Whether the nitrates in your diet are linked to higher or lower dementia risk may depend on where they come from. A large, 27-year study of Danish adults published in Alzheimer’s & Dementia suggests that people who consume more nitrates from vegetables have a lower risk of developing dementia, whereas those who consume more nitrates from animal products, processed meats, or tap water face a higher risk. These opposing patterns appeared stronger for early-onset dementia diagnosed before age 65.

Nitrates and nitrites are naturally occurring chemical compounds made of nitrogen and oxygen atoms found in soil, water, plants, and animal tissues. Food manufacturers also add them to cured and processed meats, such as bacon, ham, and sausages, to preserve color, add flavor, and prevent the growth of harmful bacteria. For most people, green leafy vegetables and certain root vegetables, such as lettuce and potatoes, supply the majority of dietary nitrate.

Inside the human body, nitrate can travel down two very different chemical pathways, and which one predominates may depend on what else is eaten with it. When people eat nitrate-rich foods, helpful bacteria in the mouth convert nitrate into nitrite, which is swallowed and turned into nitric oxide. Nitric oxide is a signaling gas that relaxes blood vessels, lowers blood pressure, supports healthy blood flow into the brain, and helps nerve cells communicate with one another.

Under different chemical conditions, nitrate and nitrite can instead combine with nitrogen-containing compounds called amines, which are abundant in meat, to form N-nitrosamines, a family of reactive chemicals long studied for their link to cancer. Laboratory research suggests these compounds may also harm brain cells. A 2009 study of rat brain cells grown in a dish found that exposure to a nitrosamine present in some preserved foods impaired the cells’ energy production, disrupted their insulin signaling, and raised levels of proteins associated with Alzheimer’s disease.

Because of these two competing pathways, nutrition scientists have proposed that the health effects of nitrate depend on its food source. Vegetables come packaged with vitamin C, folate, and plant antioxidants called polyphenols, which can block the formation of nitrosamines. Meat, by contrast, contains amines and heme, an iron-containing molecule that gives red meat its color, both of which may promote nitrosamine formation, especially during high-heat cooking.

Population studies have begun to test whether this chemical divide shows up in human health outcomes over time. In a 2021 prospective study by several of the same authors that analyzed data from the same Danish cohort used in the new research, adults who habitually ate moderate amounts of vegetable nitrate had lower blood pressure and a reduced risk of cardiovascular disease. Extending that line of inquiry to brain aging, a 2024 study of Australian adults co-authored by several members of the same research team found that higher plant-sourced nitrate intake was associated with a lower risk of dying from dementia, whereas animal-sourced nitrate was not.

To find out whether different sources of nitrate and nitrite are tied to new diagnoses of dementia, Catherine P. Bondonno of Edith Cowan University and the University of Western Australia and her colleagues examined data from the Danish Diet, Cancer and Health Cohort Study. The team also wanted to see whether drinking water nitrate was linked to dementia risk, and whether these dietary associations were stronger for early-onset dementia diagnosed before age 65.

The researchers analyzed data from 54,804 residents of Copenhagen and Aarhus who were between 50 and 64 years old and free of dementia when they enrolled between 1993 and 1997. Participants were followed through the end of 2020, a median of 24 years and a maximum of about 27 years, with new dementia cases identified through hospital diagnoses or prescriptions for dementia medication in national registries. Because the study relied on observational tracking rather than an experimental intervention, the authors noted that their analyses should be viewed as hypothesis-generating.

At the start of the study, participants completed a 192-item food frequency questionnaire detailing what they ate and drank over the preceding year. The researchers cross-referenced those answers with specialized food databases to calculate daily nitrate and nitrite intake from plants, accounting for the roughly 50% loss of nitrate when vegetables are boiled. Separate databases were used to estimate naturally occurring nitrate and nitrite in animal products like fresh meat, poultry, fish, and dairy, as well as regulated preservative additives in processed meats. To estimate nitrate intake from tap water, the scientists linked each participant’s residential address history from 1978 to 2016 with Denmark’s national drinking water monitoring database.

Across the cohort, participants consumed a median of 44 milligrams of plant-sourced nitrate per day, compared to 5.8 milligrams from animal foods and 0.8 milligrams from tap water. Along with routine demographic and lifestyle factors, the researchers adjusted their statistical models for other major food groups, such as red and processed meat when evaluating plant nitrate, and fruits, vegetables, and whole grains when evaluating animal or water nitrate, to try to separate nitrate from overall diet quality.

Over the follow-up period, 4,750 participants (about 9% of the cohort) were diagnosed with dementia at a median age of 77, including 191 who developed early-onset dementia before age 65. People in the highest fifth of plant-sourced nitrate intake, who consumed a median of 77 milligrams per day, had a 10% lower rate of developing dementia than those in the lowest fifth, who consumed a median of 22 milligrams. The lower rate appeared at moderate intakes and did not keep falling at higher ones. The authors estimate that the range where lower rates appeared, about 34 to 77 milligrams a day, corresponds to roughly half a cup to one and a half cups of raw leafy greens daily.

Looking specifically at vegetable-sourced nitrate and plant-sourced nitrite yielded nearly identical results, with the highest intake groups showing a 10% to 11% lower rate of dementia. Because roughly 9% of participants developed dementia over the study period, a 10% lower relative rate represents a modest difference in absolute risk. The authors note that established risk factors such as smoking, high blood pressure, and education remain much stronger influences on dementia risk, though vegetable nitrate may still be a meaningful contributor within a healthy diet.

Nitrate and nitrite from animal sources pointed in the opposite direction. Participants in the highest fifth of naturally occurring animal-sourced nitrate (a median of 10.7 milligrams per day, an amount found in roughly 350 grams of beef) had a 13% higher rate of dementia than those in the lowest fifth, while the highest consumers of naturally occurring animal nitrite had a 19% higher rate. Even though preservative additives in cured meats accounted for a much smaller fraction of daily intake, participants in the highest fifth of additive-permitted meat nitrate and nitrite each had an 11% higher rate of dementia compared to the lowest fifth.

Nitrate in drinking water was also associated with elevated dementia rates. Participants in the highest fifth of baseline tap water nitrate intake had a 14% higher rate of developing dementia than those with the lowest intake. When the researchers tracked 15-year rolling averages of residential water quality over time, people whose tap water contained higher nitrate concentrations between 1.8 and 5.1 milligrams per liter had a 12% to 16% higher rate of dementia than those with the lowest concentrations, even though those levels sit well below current international safety limits of 44 to 50 milligrams per liter.

The associations were larger in size for early-onset dementia, though they rest on just 191 cases. People in the highest fifth of vegetable-sourced nitrate intake had a 39% lower rate of early-onset dementia than those in the lowest fifth, while those in the highest fifth of naturally occurring animal nitrate had a 73% higher rate. Estimates for plant nitrate overall, additive meat nitrate, and tap water pointed in the same directions but were too uncertain to rule out no difference.

In a separate analysis limited to the first 10 years of follow-up, when the authors suggest the diet questionnaires likely best reflected participants’ actual eating habits, the associations grew stronger. The highest vegetable nitrate intake was linked to a 43% lower rate of dementia, and the highest naturally occurring animal nitrate intake to a 37% higher rate. The association for additive meat nitrate was no longer statistically significant.

The researchers also tested whether smoking or consuming nitrosamine-blocking antioxidants like vitamin C, vitamin E, folate, and polyphenols altered the results. Overall, these factors did not consistently change the associations. One exception ran counter to the original hypothesis, which predicted these nutrients would work in nitrate’s favor by blocking nitrosamine formation: the lower dementia rate linked to plant nitrate appeared only among participants with lower intakes of vitamin C and polyphenols. The authors note that these subgroup comparisons were meant to look for patterns rather than serve as formal statistical tests.

In the authors’ reading, people who already consume abundant vitamin C and polyphenols may reach a ceiling of blood vessel protection where extra nitrate provides little added benefit. Individuals with poorer overall diets, by contrast, may gain more noticeable vascular support from plant nitrate.

These patterns align with an earlier study of U.S. healthcare workers, which found that eating more processed red meat was tied to faster cognitive aging and higher dementia risk. In that study, replacing processed red meat with plant proteins such as nuts, beans, or tofu was linked to lower risk, though it examined whole food categories rather than nitrate and nitrite specifically.

At the same time, the animal-source results stand in tension with a separate study of older adults, which found that higher consumption of unprocessed meat was linked to slower cognitive decline and better memory retention among people carrying the high-risk apolipoprotein E e4 gene variant for Alzheimer’s disease. That study, however, examined overall and unprocessed meat intake in relation to specific genetic profiles rather than isolating nitrate and nitrite compounds across the general population.

As with all research, there are some caveats to consider when interpreting these findings. Because the study is observational, it cannot establish that nitrates or nitrites directly cause or prevent dementia, and it is difficult to completely untangle a single chemical compound from the rest of a person’s diet. Vegetables contain flavonoids and other bioactive compounds that independently support brain and blood vessel health, while meats cooked at high temperatures can contain other harmful compounds. People who eat plenty of vegetables also tend to have healthier lifestyles and socioeconomic advantages that statistical adjustments may not fully capture.

There are also a few practical limitations in how diet, water, and dementia outcomes were measured. Food intake was recorded only once at enrollment, which the authors argue likely weakened the long-term associations as diets shifted over nearly three decades. The water analysis relied on residential records without accounting for workplace water or other potential water contaminants like aluminum or arsenic. Relying on hospital and prescription registries also meant some cases may have been missed and specific subtypes of dementia, such as Alzheimer’s disease versus vascular dementia, could not be separated.

Future studies in more ethnically and geographically diverse populations, along with clinical trials that track brain biomarkers and thinking skills over time, will be needed to determine whether shifting from animal and water nitrate sources toward nitrate-rich vegetables can actively reduce dementia risk.

The study, “Source-specific nitrate intake and incident dementia in the Danish Diet, Cancer and Health Study,” was authored by Catherine P. Bondonno, Pratik Pokharel, Dorit Wielandt Erichsen, Liezhou Zhong, Jörg Schullehner, Cecilie Kyrø, Kirsten Frederiksen, Peter Fjeldstad Hendriksen, Frederik Dalgaard, Lauren C. Blekkenhorst, Stephanie R. Rainey-Smith, Samantha L. Gardener, Torben Sigsgaard, Ole Raaschou-Nielsen, Anne Tjønneland, Jonathan M. Hodgson, Christina C. Dahm, Anja Olsen, and Nicola P. Bondonno.