A Swedish study of more than 82,009 adults has linked high consumption of nitrites found in leafy green vegetables such as rocket and swiss chard to a 50 per cent higher risk of lower bowel cancer in men.
Scientists discovered that consuming large amounts of dietary nitrites increases the risk of colorectal cancer in male subjects regardless of whether the compound comes from plant sources or processed meat products.
The World Health Organisation, the international public health agency, classifies nitrites as a Group 1 carcinogen, placing them in the same danger category as tobacco smoke and asbestos. Food manufacturers traditionally add nitrites to processed meats like bacon and ham to extend shelf life and preserve a pink color, where they react with protein to form cancer-causing nitrosamines.
Health experts have warned that the failure of authorities to ban nitrites from cured meats has already cost thousands of lives. However, the new findings suggest that the dietary source of nitrites may matter far less than previously believed.
Nitrite sources and male cancer risk
The study, published in the Journal of the National Cancer Institute, tracked 82,009 middle-aged adults in Sweden who completed detailed food questionnaires in 1997, 2009, and 2019. Researchers excluded any participants with a history of cancer prior to the study, with the exception of non-melanoma skin cancer.
To calculate average daily intake, scientists combined data from the Swedish Food Agency with direct measurements of nitrite levels across hundreds of different foods. The dietary analysis showed that meat products accounted for just over half of men's total nitrite intake, with processed meats making up roughly a quarter. Vegetables, cheese, and other dairy products contributed around 20 per cent.
Over a 20-year follow-up period, 3,170 participants developed colorectal cancer. The study recorded a higher incidence among male participants, with men who consumed the highest levels of nitrites facing a 23 per cent higher overall diagnosis rate compared to those with the lowest intake. No statistically significant association was observed in female participants.
After adjusting for age, weight, smoking status, family history, and dietary fibre intake, which is known to protect against bowel disease, researchers concluded that high nitrite intake increases male cancer risk regardless of food origin. The risk peaked at a 50 per cent increase for cancers in the lower part of the bowel. Scientists theorise that the lower bowel may be especially vulnerable to toxic compounds created as nitrites break down during digestion, as earlier research noted elevated DNA damage in that region, though further research is needed to confirm the mechanism.
Processed meat guidelines and vegan diet risks
In the UK, up to 90 per cent of bacon sold in shops is thought to contain nitrites, which have been linked to prostate and breast cancers in addition to bowel disease. Official National Health Service guidelines recommend limiting processed meat consumption to no more than 70 grams a day, equivalent to about two bacon rashers. Health officials have also flagged red meat as probably carcinogenic to humans.
The findings follow the largest study to date on meat-free diets, published in February by researchers at the University of Oxford. That study warned that vegans face a 40 per cent higher risk of bowel cancer than meat eaters. The Oxford team noted that while cutting out meat brings health benefits, plant-based diets are not risk-free because other food sources contain potentially harmful nitrites.
Rising bowel cancer rates and symptoms
Bowel cancer is the fourth most common cancer in the UK and the second biggest cause of cancer deaths, claiming more than 17,000 lives annually out of more than 48,000 new diagnoses each year. It has now become the leading cause of cancer death among people under 50.
A major global analysis across 42 countries revealed that bowel cancer is the only form of cancer currently increasing exclusively among adults under 50. In England, annual diagnoses among 25 to 49-year-olds have increased by more than 50 per cent since the early 1990s. Cancer Research UK estimates that more than half (54 per cent) of all British bowel cancer cases are preventable through lifestyle and dietary changes.
Common symptoms of bowel cancer include persistent changes in bowel habits such as diarrhoea or constipation, blood in the stool, and a feeling of incomplete bowel emptying. Patients may also experience stomach pain, abdominal bloating, fatigue, and unexplained weight loss.
