Last week, I highlighted the importance, when assessing the effects of a diet or medical treatment, of focusing not so much on parameters such as blood cholesterol and blood pressure, but on health [click here]. And this week, one of my posts makes the point that it makes sense to concentrate on broad health effects of a treatment (such as overall risk of death) than more narrow ones such as, say, risk of cancer of the gallbladder [click here]. Once we pan out from the tight focus many researchers have, it’s not uncommon for a very different picture to emerge.
I came across a classic example of this mid-week in the form of a study which assessed the effects of vegetarian eating. The study, published in this month’s edition of the American Journal of Clinical Nutrition, looked at the relationship between protein intake and the levels of a variety of substances in the blood believed to be ‘markers’ for risk of cancer. Vegetarians eating low levels of protein were found to have lower levels of several these substances compared to individuals eating a standard American diet. This led the lead author of the study, Dr Luigi Fontana, to go on the record to warn people off eating red meat, though he did heartily endorse fish (also a very high protein food). Strange.
Anyway, the point I want to make is that this study is another example of research which has looked at the effect of diet on a supposed marker for health, not health itself. Nevertheless, it is likely that this research will end up being used by the vegetarian lobby to support their chosen diet. It will also, I suspect, go towards further propping up the widely held notion propagated by certain factions in the vegetarian community that vegetarianism is ‘healthier’ than eating a more omnivorous diet. But is this really the case?
The studies that are sometimes used to support vegetarianism appear to show that vegetarians are at a health advantage, particularly with regard to their risk of heart disease. However, such studies are known as ‘epidemiological’ in nature, which means that while they can show associations between two factors (in this case, vegetarian eating and better health). However, they cannot be used to ‘prove’ that vegetarianism is healthy.
Part of the reason for this relates to the fact that any apparent benefits to health that appear to come from vegetarianism may come not from the absence of meat and fish from the diet, but from other factors associated with vegetarian living such as a reduced tendency to smoke and healthier exercise habits. These so-called ‘confounding’ factors need to be taken into consideration in order to make a fair assessment of the relative merits of vegetarian and non-vegetarian diets.
Some researchers have attempted to make a more accurate assessment of the benefits (or otherwise) of vegetarian eating by taking into account these confounding factors. 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 a fairer appraisal of the impact of vegetarian or non-vegetarian eating. 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). However, 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, this study found that vegetarians and non-vegetarians had risk of deaths lower than that of the general population. However, again, death rates for vegetarians and non-vegetarians were essentially the same [3].
Another piece of research comparing vegetarians and non-vegetarians involved quite detailed analysis of the dietary habits of some 56000 individuals [4]. This study, yet again, found that the overall risk of death in vegetarians was not lower than in non-vegetarians. And what of that claim about vegetarian diets being better for the heart? None of these studies found any evidence for this contention either. So, the plain facts show that, overall, there is no broad health advantage to be had from eating a vegetarian diet.
The problem with so much research we have thrown at us through the media is that it is focused too much on the detail, and not enough on the big picture. I accept that it is the job of researchers like Dr Fontana to look at the ‘trees’. I would say, however, that he and others like him might want to take a few paces back every so often to consider the wood too.
References:
1. Fontana L, et al. Long-term low-protein, low-calorie diet and endurance exercise modulate metabolic factors associated with cancer risk Am J Clin Nutr 2006;84:1456-1462
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. Davey GK, et al. EPIC-Oxford: lifestyle characteristics and nutrient intakes in a cohort of 33,883 meat-eaters and 31,546 non meat-eaters in the UK. Public Health Nutr 2003;6:259″68
