Vegetarian diet raises cancer risk?

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The media manages to misinterpret research yet again.

So you’ve been eating a more plant-based diet thinking it was healthy? And along comes the onslaught of scary headlines in recent days.  Vegetarian diets cause cancer!  Really?  I had to check it out, starting at the source, the original research from the research group led by Dr. Tom Brenna, noted omega-3 fatty acid expert.

Here’s the title of his study, published in the research journal Molecular Biology and Evolution:

Positive selection on a regulatory insertion-deletion polymorphism in FADS2 influences apparent endogenous synthesis of arachidonic acid

Whoa!  That’s a mouthful.  How did it morph into this gloomy headline from The Times of India:

Long-term vegetarian diet could increase cancer, heart disease risk

or this more cheerful headline from The Washington Post:

Cornell study finds some people may be genetically programmed to be vegetarians

Good question.  Headlines in the main stream media, are aimed at a readership that isn’t well versed in the intricacies of nutritional science and genetic technology.  Headlines also need to grab readers’ attention quickly.  And unfortunately many readers don’t go beyond the headline, in which case you might conclude that a vegetarian diet causes cancer, and that some people can’t help being vegetarian (even if it causes cancer).  You’d be wrong on both counts.

The two part study is pretty complex, but I’m going to try to pare the findings down to something understandable.  Here are some basics premises:

  • Long chain omega-3 fats (EPA and DHA) are found only in animal-sourced foods. These fatty acids are essential to numerous metabolic systems and are inherent parts of the structure of cell membranes and other tissues.  They also have anti-inflammatory properties.
  • Plant foods contain the short chain omega-3 fat alpha linolenic acid (ALA), which humans can convert — at a low rate — to EPA and DHA.
  • Plant foods are also high in the omega-6 fatty acid linoleic acid (LA).  LA is converted to the long chain omega-6 fatty acid arachidonic acid (ARA), which plays a role in inflammation, among other things.
  • Inflammation is a natural and essential body process, but excess inflammation is suspected of causing a host of problems, like heart disease and cancer.

So far so good.  Now we get to the interesting part.  The researchers knew that certain populations in India have a very long history of a vegetarian diet.  How have those people managed to obtain sufficient long chain omega-3 fats — DHA and EPA — to sustain health?  Could there be a genetic connection?  The team compared genes of 234 people of Indian ancestry to 311 people of American ancestry.  Result: compared to Americans, the Indians had a genetic variation that ramped up conversion of LA and ALA to long chain fatty acids like EPA, DHA and ARA.  In other words, over hundreds of generations of a vegetarian diet, people with that genetic variation had better prospects for health and reproduction.  They evolved to thrive on a vegetarian diet.  In fact, this genetic variation is common in East Asia and Africa, too.

This sounds like a good thing.  People have adapted to produce more of a necessary fatty acid from a limited diet.  How did we make the jump to the cancer headlines?  Here’s how.  People with this genetic variation have a higher capacity to convert omega-6 LA to long chain ARA, which is a pro-inflammatory molecule.  Not such a problem on a traditional local vegetarian diet.  But modern processed diets are loaded with omega-6 fats, from refined vegetable oils.  A diet high in vegetable fats from these foods could, theoretically, result in an increased level of inflammatory ARA, which could, theoretically, increase disease risk.  It’s all theoretical, and it’s all dependent on having those particular genetic variations.  So just eating a vegetarian or vegan diet does not result in cancer.  There are a whole lot of variables that affect cancer risk.  The researchers conclude that people with the genetic variation might have a higher rate of inflammation, depending on dietary intake, and that balanced consumption of LA and ALA would be a good idea to reduce that possibility.

This study did not say that vegetarian diets cause cancer.  What it does suggest is that some people with certain genetic variations, primarily from Indian and Asian backgrounds, may reduce the potential for inflammation by balancing their intake of omega-6 and omega-3 plant fats.  And of course, everyone — vegetarian, vegan, meat eaters, with or without gene variations — needs adequate omega-3 fatty acids, and would do better to balance intake of omega-6 and omega-3.

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