Vegetarian diets lower cancer risk by 14%, study says

Vegetarians and vegans have a 14% lower risk of developing common cancers related to lifestyle, a new study published in the journal BMC Medicine has found. Researchers at Oxford University analyzed the relationship between different levels of meat consumption, fish consumption, a vegetarian diet and the resulting cancer risk. The team used data compiled by the UK Biobank, a repository of medical and genetic data from half a million volunteers.

Data from 472,377 adults between the ages of 40 and 70 was studied, of whom 247,571 (52%) ate meat more than five times a week, 205,382 (44%) consumed meat five times a week or less, 10,696 (2.2%) ate fish but not meat, and 8,685 (1.8%) were vegetarian or vegan. During the 11 years they provided data, 54,961 (12%) of the volunteers developed some type of cancer. Overall, the risk of developing the disease for those who ate meat five times a week or more was 2% higher than for those who ate meat five times a week or less. For those who ate fish but not meat, it was 10% lower. And the risk was 14% lower among vegetarians and vegans.

In 2015, the WHO’s International Agency for Research on Cancer put processed meat on its list of carcinogenic products, and red meat on the list of substances likely to be carcinogenic. The uproar was immediate as processed meat joined tobacco, alcohol and even plutonium as substances hazardous for humans to consume, and since then meat has continued to be at the center of public health debates. The high level of CO2 emissions associated with meat production, especially in the case of beef, alongside concerns about animal welfare have also contributed to emotive and often angry exchanges about its consumption.

The new Oxford study looked specifically at the most common tumors (colon, prostate and breast) largely linked to lifestyle, which account for 39% of total new cancer cases in the United Kingdom. There was significant variation among types of cancer, with the risk of prostate cancer 20% lower among men who ate fish but not meat, and 31% lower among vegetarians and vegans. An 18% reduction in breast cancer risk was observed among vegetarian women versus those who ate meat more than five times a week, but the higher body mass index of the meat-eaters, and the fact that obesity is a risk factor, must also be taken into account. The authors caution in their conclusions that a cause and effect relationship cannot be directly drawn between diet and cancer risk, given the other factors that influence the acquisition of the disease.

Marina Pollán, director of Spain’s National Epidemiology Center, points out that it is not easy to obtain clear and definitive conclusions from this type of study. “It happens with almost all risk factors when you analyze lifestyle, except for things such as tobacco, which is a definitive carcinogen,” she said. “Studying diet is complicated, because it is done through a questionnaire on how often a food is consumed, and it is not easy to answer them,” she added. “In epidemiology, we see that these types of questionnaires sometimes underestimate the real effect of things like diet.”

If you consume a lot of meat you stop consuming other things, such as legumes or fish, which can have beneficial effects

Marina Pollán, director of Spain’s National Epidemiology Center

Pollán clarified that governments are “very conservative” when using this type of evidence to make dietary recommendations or warnings, and that toxicology or animal studies can be better controlled. Studies done in mice also point to the carcinogenic risk of products such as processed meat. In any case, Pollán believes recommendations to reduce meat intake are sensible. “They don’t say ‘don’t eat meat,’ they say moderate meat consumption,” she added. “If you consume a lot of meat you stop consuming other things, such as legumes or fish, which can have beneficial effects that you are not going to profit from,” she concluded.

Although there is significant support for the WHO dietary recommendations, there are also scientists who believe that the evidence from observational studies does not provide sufficient certainty to issue such guidelines, and claim the results of animal studies cannot be extrapolated directly for humans. Pablo Alonso Coello, a researcher at the Iberoamerican Cochrane Center in Barcelona, said: “This type of research can be used to make stronger recommendations if the effects observed are large, as is the case with the relationship between tobacco and cancer, but this is not what we see with meat consumption.”

Alonso Coello published an extensive and controversial review of studies linking meat consumption and cancer in 2019 in the Annals of Internal Medicine. It stated that the certainty of evidence linking meat consumption and cancer was low or very low (though for processed meat the certainty was moderate). “The risk of colon cancer is low. What we see in this study is that this low risk could be reduced to a slightly lower level, but even then the change in risk is very low, and the certainty of this information is also low,” he asserted.