Nitrate is found in high amounts in green leafy vegetables, beetroot, radish, rhubarb, and more.^[11][12][13] See the table below for more information.

This table is composed of averages from multiple samples. Farming techniques, transport, storage conditions, and cooking methods can all greatly affect the actual nitrate and oxalate content of food.
References: Jackson et al. Nutr Res Rev. 2017.^[49] ● Lidder and Webb. Br J Clin Pharmacol. 2013.^[50] ● Griesenbeck et al. Nutr J. 2009.^[51] ● Tamme et al. Food Addit Contam. 2006.^[52] ● Siener et al. Food Chem. 2006. DOI:10.1016/j.foodchem.2005.05.059 ● Hönow and Hesse. Food Chem. 2002.^[53] ● Santamaria et al. J. Sci. Food Agric. 1999.^[54] ● Dr. Duke's Phytochemical and Ethnobotanical databases ● FoodData Central

Some studies have shown that altering the quantity or function of bacteria in the mouth blunts the beneficial effects of dietary nitrate. Such effects have been observed in animal studies of germ-free mice^[55] and in clinical studies that have manipulated the oral microbial environment by adding anaerobic bacteria into the mouth^[56] or by using antiseptic mouthwash.^{[57][58][59][58][60]} The typical rise in plasma nitrite after consuming nitrate is also blunted after using antiseptic mouthwash.^[61][58]

Interestingly, one epidemiological study that examined 945 individuals over 3 years found that frequent mouthwash users (at least twice daily) had a 55% greater risk of developing prediabetes or type 2 diabetes compared to less frequent users and a 49% greater risk compared to nonusers.^[62] While this study did not examine mechanisms, the disruption of the nitrate-to-nitrite conversion pathway in the mouth, which is catalyzed by oral bacteria,^[33] was discussed as one possible explanation for the findings.

Because supplementation with nitrate has been shown to improve exercise performance for some types of exercise (see What are nitrate’s main benefits?), it is interesting to explore whether nitrate-rich foods have similar effects. The current evidence shows that beetroot (predominantly administered using beetroot juice) can cause a very small improvement in exercise performance, whereas red spinach, Swiss chard, and rhubarb cannot.^[63] However, a limited number of studies have examined this topic, so further research is needed to fully understand the effects of nitrate-rich foods on exercise performance (and indeed on other outcomes like blood pressure and vascular function).

Current evidence shows that nitrate supplementation can lower subjective perceptions of exertion during exercise in people with chronic obstructive pulmonary disease (COPD).^[64] Therefore, nitrate might improve exercise tolerance in this population. However, while some trials have found that nitrate/beetroot juice can increase the distance people with COPD can walk,^[65][66][67] all of the current evidence compiled in systematic reviews and meta-analyses shows large variability in the effect of nitrate on exercise capacity between studies.^[68][64][69] Consequently, while there is potential for nitrate supplementation to improve exercise capacity in people with COPD, further high-quality randomized controlled trials are needed to fully understand its efficacy.^[68][64][69]

A systematic review and meta-analysis published in 2020 found that supplementation with nitrate did not increase the length of time people with heart failure could exercise or the distance they could walk.^[70] However, currently only three randomized controlled trials have examined this topic,^[70] so further research is needed to make conclusions about the effect of nitrate on exercise capacity in people with heart failure.

Systematic reviews and meta-analyses have found that supplementation with nitrate tends to work best when taken approximately 2 to 3 hours before exercise.^[30] Meta-analyses have also found that the effects of nitrate might be influenced by training status. For example, some meta-analyses have found that effects tend to be diminished in endurance athletes with a high performance level (e.g., elite athletes) and that nitrate tends to work best in recreational athletes or nonathletes during longer-duration time-to-exhaustion tests (i.e., tests measuring how long a person can run/cycle at a low to moderate intensity).^[29][31][32] That said, some analyses have shown nitrate to work best during shorter exercise bouts (lasting between 2 and 10 min).^[30] Nitrate might also be more effective in men than women; however, very few studies in women have been published, so more research is needed to understand whether a sex difference exists.^[31][32][30]

The major problem with such analyses is that very few randomized controlled trials have been specifically designed to examine dose-response effects, time course effects, sex differences, etc. Plus, the existing studies include a small number of participants. Further high-quality research is needed to understand when and in whom nitrate works best.

The International Agency for Research on Cancer (IARC) has classified ingested nitrate as “probably carcinogenic to humans” (a Group 2A carcinogen).^[8][45] 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).^[71] 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.^{[41][42][43][44]} 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.^[72] Similarly, an excessive nitrate intake from processed foods has also been associated with an increased risk of conditions, including gastrointestinal cancer.^[11] 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”.^[45] Interestingly, drinking water containing nitrate can lead to increased levels of NOCs,^[73][74] and high levels of red meat ingestion are also associated with higher levels of NOCs in the body.^{[73][75][76][77]} One clinical study has even shown that drinking beetroot juice can cause a small increase in the formation of NOCs.^[78]

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.^[79] 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.^[8] 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.^[80][81]

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.

From an epidemiological perspective, nitrates are commonly thought to be adverse to health due to a long-standing restriction on how much nitrate can be in drinking water (50mg/L) or ground water (due to accumulating in vegetables) due to the risk of infantile methemoglobinemia (baby blue syndrome)^[82][83] which does appear to apply to vegetables.^[84][85] More commonly, the association between processed meat products that are pink in color (which very commonly use sodium nitrate as a preservative) and cancer occurrence also paints nitrates in the same negative light.^[86] Conversely, vegetables are commonly stated to not be associated with cancer risk (which then raises concern about whether or not they are the same molecules).

Both consumption of vegetables (most commonly researched is beetroot^[87]) and consumption of the sodium nitrate preservative cause increases in serum nitrate and nitrite, and are considered bioequivalent. In some studies that assess nitrate's bioactivity, liquid solutions of sodium nitrate are even used.^[88][89][88]

There are still differences between eating processed meat and vegetables in this regard (intake of sodium through the preservative sodium nitrate and higher intake of potassium via vegetables; less conversion of nitrate into nitrosamines with coingested antioxidants) but the molecule itself is the same.