Vegetarians and cancer risk

A few weeks ago I wrote a letter which will be published in the next Vegan Voice magazine (out in September) commenting on this article on the BBC health website. The article describes how a report in the British Journal of Cancer called "Cancer incidence in British vegetarians" indicates that like meat eaters, vegetarians are at risk of developing bowel cancer¹; contradicting an abundance of earlier information. This is why I decided to start this blog, although since doing so I have found another one from a vegan dietician which already looked at this report and is produced by someone who is more qualified than me. In any case, I will continue to publish my thoughts on relevant material but please keep an eye on Dina's blog Vegan RD as it looks like she really knows her stuff.

The data for the Key et. al BJC report was obtained from two studies, the Oxford vegetarian study and the more recent EPIC-Oxford study. These two studies used different methods of obtaining information from the subjects. The Oxford study used a simple questionnaire, while the EPIC-oxford study followed up a questionnaire with a food diary which provides more accurate details of variations in the diet, and allows differences in a dietary cohort to be quantified. For example, a person categorized as a "meat eater" answers how many times per week they eat meat in the questionnaire, but all this indicates for a vegetarian is that they don't eat meat. A food diary indicates how often a vegetarian ate processed foods, which is crucial for a dietary cancer study since fat intake is linked to cancer.

In fact, in the previous issue of the BJC another report described how fats were linked to bowel cancer due to their effects on a particular class of proteins². Basically this could not be assessed in the vegetarians among the Oxford study as the wrong questions were asked, which means that the findings of increased bowel cancer risk in vegetarians is imprudent as they were unable to compare the rates of cancer among vegetarians who consumed relatively little dietary fat with those who ate lots of it. In fact, the relevant variations which may have been indicated by the EPIC-Oxford cohort were disregarded.

Other issues in the study relate to the lower risk of cancers of the stomach, bladder and blood in vegetarians. There is evidence indicating fat intake is linked to the increased risk of bladder cancers³, but it may be that the impacts of fat intake on bladder cancer in vegetarians are offset by some other dietary component. It has been shown that people with high levels of vegetable fat in the diet have reduced risk of developing gastric cancers⁴. So while a vego diet which is high in fat may be protecting against stomach cancer, it may also be increasing your risk of colon cancer. You can see why it is important to disentangle these relationships when we are talking about something as serious as cancer.

People with a family history of bowel cancer must be more careful than the rest of us about what they eat because they may have a genetic predisposition for the development of colorectal cancers, so they may read the BBC article and conclude that they are at no less risk sticking to a vego diet than eating meat; which simply isn't true. It is fats (in particular saturated fats) which must be avoided; and vegetables provide necessary fibre which is essential in colon health. Also the type of iron in meat (heme iron) is indicated to be a major factor in the development of colorectal cancers as it affects the lining of the colon. The best way to reduce your risk of colorectal cancer is to adhere to a vegetarian diet, and focus on fresh fruits and vegetables with only moderate intake of processed foods.

References

1: Cancer incidence in British vegetarians

TJ Key, PN Appleby, EA Spencer, RC Travis, NE Allen, M Thorogood and JI Mann

British Journal of Cancer (2009) 101, 192 – 197

2: Association of apolipoprotein E polymorphisms and dietary factors in colorectal cancer

M Mrkonjic, E Chappell, VV Pethe, M Manno, D Daftary, CM Greenwood, S Gallinger, BW Zanke, JA Knight and B Bapat.

British Journal of Cancer (2009) 100, 1966 – 1974

3: Nutritional Factors and Gastric Cancer in Spain

CA Gonzalez, E Riboli, J Badosa, E Batiste, T Cardona, S Pita, JM Sanz, M Torrent and A Agudo

American Journal of Epidemiology (1994) 139, 466-473

4: Dietary patterns, nutrient intake and gastric cancer in a high-risk area of Italy

D Palli, A Russo and A Decarli

Cancer Causes and Control (2001) 12, 163-172