Vegetarianism doesn’t cause cancer, despite what many news outlets reported this week

bissonGuest blogger Robin Bisson is the director of the Genetic Expert News Service, a non-profit that aims to provide journalists with rapid access to scientific expertise in genetics and biotechnology. He tweets as @RobinBisson.

A neat little study was published this week. It reported a gene variant that is found more commonly in traditionally vegetarian societies enables the body to make essential compounds that would usually be obtained by eating meat and fish.

Handsome man chooses between healthy and unhealthy foodThe researchers found the mutation in 68% of residents in the city of Pune, India, a community which has been largely vegetarian for around 2,500 years. This same mutation was found in 18% of Americans from Kansas. They made a strong case that, through the mechanism of evolution, the “vegetarian” gene variant became more common in communities that didn’t get enough long-chain omega-3 and omega-6 fatty acids from their diet.

So you might wonder why coverage of the study was headlined, “What could cause cancer now? Vegetarianism” (The Houston Chronicle) and “Being a vegetarian could kill you, science warns” (The New York Post).

It all started with a news release

There are usually three basic elements in news reports about research:

  • the study and its authors;
  • a news release;
  • and the journalist.

Often these elements come together to inform the public about new science in an accessible and accurate way. But sometimes the machine cranks out a stink bomb that raises nonsensical fear, yet with an apparent stamp of scientific authority.

It all starts with the news release. News releases exist to catch a journalist’s eye and highlight things that are of most interest to the public. In this case, the journal Molecular Biology and Evolution put out a release with the subheading, “Evidence of a vegetarian diet permanently shaping the human genome to change individual risk of cancer and heart disease.”

Diet and cancer? That’s an easy sell to an editor.

Researchers didn’t collect any data linking the gene variant to disease risk

The release goes on to say “scientists have found tantalizing evidence that a vegetarian diet has led to a mutation that… may make people more susceptible to inflammation, and by association, increased risk of heart disease and colon cancer.” The problem is that it’s just not true that they found evidence for a disease link, tantalizing or not. What they “found” was a hypothesis, and that’s very different.

This takes a little explaining. The gene variant which the researchers were interested in enables the body to increase the amount of arachidonic acid, a long-chain omega-6 fatty acid, that it can make from smaller omega-6 molecules found in plants. The researchers note that modern diets have seen a huge increase in the ratio of omega-6 to omega-3 due to widespread adoption of seed oils like sunflower oil, and that there is some evidence this shift could play a role in lifestyle-related diseases like colon cancer and heart disease.

The researchers hypothesized that, since arachidonic acid has been linked to inflammation, people with the gene variant may therefore be at higher risk of inflammatory-related diseases. But, crucially, they didn’t actually collect any data linking the gene variant to disease risk.

Key assumptions in dispute

It’s not even clear that higher levels of omega-6 pose a health risk. I spoke with Bill Harris, a professor at the University of South Dakota who has spent his career investigating the health effects of fatty acids. He told me the jury is very much out on omega-6, pointing me toward a review of the scientific literature which found that a higher level of arachidonic acid in the bloodstream was in fact associated with a reduced risk of heart disease.

Regarding cancer, Harris said “you can cause cancer in animals to be worse with omega-6, but when you look at human data on intakes of omega-6, there’s no relationship.”

So to recap: The researchers came up with an interesting hypothesis about which there is not yet any data, which concerns a conflicted field of science. I think it’s fair to say the news release implied far more than the paper actually found.

The importance of independent sources

But any journalist worth their salt won’t rely on a news release; they would at least talk to the study’s author. The UK Daily Telegraph first reported lead researcher Tom Brenna of Cornell University as saying:

“Those whose ancestry derives from vegetarians are more likely to carry genetics that more rapidly metabolise plant fatty acids. In such individuals, vegetable oils will be converted to the more pro-inflammatory arachidonic acid, increasing the risk for chronic inflammation that is implicated in the development of heart disease, and exacerbates cancer.”

This quote, which re-appeared again, again and again in news reports, would seem like a license to report a link between the gene variant and disease risk, especially for journalists on a deadline. But is it too much to ask journalists to investigate such claims? Study authors generally enthuse about their own research and I would have liked to hear from at least one third-party scientist.

Independent experts see no health link

Talking to third-party scientists about new studies is my day job, and according to David Cutler, a geneticist at Emory University, “while the results are interesting, and very likely to be correct, it is far from clear that they have any important health implications.”

Marcus Feldman, a theoretical biologist at Stanford University, also said “the paper gives powerful evidence of some kind of selection over the past 2,500 years but it doesn’t help an individual to decide whether being a vegetarian is healthier than being a non-vegetarian.”

So you can understand that my heart sunk when almost every headline included the words “health risk,” “harmful,” “cancer” or “disease.”

It’s worth repeating: There is literally no data suggesting the “vegetarian” gene variant is linked to cancer. Reports suggesting otherwise amount to little more than nonsense.

Related toolkit resources worth reading:

Earlier this week we featured a post by Arvind Suresh, Science Media Liaison at the Genetic Expert News Service (GENeS). Suresh explained how journalists can avoid being taken in by unsupported “biohype” claims.

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