The International Agency for Research on Cancer (IARC) has classified ingested nitrate as “probably carcinogenic to humans” (a Group 2A carcinogen).[1][2] This means that “there is limited evidence of carcinogenicity in humans and either sufficient evidence of carcinogenicity in experimental animals or strong mechanistic evidence” from cell-culture experiments (in vitro studies).[3] In humans, this evidence comes from epidemiological observational studies that have shown that higher dietary intakes of nitrate are associated with an increased risk of some types of cancer.[4][5][6][7] Further human evidence is derived from studies that show that drinking tap water that contains high levels of nitrate is associated with several conditions, including colorectal cancer.[8] Similarly, an excessive nitrate intake from processed foods has also been associated with an increased risk of conditions, including gastrointestinal cancer.[9] However, the strength of the association between nitrate intake and cancer risk is variable between the studies, and some studies show that a greater nitrate intake is associated with a lower risk. Consequently, it is unclear whether consuming nitrate directly causes cancer in humans.
The associations between nitrate intake and cancer risk might be explained by the formation of N-nitroso compounds (NOCs) caused by nitrate. This is important because, like nitrate, several N-nitroso compounds have also been classified as “possibly carcinogenic to humans”.[2] Interestingly, drinking water containing nitrate can lead to increased levels of NOCs,[10][11] and high levels of red meat ingestion are also associated with higher levels of NOCs in the body.[10][12][13][14] One clinical study has even shown that drinking beetroot juice can cause a small increase in the formation of NOCs.[15]
The above-described information indicates that ingesting nitrate has the potential to cause cancer. It even suggests that drinking beetroot juice in excessive amounts might pose a health risk due to the possible formation of NOCs. However, there is no known association between drinking beetroot juice and cancer risk.[16] Additionally, nitrate intake through plant-based food sources (green leafy vegetables, beetroot, etc.) has never been found to be associated with health risks. Furthermore, nitrate-containing vegetables (including beetroot) also contain many other nutrients — vitamins and phytochemicals — that function as antioxidants and can inhibit the conversion of nitrate into NOCs.[1] Therefore, the cancer risk from drinking beetroot juice or eating nitrate-containing vegetables is likely to be very low. Furthermore, it is generally agreed that consuming a variety of fruits and vegetables can protect against chronic conditions, including cancer.[17][18]
To summarize, while consuming nitrates in excessive levels may increase the risk of cancer, it is currently believed that nitrate is unlikely to pose a major risk in the amounts people typically consume. However, if people want to reduce their exposure to any potential hazard posed by nitrate, they can choose to refrain from eating processed meats and from drinking tap water in nitrate-contaminated areas.