Nitrate consumption may be linked to the risk of dementia - a decline in cognitive function - but the effect depends on the source. That is the conclusion of the authors of a 27-year study of Danish residents, published in the journal Alzheimer's & Dementia.
Nitrates are chemical compounds that occur naturally in soil, water and plants. They are also added to sausages and frankfurters to preserve color and protect against bacteria. In the body, nitrates can be converted into nitric oxide, which dilates blood vessels and improves blood flow, including in the brain. But under other conditions they can form nitrosamines - substances that have been linked to cancer risk and possible harm to nerve cells.
Researchers from Edith Cowan University and the University of Western Australia analyzed data from residents of Copenhagen and Aarhus aged 50-64 who did not have dementia at the start of observation in 1993-1997. Participants were followed until the end of 2020 - on average 24 years, a maximum of about 27 years. New cases of dementia were identified from hospital diagnoses and prescriptions for medicines.
At the start of the study, participants completed a detailed questionnaire about their diet over the previous year. The scientists matched the answers with specialized databases to calculate daily consumption of nitrates and nitrites from plants, animal products and tap water. Residential addresses were linked to the national drinking water quality monitoring database.
During the observation period, dementia was diagnosed in 4,750 people (about 9% of participants), with the average age at diagnosis being 77. Those who consumed the most nitrates from plants (on average 77 mg per day) had a 10% lower risk of dementia than the group with the lowest consumption (22 mg per day). This effect was observed at moderate doses and did not increase with further increases in consumption. The authors estimate that the range of 34-77 mg per day corresponds roughly to half to one and a half cups of raw leafy vegetables.
Nitrates and nitrites from animal products showed the opposite association. Participants with the highest consumption of natural nitrates from animal products (on average 10.7 mg per day, roughly as in 350 g of beef) had a 13% higher risk of dementia, and those who favored nitrites from animal products had a 19% higher risk. Added nitrates and nitrites from processed meat, despite the smaller amount, were also associated with an 11% increase in risk.
Nitrates in tap water were also linked to an increased risk: the group with the highest consumption had a 14% higher risk of dementia than the group with the lowest. In the analysis of water quality by address over 15 years, concentrations of 1.8-5.1 mg/l corresponded to a 12-16% higher risk, although these levels are significantly below international standards (44-50 mg/l).
For early dementia (before age 65), the associations were stronger, but they rest on only 191 cases. Thus, high consumption of plant nitrates was accompanied by a 39% lower risk, and animal nitrates by a 73% higher risk.
It is important to emphasize: this is an observational study, not an experiment. It does not prove that nitrates directly cause or prevent dementia. Correlation - a statistical association - by itself does not indicate causation. The authors note that factors such as smoking, hypertension and education influence dementia risk far more strongly. In addition, people who eat a lot of vegetables often lead healthier lifestyles, and this could have influenced the results.
Among the limitations: diet was assessed only once at the start, water quality was taken into account only by place of residence, and diagnoses were taken from registries where some cases may have been missed. The scientists conclude that their findings require further research, including clinical trials, to understand whether switching from animal sources of nitrates to vegetable sources can actually reduce the risk of dementia.