrosohexamethyleneimine. Z Krebsforsch Klin Onkol Cancer Res Clin Oncol. 1976 May 3;86(1):69-75.
- Gruener et al. Methemoglobinemia induced by transplacental passage of nitrites in rats. Bull Environ Contam Toxicol. 1973 Jan;9(1):44-8.