While cutting out meat has long been associated with various health benefits, a significant new study from the University of Oxford has introduced a nuanced perspective on the relationship between meat-free diets and cancer risk. The findings suggest that while vegetarian and vegan diets may reduce the risk of some cancers, they could surprisingly be linked to a higher incidence of bowel cancer, particularly among vegans, and also an increased risk of oesophageal cancer for vegetarians.

This research emerges more than a decade after the World Health Organization (WHO) classified processed meats as Group 1 carcinogens, placing items like bacon and sausages in the same evidence category as known cancer-causing agents such as cigarettes, alcohol, and asbestos. Earlier studies highlighted that consuming merely two rashers of bacon (approximately 50g) daily could elevate bowel cancer risk by nearly 20 percent. Red meat was also flagged as probably cancer-causing, though at a lower classification level. The new study, recognized as the most extensive investigation into meat-free diets and cancer to date, analyzed the risk of 17 different cancers across five distinct dietary groups, ranging from regular meat-eaters to strict vegans. The study incorporated pooled data from previous research conducted between 1980 and 2010, encompassing over 1.8 million individuals across the UK, US, Taiwan, and India, with participants followed for an average of 16 years. During this period, 220,387 cancer cases were recorded, predominantly breast, prostate, and colorectal cancers.

The Oxford scientists identified several protective benefits associated with vegetarian diets. Vegetarians exhibited lower likelihoods of developing pancreatic, breast, prostate, and kidney cancer, and also showed reduced rates of multiple myeloma, a form of blood cancer. Specifically, men who avoided meat had a 12 percent lower risk of prostate cancer, and vegetarians and pescatarians were more than 25 percent less likely to develop kidney cancer. The reduced risk of blood cancer was linked to healthier body weight among vegetarians, given that obesity is a major risk factor for such conditions. Other studies also suggest that plant-based diets might slow the progression of prostate cancer and alleviate side-effects like urinary incontinence and erectile dysfunction.

However, the study presented some unexpected and concerning findings regarding other cancer types. Vegetarians were found to be more susceptible to bowel cancer, with vegans facing a significantly higher risk—40 percent greater than meat-eaters. This outcome was particularly surprising, given that vegan diets are typically lower in saturated fat and higher in fibre, factors generally considered protective against bowel cancer. Furthermore, vegetarians were almost twice as likely to be diagnosed with oesophageal cancer, specifically squamous cell carcinoma of the oesophagus, one of the two main types of the disease. This is particularly relevant as around 9,000 people are diagnosed with oesophageal cancer annually in the UK, with rates significantly higher in parts of Asia.

The researchers offered several explanations for these seemingly contradictory results. Professor Tim Key, a study co-author and expert in diet and cancer, noted that the findings are not incompatible with the established link between processed meat and bowel cancer. He suggested that the meat-eaters in their study were generally health-conscious and consumed far less meat than the average UK adult, who typically eats about 34g of meat daily. Had the study included more heavy consumers of processed meat, the results might have differed, potentially showing a clearer link between higher meat consumption and increased bowel cancer risk.

Regarding the increased risk of bowel cancer in vegans, a key hypothesis points to low calcium intake, which is a known risk factor. The study found that across all dietary groups, vegans consumed the least calcium, despite having the highest fibre intake and lowest alcohol consumption—both factors generally thought to be protective against bowel cancer. For the higher risk of oesophageal cancer among vegetarians, researchers suggested that restricted diets, coupled with low intakes of animal protein and micronutrients such as riboflavin and zinc, might play a significant role.

The broader context of bowel cancer trends also received attention. While bowel cancer rates have declined among older adults, largely due to screening efforts, cases are escalating at an alarming pace among younger, otherwise healthy individuals. Experts suspect a link to ultra-processed foods, whose additives and preservatives strip them of gut-protective fibre. Processed meat, in particular, contains nitrites that preserve color and extend shelf life; when cooked, these can form nitrosamines, compounds definitively linked to cancer. Cancer Research UK estimates that approximately 5,400 bowel cancer cases annually in the UK are attributable to processed meat consumption.

Prof. Key concluded that, overall, the outlook for vegetarians remains "quite optimistic." However, he stressed the importance for vegetarians, like everyone else, to ensure they adequately replace any missing nutrients through supplements or fortified foods. The study's authors emphasized that their research was observational and thus cannot definitively prove cause and effect. They also cautioned that vegetarian diets are highly varied and are often defined by what people exclude rather than what they consume. Professor Tom Sanders, a nutrition expert not involved in the study, underscored the findings' importance but advised caution due to the relatively low meat consumption of the control group. Professor Jules Griffin added that a comparison with NHS Eatwell guidelines, which might represent an optimal cancer-protective diet, was missing from the analysis. Cancer continues to be a leading global cause of death, accounting for nearly one in six fatalities worldwide, with colon cancer uniquely rising among under-50s across 42 countries.