Nitrate Exposure in Drinking Water May Increase Prostate Cancer Risk

A new study has suggested that long-term exposure to nitrates in drinking water could be linked to an increased risk of developing prostate cancer, especially for more aggressive cancers and younger men. The research is published in Environmental Health Perspectives.

Nitrates in drinking water

Nitrates and trihalomethanes (THMs) are some of the most common contaminants found in drinking water, resulting from the use of agricultural fertilizers and from manure produced by livestock farming. THMs are produced after water disinfection processes and can be inhaled or absorbed through the skin when exposed to water – for example, while showering or swimming.

Rainfall washes these contaminants into rivers and other water sources, eventually ending up in our drinking water. “Nitrate is a compound that is a part of nature, but we have altered its natural cycle,” said Cristina Villanueva, senior author of the study and associate research professor at the Barcelona Institute for Global Health.

Previous studies have linked long-term THM exposure to elevated bladder cancer risk, but there is a limited amount of information on associations between nitrates and THMs for any other cancer types. Villanueva and colleagues investigated if there were possible links with prostate cancer, the most common cancer among Spanish men.

Consequences of long-term exposure

To identify possible links to long-term nitrate exposure, researchers analyzed data from almost 700 prostate cancer cases (including 97 “aggressive” cancers) treated in Spanish hospitals from 2008–2013, and just over 900 men between 38–85 years of age with no prostate cancer diagnosis were included as a control group.

The participants’ average exposure to nitrates and THMs since the age of 18 were predicted from information on the locations in which they lived and the amount and type of water they had consumed – for example, tap, bottled or well water. Estimates of nitrate and THM levels were produced from data on drinking water, analyses of widely distributed brands of bottled water and measurements of water supplies from groundwater in different locations in Spain.

Overall, the analysis showed that the higher the nitrate consumption, the stronger the association with prostate cancer risk. High consumption of waterborne nitrates was defined as a lifetime average of over 14 mg a day, while low consumption was defined as less than 6 mg a day.

Those consuming a high amount were 1.6 times more likely to develop low- to medium-grade prostate cancers compared to those with a low intake. These cancers grow at slow to moderate speeds and do not always need immediate treatment. However, high nitrate consumption was also associated with a three times greater risk of developing high-grade prostate cancers, which are more likely to grow and spread, and therefore require more urgent interventions.

“It has been suggested that aggressive prostate cancers, which are associated with a worse prognosis, have different underlying etiological causes than slow-growing tumors with an indolent course, and our findings confirm this possibility,” explained lead author Dr. Carolina Donat-Vargas. “The risks associated with waterborne nitrate ingestion are already observed in people who consume water with nitrate levels below the maximum level allowed by European directives, which is 50 mg of nitrate per liter of water.”

Information on the participants’ diets produced some striking results. Food frequency questionnaires revealed that the association between nitrate ingestion and prostate cancer occurred only in men with diets low in fiber, fruit, vegetables and vitamin C. Low dietary fiber (defined as ≤ 11 g per day) combined with higher nitrate intake increased the risk of prostate cancer by a factor of 2.3 – however, high dietary fiber intake (> 11 g per day) combined with higher nitrate intake was not associated with elevated prostate cancer risk.

“Antioxidants, vitamins and polyphenols in fruits and vegetables may inhibit the formation of nitrosamines – compounds with carcinogenic potential – in the stomach,” said Donat-Vargas. “Moreover, vitamin C has shown significant anti-tumor activity. And fiber, for its part, benefits the intestinal bacteria, which protect against food-derived toxicants, including nitrosamines.”

Furthermore, while the findings showed that exposure to THMs in residential tap water through the skin and through inhalation was indeed linked to prostate cancer development, there was no link to the ingestion of waterborne THMs. As a result, further research is required to investigate the effect of different routes of exposure.

Correlation or causation?

This research provides evidence only for correlation, not causation, and further research is required to confirm these observations. “Being exposed to nitrates through drinking water does not mean that you are going to develop prostate cancer,” said Donat-Vargas. “Our hope is that this study, and others, will encourage a review of the levels of nitrate that are allowed in water, in order to ensure that there is no risk to human health.”

These findings underscore the impact of waterborne pollutants and their influence on both the environment and human health. The researchers recommend “putting an end to the indiscriminate use of fertilizers and pesticides”, possibly with the help of reducing the amount of animal-based foods in our diet, in an effort to reduce levels of contamination and improve the health of our planet.

Reference: Donat-Vargas C, Manolis K, Kogevinas M et al. Long-term exposure to nitrate and trihalomethanes in drinking water and prostate cancer: A multicase–control study in Spain (MCC-Spain). Environ. Health Perspect. 2023. doi: 10.1289/EHP11391

This article is a rework of a press release issued by the Barcelona Institute for Global Health. Material has been edited for length and content.