Thursday, 3 August 2017

Fast food, obesity and junk science

I was on the radio yesterday debating with some busybody from the NHS who wants to limit the number of takeaway shops in Britain's high streets. The feeble hook for this story was the news that the number of takeaway outlets in the country has increased by eight per cent in the last three years, according to a pearl-clutching Guardian analysis.

This would seem to reflect the resurgent economy and population growth but for the 'public health' lobby it is a crisis. Why? Because fast food causes obesity, dunnit?

But does it? Leaving aside the fact that takeaway food shops do not necessarily sell 'junk' or 'fast' food, the assumption that takeaways cause obesity has always been just that: an assumption.

What empirical basis does it have? A 2010 evidence review identified 12 studies looking at fast food outlet availability and obesity. Six found a positive association, five found no association and two found a negative association.

It also looked at six studies of fast food consumption and obesity. Three found an association, the other three didn't.

This is hardly compelling and I recommend reading this post by Mike Gibney in which he discusses some of the more recent evidence, starting with a 2017 study from the USA which concluded:

Our a priori prediction that FFRs [fast food restaurants] and FSRs [full service restaurants] would be positively linked to obesity prevalence was not supported

He also mentions a study of Europeans which found:

Our results suggest, contrary to normative views, that away from home food expenditures negatively affect BMI and that BMI is negatively related to the percentage of the food budget spent away from home.

And he mentions another recent study from the USA which found that...

Neighbourhood convenience stores and fast-food restaurants were not associated with BMI in any model.

The authors of that study argued that 'weak findings in the literature [which report an association between fast food and obesity] may be due to residual confounding'.

If you want to get an idea of how weak the evidence is for the fast food/obesity link, take a look at this study from the British Medical Journal. This is the one that campaigners like to cite because it concluded:

Exposure to takeaway food outlets in home, work, and commuting environments combined was associated with marginally higher consumption of takeaway food, greater body mass index, and greater odds of obesity.

The study looked at people's 'exposure' to takeaway food in Cambridgeshire and claimed that people who were heavily 'exposed' were 80 per cent more likely to be obese. The study concludes with a call for 'policies designed to improve diets through restricting takeaway food availability' which is a bit of a red flag for activist-driven research. This response from a statistician (which the authors never addressed) is devastating. It reveals that the finding was entirely dependent on adjustments to the data.

After reading the interesting article by Burgoine et al. I was at first irritated by the lack of a table to compare the characteristics (as shown in Table 1) of participants grouped according to quarters of take-away environment. Further, I missed a simple presentation of outcome variables (mean take-away consumption, mean BMI, percentage overweight and obese) grouped according to these same quarters. Usually one would expect such tables in order to assess the comparability of the groups with respect to possible confounders and for a direct, unadjusted comparison of outcomes, respectively.

Then I discovered this information in Web table 3 of the online appendix. Here, we see systematic differences between quarters with respect to education, smoking and car ownership. I think the authors should have presented these tables and drawn attention to these differences in the main printed article, even if the multiple linear regression models adjusted for the covariables concerned.

What surprised me even more in Web table 3 was the fact that mean take-away consumption was slightly inversely correlated with combined take-away availability, varying between 36.3 g/day in Q1 and 34.2 g/day in Q4. This contrasts completely with the results of the multivariate analysis (Fig. 1) in which a significant positive correlation between take-away availability and consumption was obtained. Moreover, In Web table 3 mean BMI is almost constant in all quarters of take-away availability, contrasting with the significant positive correlation between take-away availability and BMI derived from the multiple linear model (Fig. 2). While I accept that the multivariate analysis adjusting for potential confounders is the analysis of choice for such an observational study, the complete lack of agreement with the simple univariate analysis is worrying and should be presented and discussed.

A hint on the possible explanation for these inconsistencies is given under ‘sensitivity analyses’. ‘In models that omitted supermarket exposure as a covariate, the associations between combined take-away food outlet exposure, consumption of take-away food and body mass index were attenuated towards the null…’. These sensitivity results are given in Web figures 5 and 6. The expression ‘attenuated towards the null’ is an understatement: no association remains at all, in agreement with the simple univariate comparison.

If you look at the supplementary data, you can see that he is correct. There is no difference in average weight between those who have the greatest exposure to takeaways (4) and those who have the least (1). Moreover, the people who had the easiest access to takeaway food ate less of it than those who had the least access.

Your faith in the authors' conclusion therefore depends on how much faith you have in their numerous adjustments to the data. To my mind, it is junk science, but whatever you think of it, it is simply untrue to claim that the people in this study who were most 'exposed' to takeaways were fatter than other groups.

It brings to mind a study I wrote about five years ago which looked at fast food consumption among teenagers in Tower Hamlets. It concluded:

This study revealed a very high frequency of fast food consumption among the schoolchildren. Taste, quick access and peer influence were major contributing factors. These schoolchildren are exposed to an obesogenic environment, and it is not surprising that in this situation, many of these children are already overweight and will likely become obese as adults.

Notice how the authors beg the question. It is assumed that those who eat the most fast food will be most likely to be obese. They say that 'many of these children' are already overweight or obese but they do not say how many, nor do they compare obesity rates between those who eat a lot of it and those who do not.

You have to dig into the study to discover why they are so coy about this. It goes unmentioned in the text of the study, but we can see in Table 2 that there is an inverse relationship between frequency of takeaway consumption and body weight. The children who ate the most of it weighed the least and those who ate the least of it weighed the most.

This was clearly not what the authors were hoping to find so they ignored it and editorialised with a blatantly activist conclusion:

Clearly, actions need to be taken to either limit the ability of these children to access fast food outlets or to change the foods they purchased at these outlets (eg, less calorie dense, with more fruit and vegetables, with less fat and salt) and to have a ban on the sale of sweetened soft drinks at these outlets.

 Ever get the feeling you're being cheated?

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