Study finds link between vegetarian diets and lower cancer risk

In 2015, the WHO International Agency for Research on Cancer placed processed meat on its list of carcinogenic products and red meat among those that probably are.

Processed meat was thus placed among some of the most dangerous substances for health, such as tobacco, alcohol or even plutonium.

The uproar was immediate and simplistic interpretations of the decision abounded, mocking the comparison between the key element of atomic bombs and a hamburger, or claiming that tobacco and bacon are just as dangerous.

Since then, meat has continued to star in public debate, since for many individuals it touches on aspects of identity that go far beyond its value as food.

In addition, the high level of CO2 emissions associated with its production,

Today, the journal

publishes a study by researchers at the University of Oxford in which the relationship between different levels of meat consumption, fish consumption, vegetarian diet and cancer risk is analyzed.

The team, whose principal investigator is the Spanish Aurora Pérez-Cornago, used for its analysis information compiled by the United Kingdom Biobank, a repository that includes information of medical interest from half a million volunteers, from data on lifestyle or electronic health records to genome data or your cardiovascular health.

Of the half million individuals present in the biobank, data from 472,377 adults between 40 and 70 years of age were used.

Of these, 247,571 (52%) ate meat more than five times a week, 205,382 (44%) ate meat five times a week or less, 10,696 (2.2%) ate fish but not meat, and 8,685 (1 .8%) were vegetarians and/or vegans.

During the 11 years of follow-up, 54,961 (12%) developed some type of cancer.

In total, the risk of developing a tumor for those who ate meat five times a week or less was 2% lower than that of those who ate meat five times a week or more.

That of those who ate fish, but not meat, was 10% lower.

The risk was 14% lower among vegetarians and vegans.

The study also looks specifically at the most common tumors (colon, prostate and breast) which, in the United Kingdom, account for 39% of the total.

Among these types of diseases, the risk of prostate cancer was 20% lower among men who ate fish but not meat, and 31% lower among vegetarians and vegans.

Here, too, an 18% reduction in breast cancer risk was seen among vegetarian women versus those who ate meat more than five times a week, but the difference was related to the higher body mass index of carnivores and the fact that obesity is a risk factor for these diseases.

The results of this observational study agree, at least in trend, with many others that try to assess the role of diet in tumor development.

However, like those other studies, it has limitations.

The authors caution in their conclusions that the nature of their work does not allow a cause-and-effect relationship to be established between diet and cancer risk.

Marina Pollán, director of the National Center for Epidemiology, points out that in this type of study it is not easy to obtain clear and definitive conclusions.

"It happens with almost all risk estimators when you analyze lifestyles, except in matters such as tobacco, which is a total carcinogen," she points out.

“Studying the diet is complicated, because it is done through a questionnaire about the frequency with which a food is consumed and it is not easy to answer these questionnaires.

You have to do a mental extrapolation that conditions the results,” she continues.

This makes, according to Pollán, the result of this type of study seem less conclusive than it is.

“In epidemiology we have seen that these types of questionnaires sometimes tend to underestimate the real effect of things like diet,” she adds.

The epidemiologist affirms that the organisms that use this type of evidence to make recommendations or warnings about the diet are "very conservative" and that they add to the observation in humans toxicology studies or with animals that can be better controlled.

Unlike a human, who cannot control all the factors of his lifestyle or the amount he eats of each food, this can be done in mice and the results also point to the carcinogenic risk of products such as processed meat .

In any case, Pollán believes that “the recommendations are sensible”.

“It is not said do not eat meat, it is said moderate meat consumption,” she indicates.

"This is also interesting because if you eat a lot of meat you stop eating other things, such as legumes or fish,

Although there is significant support for dietary recommendations such as those of the WHO, there are also critical scientists who consider that the evidence from observational studies does not provide sufficient certainty and that the results of animal studies cannot be directly extrapolated to humans.

Pablo Alonso Coello, a researcher at the Ibero-American Cochrane Center (IIB Sant Pau), in Barcelona, ​​points out that although "the effect in these studies is repeated, it is a very small effect."

“This type of research can help make stronger recommendations if the effects you see are large, as is the case with the link between tobacco and cancer, but not what we see with meat consumption,” he says.

In 2019, Alonso Coello signed an extensive and controversial review of this type of study on the association between meat consumption and cancer, which was published in the journal

.

It stated that the certainty of the evidence linking meat consumption and cancer was low or very low (for processed meat, the certainty is moderate).

In the researcher's opinion, these types of results should make "the recommendation to eat less meat conditional or weak."

“The risk of colon cancer is low.

What we see in this study is that that low risk could be reduced to a slightly smaller magnitude, but the change in risk is very low, and the certainty of that information is also low,” he explains.

In addition, the researcher considers it important to differentiate what information means from the point of view of public health and from the individual.

Something that does not represent a large variation in risk for an individual, can become a large figure at the population level.

“In this diet there is a lot of paternalism and, despite the uncertainty, many recommendations are made applying the precautionary principle,” he says.

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