Questioning the nitrate-colon cancer link

Fish & Game recently carried out tests to determine whether drinking water on the Canterbury Plains contained nitrate levels in excess of 3.87 mg/1000ml.

Why this specific number?

Well, in January of this year, a study was released in Denmark which highlighted a link between the consumption of water containing nitrates exceeding that level and an increased risk of colorectal cancer (CRC).

Based on drinking water quality analyses at public waterworks and private wells between 1978 and 2011, the study calculated nitrate exposure for 2.7 million Danish adults, 5,994 of which were found to have CRC.

For those unaware, nitrate can become a carcinogenic concern for humans when it is ingested and converted to nitrite by gut bacteria. Nitrite is structured similarly to nitrate, but, rather than containing one nitrogen atom and three oxygen atoms, it contains one nitrogen atom and only two oxygen atoms. The process of nitrate becoming nitrite, therefore, is facilitated by oxygen reduction. However, for nitrite to become carcinogenic in the gut, certain dietary amino compounds are required (we’ll come back to this point a bit later).

The authors of the study in Denmark used proportional models to estimate hazard ratios (HRs) of nitrate exposure on CRC risk, and found statistically significant increased risks of the disease in people exposed to drinking water with nitrate levels exceeding 3.87 mg/L.

More specifically, they found that people who were exposed to the highest concentration of nitrate in drinking water (above 9.3 mg per litre of water) had a 15 per cent greater risk of getting colorectal cancer compared to those who had least exposure (less than 1.3 mg per litre of water).

These number are, of course, well below drinking water standards in Europe and New Zealand, where the maximum acceptable level for nitrate is 50mg/L.

In their testing on the plains, Fish & Game found more than half of the 114 water samples tested to be above the 3.87mg/L level, something they called a “major concern”.

Indeed, if risk of colorectal cancer really does increase with consumption of drinking water containing nitrate in excess of that number, it would be worrying, and could perhaps explain why our bowel cancer rates are so high.

Each year roughly 3,000 people are diagnosed with bowel cancer (also called colon cancer) in New Zealand, which, according to the World Cancer Research Fund, puts us 14th globally (equal with Belgium). In fact, last year, New Zealand women ranked inside the top ten for bowel cancer cases amongst the female population worldwide.

But before we get too far ahead of ourselves, it’s important to look at the breadth of scientific research that has examined this nitrite-bowel cancer link over the past 40 years, and furthermore, to see what New Zealand’s current scientific community have to say about its validity.

Most of the international research conducted throughout the past four decades on this topic has found either a negligible or only slight correlation between nitrates in drinking water and colon/bowel cancer rates.

In 1994, a study was conducted in Valencia, Spain (which has a higher concentration of nitrates in its public drinking water than any other city in Europe) to determine whether exposure to nitrates in increased mortality in the area due to increased instances of stomach, bladder, prostate or colon cancer.

It found an increased mortality rate in gastric and prostate cancer, but not in colon cancer.

In 2007, a study was conducted which looked at all colon cancer deaths of Taiwan residents for the past four years and pair matched them to deaths from other causes during the same period. Each matched control was selected randomly from the set of possible controls for each case.

The odds of colon cancer death for those with high nitrate levels in their drinking water was found to be 0.98, the same odds as those with the lowest nitrate levels in their water. In other words, no statistically significant association was found between NO3–N in drinking water and risk of death from colon cancer.

A study was conducted in Iowa by a selection of environmental scientists, ecologists and epidemiologists between 1986 and 1989, entitled “Nitrate in Public Water Supplies and the Risk of Colon and Rectum Cancers.”

It found negligible overall associations between the two, concluding that any increased risk of colon cancer associated with nitrate in public drinking water might occur only among susceptible populations.

Somewhat similar conclusions were drawn by a study published in the International journal of Cancer Research in 2016, though it leaned more toward there being a positive correlation.

Conducted by a large group of epidemiologists, it took place in Spain and Italy between 2008 and 2013 and evaluated the association between CRC risk and exposure to nitrate through drinking water and diet.

Ultimately, the study found a suggested positive association between CRC risk and long‐term exposure to nitrate in drinking water at levels below the European regulatory limit, particularly among subjects with other risk factors. Those factors included (but were not limited to) alcohol intake, consumption of processed meat, body mass index, and smoking.

The idea that colon cancer is heavily influenced by diet surfaces in many of the studies evaluating its link to the intake of nitrate through drinking water.

Ian Shaw, professor of toxicology at the University of Canterbury, says it is this very factor that makes the associations between water nitrate and colon cancer unconvincing:

“In my opinion nitrate is associated with colon cancer because it can be converted to nitrite by gut bacteria and form nitrosamines with dietary amino compounds. Nitrosamines are profound carcinogens. Links with water nitrate would, therefore, not be definitive because other components of the diet would be necessary to facilitate carcinogenesis. If exposure to an appropriate dietary mixture, plus the right bacterial species in the microbiome do not coincide carcinogenesis will not occur. This is a complex scenario that cannot be attributed to a single exposure to a single chemical.”

In other words, attributing high colon cancer rates to nitrates in drinking water would be oversimplifying things to a considerable level. One must consider the variations of diet and lifestyle also considered potential factors for increasing colon cancer risk, and this is something the Danish study failed to do.

Canterbury Medical Officer of Health Dr. Raymond Pink discussed this in his response to questions regarding the validity of the study:

“While it was compelling and well-presented, with some clear strengths such as the size of population involved in the study and the long period over which the study was undertaken, it also had some shortcomings that were acknowledged by its authors, specifically that it didn’t take sufficient account of other potential factors that are known to contribute to the development of colorectal cancer – smoking, alcohol consumption, sedentary lifestyles and obesity, for example.”

It is important to note that Denmark, which ranks sixth in the world for colorectal cancer cases, also ranks 13th for consumption of red meat and 26th for alcohol. Additionally, within Europe, it sits second from the bottom in terms of sufficient physical activity amongst adolescents and has the third highest percentage of raised cholesterol among adults. These are all factors considered potential contributors to developing colon cancer.

We here in New Zealand also rank high in several of the colon cancer-related health categories. According to, we are in the top four for obesity in the developed world, with over 30% of our adult population falling into the ‘obese’ category. We’re also, unsurprisingly, inside the top ten for red meat consumption.

The Danish study was compelling, certainly, had the benefit of a huge population size, and raised a point of interest regarding nitrates in drinking water and colon cancer which demands attention and additional research. However, in the words of Dr. Pink, “it is not enough to signal the need for a change to the internationally accepted maximum level of nitrate concentration for drinking water.”

The Fish & Game study too, while including some interesting findings, was hardly definitive. Dr Tim Davie points out that, as a technical science investigation, it was “lacking several things that would allow a fuller interpretation of the data”. These, according to Davie, include more extensive and specific information on where the readings were taken, what depth the groundwater was extracted from, and whether the measurements were repeated or taken from different locations.

The tests in Canterbury have shed light on the water nitrates/colon cancer topic and made many in this country aware of the Danish study which, while not definitive, certainly makes for interesting reading and raises questions around how the toxins in our water might be harming us, even at low levels.

Before any changes are made to nitrate limits, however, further research is needed. As Ramon Pink says, “the study is worthy of consideration in conjunction with other studies available and with future studies to provide an overall better picture and together inform change or whether to leave the current standard unchanged, through the appropriate international authority, the World Health Organisation.”