Nitrates: Are they unhealthy or protective?

May 08, 2023

Nitrates are compounds made from nitrogen and oxygen atoms. They are commonly found in vegetables, meat, and drinking water.

In 1976, two studies showed that nitrates can form N-nitrosamines, which are highly carcinogenic in lab animals and linked to cancer in humans, too. These studies and others have formed the basis of guidelines monitoring nitrate intake.

However, other research suggests that vegetables high in nitrates may protect against cardiovascular disease.

Studies also suggest that certain sources of nitrate may inhibit the production of N-nitrosamines. One study found that nitrates from vegetables that contain vitamin C or polyphenols may prevent its production, meaning consumers can enjoy the nutrient's benefits while avoiding its negative consequences.

Further study on the health impact of nitrates could lead to the development of healthier diets and prevention strategies for various conditions.

Recently, a team led by researchers from Edith Cowan University, Australia, reviewed studies investigating the health benefits and disadvantages of dietary nitrates.

They concluded that evidence is insufficient to say that nitrates in food and water are carcinogenic and that more studies are needed to understand the scale of their effects.

"Nearly 80% of our dietary nitrate intake comes from vegetable consumption," Dr. Kelly Johnson-Arbor, medical toxicologist, co-medical director, and interim executive director at the National Capital Poison Center, who was not involved in the study, told Medical News Today.

"When food sources of nitrate are consumed, the nitrate is absorbed by salivary glands, where it is converted into nitrite. From there, the nitrite is absorbed into the bloodstream and transformed into nitric oxide. Nitric oxide plays a key role in many functions within the human body, including blood pressure regulation and heart health," she added.

The study was published in Trends in Food Science and Technology.

Current guidelines suggest nitrate intakes of 0–3.7 milligrams per kilogram (mg/kg) of body weight — or around 260 milligrams (mg) for an adult weighing 70 kilograms (kg).

Plants are major sources of nitrates. Leafy vegetables such as arugula, Chinese spinach, and butterhead lettuce contain the highest levels of nitrates at over 2,500 mg/kg. Fruits such as nectarines and peaches have the lowest quantities of nitrates at less than 25 mg/kg.

Nitrates are found in lower quantities in animal-based food products. Most animal-based products, such as red meat, poultry, and fish contain less than 50 mg/kg. Dairy products also contain lower levels, with skimmed milk containing less than 0.5 mg/kg.

Processed meat products contain higher levels of nitrates as sodium and potassium nitrate are often used as additives. Chorizo, for example, contained a median of 101 mg/ kg, and fresh sausage had a median of 77 mg/kg.

Dr. Johnson-Arbor told MNT: "Nitrates and nitrites are added to processed meats to reduce bacterial contamination and prevent foodborne diseases like botulism. This process is called ‘curing.’ Since cured meats contain nitrates, many people choose to avoid consumption of ham, bologna, bacon, or other processed meats in an effort to avoid nitrate consumption and reduce cancer risk."

"Some people choose to eat ‘uncured’ or ‘naturally cured’ meat products instead. These meat products are cured with celery or other vegetable juices, instead of nitrates and salt. However, since vegetables are a good source of nitrates, ‘naturally cured’ or ‘uncured’ meats will still often still contain significant amounts of nitrates, and there are likely little health benefits to consuming those products."

– Dr. Kelly Johnson-Arbor

Nitrates are also found in both surface and groundwater. The World Health Organization's (WHO) guideline value for nitrate in drinking water is of 50 milligrams per liter (mg/L).

A report by the European Commission in 2021 found that 14% of groundwater monitoring sites had above 50 mg/L although the majority contained less than 25 mg/L or nitrates.

Several comprehensive reviews have found that dietary nitrates improve cardiovascular function and health and lower long-term risk for cardiovascular disease. Most of these studies investigated the effects of nitrate intake from plant sources.

In addition, dietary nitrate intake can increase nitric oxide levels, a molecule involved in the regulation of the central nervous and cardiovascular systems, both of which affect cognition and brain function.

The researchers note that nitric oxide inhibits the phosphorylation of the protein tau, which may be relevant given that Alzheimer's disease is associated with a hyperphosphorylation of tau.

They added that seven out of 12 clinical trials observed that dietary nitrate from beetroot juice is linked to improved cognitive function and cerebral blood flow. Four of the studies, however, found that nitrate had no effect on cognitive function.

Studies also show that nitrates taken either as supplements or from dietary sources:

Few trials have explored the direct effects of nitrates on type 2 diabetes. However, one trial of beetroot powder as a source of dietary nitrate found no significant impact on glucose or insulin parameters.

"Dietary nitrates found in fruits/vegetables have healthful cardiovascular effects by opening up blood vessels, as well as acting as antioxidants and anti-inflammatory compounds as well that may inhibit the formation of harmful N-nitrosamines which are associated with cancer," Dr. Dana Ellis Hunnes, a senior clinical dietitian at the UCLA Medical Center and an adjunct assistant professor at the UCLA Fielding School of Public Health in Los Angeles, not involved in the study, told MNT.

Foods are among the lowest sources of the carcinogenic compounds N-nitrosamines. The highest include tobacco products — containing over 16000 micrograms per kilogram (μg/kg) — and some personal care products, which contain over 1,500 μg/kg.

Foods commonly contain N-nitrosamine levels between 0 and 373 μg/kg. They include:

Increasing the cooking temperature of foods via frying, grilling, and roasting can promote the formation of various N-nitrosamines.

One meta-analysis found that higher dietary nitrate is linked to an increased risk of colorectal cancer. However, a previous meta-analysis from 2016 found no link.

Another study found that higher nitrate intake is linked to a lower risk for gastric cancer. Meanwhile, other studies have also found no link between dietary nitrate intake largely from vegetables and:

Some research, however, has noted that nitrates from processed meat are linked to higher risks of bladder, colorectal, breast, and prostate cancer, although other studies have identified no such association.

The effects of nitrates from drinking water on cancer risk are mixed. A meta-analysis found that high consumption is linked to gastric and not to colorectal cancer, but another study found that it is linked to colon cancer and not other cancers.

One study found that women in the highest quartile of nitrate consumption have almost double the risk of ovarian cancer than women in the lowest quartile.

Dr. Hunnes noted that some of the biggest limitations of the current analysis are that nutrition studies are currently too short to show long-term effects or significant changes. She noted that in real life, people often have more varied diets than suggested by studies.

When asked who should increase — and who should decrease — their nitrate intake, Dr. Johnson-Arbor noted that there is no "one size fits all" approach, and that the risks and benefits should be balanced according to individual characteristics.

"People who are at risk for dementia, heart disease, diabetes, or inflammatory conditions may benefit from [the] consumption of nitrate-rich foods in moderation, while those at high risk for cancer may wish to avoid exposure to high levels of nitrates," she told us.

"However, as explained in this article, the association between nitrate consumption and human cancer is not definitive, and more research needs to be performed to conclusively establish a link between nitrate consumption and human cancer," she added.

MNT also spoke with Dr. Misagh Karimi, a medical oncologist specializing in gastrointestinal cancers at City of Hope Orange County Lennar Foundation Cancer Center in Irvine, CA, and director of clinical operations at City of Hope Newport Beach Fashion Island, not involved in the study.

He noted that the study "pushes back" on the assumption that nitrate consumption is equally beneficial or harmful regardless of source.

"Since nitrates form nitrosamines in processed meats, it is clear that regardless of certain health conditions or risks, all individuals should proactively take steps to reduce their consumption of this. To put it in perspective, the [WHO] found that eating approximately 5 slices of bacon daily increased the risk of colorectal cancer by 18%. This is astonishing evidence that resulted in the WHO categorizing these foods as a group 1 carcinogen," Dr. Karimi noted.

"A plant-based diet full of vegetables, fruits, beans, lentils, legumes, and soy is best for most people — and we know these foods have nutrients that can help prevent cancer," he added.

Kimberly Spatola, a registered dietitian at Novant Health Heart and Vascular Institute in Charlotte, NC, not involved in the study, agreed with Dr. Karimi's recommendations.

"Nitrates are harmless molecules that are naturally found in vegetables, and vegetables rich in nitrates will provide a benefit to the body. Nitrates used to preserve processed meats, once they are exposed to high heat, will turn into nitrosamines, which are harmful to the body," she explained.

"[The] bottom line: Eat plants, limit processed meat," Spatola concluded.