Vegetarian and vegan diets raise bowel cancer risk, alarming new study finds… but chances of five other major cancers are reduced

Cutting out meat may reduce the risk of some cancers, but a large study suggests it may be linked to a higher risk of bowel cancer, especially for vegans.

The findings come more than a decade after the World Health Organization branded processed meats a Group 1 carcinogen, putting bacon and sausages in the same evidence category as cigarettes, alcohol and asbestos.

This decision follows research showing that eating just two slices of bacon a day (about 50 grams per day) can increase the risk of bowel cancer by about 20 percent. Red meat was also flagged, but at a lower level, classified as possibly causing cancer.

Now, the largest study ever on meat-free diets and cancer has revealed a new warning.

Scientists from the University of Oxford analyzed the risk of 17 cancers across five dietary groups, from meat eaters to vegans.

They found that vegetarians were less likely to get pancreatic, breast, prostate and kidney cancer and had lower rates of multiple myeloma, a blood cancer.

But they were more likely to get bowel cancer; vegans faced a 40 percent higher risk than meat eaters. Vegetarians were also almost twice as likely to develop esophageal cancer.

The findings underscore that cutting out meat may provide benefits, but it is not without risks, the researchers said.

Professor Tim Key, co-author of the study and diet and cancer expert, said: ‘Dietary patterns that prioritize fruit, vegetables and fibre-containing foods and avoid processed meat are recommended to reduce the risk of cancer.

‘Our study helps shed light on both the benefits and risks of a vegetarian diet.’

The analysis combined data from previous studies published between 1980 and 2010, covering more than 1.8 million people in the UK, US, Taiwan and India.

Participants were followed for an average of 16 years, during which time there were 220,387 cases of cancer; the most common is breast cancer, followed by prostate and colorectal cancer.

While bowel cancer rates among older adults have fallen largely due to screening, cases are rising at an alarming rate among young, healthy people, worrying experts.

Ultra-processed foods have long been suspected of driving this trend, as additives and preservatives destroy the gut-protecting fibers in foods.

Processed meat has come under special scrutiny because it contains nitrites, which help keep bacon pink and extend its shelf life. When cooked, these can form nitrosamine compounds known to cause cancer.

Cancer Research UK estimates that around 5,400 cases of bowel cancer a year are caused by consumption of processed meat.

But the study found that vegan diets, which are generally low in saturated fat and high in fiber, were linked to the highest risk of colorectal cancer.

Researchers, who published their findings in the British Journal of Cancer, said the results were surprising.

One possible explanation is that the meat eaters in the study consumed much less meat than average. Adults in the UK generally eat around 34 grams of meat a day (roughly the size of a standard patty), but study participants ate less than half this amount.

The team also suggested that the higher risk of bowel cancer seen in vegans may be linked to low calcium intake, a known risk factor.

Across all groups, vegans consumed the least calcium, despite having the highest fiber intake and lowest alcohol consumption; Both factors were thought to protect against bowel cancer.

Prof Key said: ‘On the surface our findings may seem contradictory, but they are not incompatible with the established link between processed meat and bowel cancer.

‘Most of the meat eaters we studied were relatively health-conscious. ‘The results may have been different if we had included more heavily processed meat consumers.’

Vegetarians were also found to have a higher risk of esophageal squamous cell carcinoma, one of the two main types of esophageal cancer.

Around 9,000 people are diagnosed with the disease each year in the UK, but rates are much higher in parts of Asia.

The researchers suggested that restricted diets and low intake of animal protein and micronutrients such as riboflavin and zinc may play a role.

A vegetarian diet has been shown to be protective against other cancers.

Men who avoided meat had a 12 percent lower risk of developing prostate cancer. Other studies suggest that plant-based diets may slow disease progression and reduce side effects such as urinary incontinence and erectile dysfunction.

Vegetarians and pescatarians were also less than 25 percent less likely to develop kidney cancer; This is likely because high animal protein intake increases levels of a biomarker linked to kidney damage, although more research is needed.

The risk of blood cancer was also lower; researchers linked this to healthier body weight, and obesity was a major risk factor.

Prof Key concluded: ‘Overall the picture for vegetarians is quite optimistic.

‘But like everyone else, vegetarians must make up for missing nutrients with supplements or fortified foods.’

The authors emphasized that the study was observational and cannot prove cause and effect. They also warned that vegetarian diets are so diverse that people are defined not by what they eat but by what they avoid.

Professor Tom Sanders, a nutritionist from King’s College London who was not involved in the research, described the findings as ‘important’ but urged caution, noting that the meat eaters studied did not consume large amounts of meat.

Professor Jules Griffin, from the University of Aberdeen, added that comparisons with NHS Eatwell guidelines were lacking and may represent the best preventive diet against cancer.

Cancer remains the leading cause of death worldwide and is responsible for almost one in six deaths.

Global data from 42 countries shows that colon cancer is the only cancer that is increasing among those under 50, leaving doctors struggling to find answers.