I saw this morning reports on a study which has found links between vegetarianism and a reduced risk of certain cancers. The study, which assess nutritional habits and cancer risk in more than 61,000 men and women in the UK found that compared to meat-eaters, vegetarians had a 53, 45 and 74 per cent reduced risk in bladder, blood and lymph system (eg. leukaemia and lymphoma) and stomach cancers respectively [1]. Looking at all cancers combined, risk reduction was found to be 12 per cent. Something tells me that those keen on the vegetarian way will wave this study around as yet more ‘proof’ that the vegetarian diet is healthiest for us.
I do not dismiss this evidence out of hand, but I think it is important to bear in mind that this study was epidemiological in nature, and can only then tell us about associations between diet and cancer. But just because two things are associated does not mean one is causing the other. The usual assumption with studies of this nature is that there’s something bad about meat that ups cancer risk. But it may be vegetables are cancer protective and vegetarians eat more of these. Or maybe it’s neither of these things. Or maybe, it’s nothing more than an association and eating meat/fewer vegetables does not cause cancer at all. We just don’t know.
One major problem with epidemiological studies is what are known as ‘confounding’ factors. So let’s say we find from a study less physically active people turn out to have a higher risk of lung cancer. We have found an association between two things that might cause people to conclude that a sedentary lifestyle causes lung cancer. But, imagine that sedentary people are more likely to smoke. Smoking is a potential confounding factor here, and may be the real reason for why sedentary individuals are at heightened risk of lung cancer. In epidemiological studies often an attempt is made to ‘control for’ potential confounding factors. It’s an imprecise science, for sure, but generally better than nothing.
Controlling for potential confounding factors is particularly important when comparing meat-eaters and vegetarians because, generally speaking, they are likely to be more health-conscious than meat-eaters. Meat-eating has an unhealthy reputation, right? And vegetarianism generally as a ‘healthy’ image too. So someone inclined to do what they can to protect or improve their health may be drawn towards eschewing meat. But these individuals may also be drawn to other habits too like smoking less and exercising more.
The study focused on here did control for a variety of potential confounding factors including age, smoking, alcohol intake, body mass index and physical activity level. But another way to level the playing field might be to attempt to compare vegetarians and non-vegetarians who have been matched for a similar level of health consciousness.
In one study, researchers attempted to counteract any confounding factors by focusing only on individuals who shopped in health food stores. The idea here is that all of these individuals are generally ‘health-conscious’, whether they are vegetarian or not. This allows, some would argue, a fairer appraisal of the impact of vegetarian or non-vegetarian eating.
This study did not focus on risk of individual conditions, but a much better marker for overall health: overall risk of death. This study found that compared to the general population, death rates in vegetarians and non-vegetarians were significantly lower than in the general population (which supports the notion that health food shoppers are a generally health-conscious bunch). Crucially, though, overall risk of death in vegetarians and non-vegetarians the same [2].
In another study, vegetarians were asked to recruit their friends and family into the study. Doing this was thought to help ensure that all individuals in the study were similarly health-conscious. Again, death rates for vegetarians and non-vegetarians were essentially the same [3].
In May this year, a study was published which assessed total mortality in a large group of vegetarians and non-vegetarians [4]. Confounding factors controlled for were age, sex, smoking, and alcohol consumption. The result? No difference in risk of death between vegerarians and non-vegetarians. Risk of death from heart disease was the same too.
I read here that Professor Tim Key, the lead author of the study being reported on today said it was impossible to draw strong conclusions from this one single study. He is quoted as saying: “At the moment these findings are not strong enough to ask for particularly large changes in the diets of people following an average balanced diet.”
Even the study’s lead author seems to have poured cold water on its findings. Perhaps Professor Key is aware of the evidence which shows vegetarianism does not offer distinct health advantages? He should be, seeing as he’s the lead author of the studies mentioned above [2,4] that show that vegetarians have mortality rates the same as non-vegetarians.
References:
1. Key TJ, et al. Cancer incidence in British vegetarians. Br J Cancer. 2009 Jun 16. [Epub ahead of print]
2. Key TJA, et al. Dietary habits and mortality in 11000 vegetarians and health conscious people: results of a 17 year follow up. Br Med J 1996;313:775″9
3. Thorogood M, et al. Risk of death from cancer and ischemic heart disease in meat and non-meat eaters. BMJ 1994;308:1667″70
4. Key TJ, et al. Mortality in British vegetarians: results from the European Prospective Investigation into Cancer and Nutrition (EPIC-Oxford). Am J Clin Nutr. 2009;89(5):1613S-1619S
