I came across an article earlier this week that said that “vegetarians are less healthy and have a lower quality of life than meat-eaters”, according to “scientists”. As a vegetarian who is used to hearing that vegetarians live on average 7 years longer than meat-eaters, this took my interest. Are we vegetarians really less healthy? It seemed, frankly, impossible.
The academic paper is published in “PLOS ONE” which claims to “rigorously peer-review submissions” and only publish papers that are technically sound. Unfortunately, I do not agree that the conclusions of this paper are supported by the evidence, and therefore, I do not agree that the methodology and related conclusions are “technically sound”.
The paper introduces the research by saying that vegetarians eat less fat, more fruit and vegetables, are more active, drink and smoke less. In my opinion, you could probably make conclusions at this stage, but the authors are interested in specifically exploring the effect of a vegetarian diet in the Austrian population. Seems to me that the effect of a vegetarian diet in Austria wouldn’t be that much different in other non-Austrian countries.
The great thing about this paper (well, let’s try and be positive) is that it illustrates how difficult it is to draw conclusions using observational studies: extreme care must be taken. The authors have identified over 300 vegetarians, and matched them with similar meat eaters, using age, gender and socio-economic status as factors in their matching process. They then analyse the data, statistically adjusting for body-mass-index, physical activity, smoking behaviour, and alcohol consumption.
The point of matching the subjects and statistically adjusting the analysis is to attempt to remove the effect of lots of other factors that will affect how healthy someone is (e.g. smoking and drinking). This point, in my opinion, invalidates the conclusions of the study. The conclusion from the analysis of this study should be “vegetarians would be more unhealthy as meat-eaters if they followed the less healthy meat-eaters lifestyle”. However, I don’t imagine this would make such a great newspaper headline.
The authors call for public health programs to reduce risks due to nutritional factors. I think they want governments to discourage vegetarian diets as being unhealthy, but they don’t come out and clearly state this. This is actually a potentially dangerous conclusion to draw from this study. If public money were diverted to discourage a vegetarian diet or lifestyle, I would guess that it would either have no effect on public health, or at worse, a negative effect. As stated by the authors, vegetarians generally have a healthier lifestyle than meat eaters, and would benefit less from more public health funds being diverted in their direction.
There are two additional methodological flaws with the report that ideally should have been picked up by statistical reviewers. Firstly, only statistically adjusted results are presented, and no raw unadjusted data. I would very much like to see the raw data on health states in the different dietary groups, both across the whole dataset and in the matched sample. Second, the statistical adjustment is carried out in a model – specifically a “general linear model”, which makes a series of assumptions about the relationships between the variables being analysed. In this example, there are assumptions that the relationship between these variables is a linear relationship (e.g. twice as bad health for twice as much smoking). I would wager that the poorer outcomes for vegetarians after adjusting using a general linear model are more about incorrect linearity in the model assumptions than any real clinical effect on different diets.
In conclusion, please don’t pay any attention to this call for more public health money to be spent on persuading vegetarians to eat meat. It’s daft.