Eating large amounts of some leafy green vegetables may be linked to a higher risk of bowel cancer, new research suggests.
Scientists found that high intake of a compound present in greens including rocket and Swiss chard could increase the risk of the disease by as much as 50 per cent.
The compounds in question, known as nitrites, have long been associated with the development of bowel cancer.
The World Health Organisation classifies nitrites as a Group 1 carcinogen, placing them in the same evidence category as tobacco and asbestos.
Until now, much of the concern has centred on nitrites used in processed meats, where they help preserve products, extend shelf life and maintain a pink colour.
Researchers believe these additives can react with proteins in meat to create nitrosamines, chemicals capable of damaging healthy bowel cells and potentially triggering cancerous changes.
However, scientists now say the source of nitrites in the diet may be less important than previously assumed.
The findings come amid growing concern over bowel cancer rates, with more than 48,000 people diagnosed in the UK each year. The disease is now the leading cause of death among under-50s.
Experts warn that failure to ban nitrites from bacon and ham has cost thousands of lives
The new study – published in the Journal of the National Cancer Institute – analysed the diet of 82,009 middle-aged Swedish adults, who completed food questionnaires in 1997, 2009 and 2019.
Anyone with a previous cancer diagnosis – with the exception of non-melanoma skin cancer – was excluded.
The researchers used data held by the Swedish Food Agency, alongside their own measurements of nitrites in hundreds of foods to estimate participants’ average daily intake.
Results showed that meat products accounted for just over half of men’s nitrite intake, with processed meats contributing around a quarter.
Around 20 per cent came from vegetables, cheese and other dairy products.
Over the 20-year follow-up period, 3,170 participants were diagnosed with colorectal cancer.
More cases occurred in men, with those with the highest nitrite-intake 23 per cent more likely to be diagnosed with bowel cancer than those with the lowest.
Interestingly, no significant association was observed in women.
After accounting for fibre intake – known to protect against bowel cancer – age, family history, weight, and smoking status, the researchers concluded that a diet high in nitrites increases the risk of the disease in men, regardless of the food source.
The association appeared strongest for cancer that affects the lower part of the bowel, with men consuming the most nitrites facing a 50 per cent increased risk.
One theory is that this part of the organ may be particularly vulnerable to harmful compounds formed when nitrites are broken down during digestion.
Previous studies have found higher levels of DNA damage linked to these compounds in this part of the bowel.
However, the researchers said more research is needed to confirm this finding.
Currently, up to 90 per cent of bacon sold in the UK is thought to contain nitrites, which have been linked not only to bowel cancer, but also breast and prostate cancers.
As such, NHS guidelines advise limiting processed meat consumption to no more than 70g a day – which is around two rashers of bacon.
Red meat has also been flagged by health officials, and classed as probably carcinogenic.
However, the biggest study yet into meat-free diets, published in February, highlighted a new warning – vegans face a 40 per cent higher risk of bowel cancer than meat-eaters.
The researchers, from the University of Oxford, said the findings underline that cutting out meat may bring benefits, but it is not risk-free, with other food sources containing potentially harmful nitrites.
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 every year.
Symptoms include changes in bowel habits such as persistent diarrhoea or constipation, a feeling of incomplete emptying, and blood in the stool.
Stomach pain, bloating, unexpected weight loss and fatigue are other warning signs.
It comes as a major new global analysis across 42 countries found that bowel cancer is the only cancer rising exclusively among under-50s.
In England, 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 bowel cancer cases in Britain are preventable.
