Friday, March 21, 2014

Does Fast Food in the Neighborhood Make You Fat?

There is evidence that 1) quick service restaurant meals,1 and sit down restaurant meals,2 provide excess calories and 2) that consuming excess calories leads to adverse health outcomes in most people.3,4  In non-experimental studies, people who consume the most fast food meals have also been the ones with health problems and these studies led the way for studies related to ‘obesogenic’ environments.5

I first heard the term ‘obesogenic environment’ when reading the research and policy recommendations of Professor Boyd Swinburn (University of Auckland).  Obesogenic environments push behavior that may lead to obesity.  One of these pushes might be the availability of cheap, high calorie foods.

Researchers have investigated the association between living, learning or working near quick service restaurants and certain outcomes. [Quick service (aka fast food) restaurants are defined as places where you purchase your food at a counter, receive it within minutes of ordering it and the food is often fried.  Pizza restaurants are included). Three questions researchers ask are 1) whether being near to quick service restaurants – convenience/marketing – leads people to purchase or consume more fast foods than not being near to them, 2) what exactly is near? (e.g., .1 mi, .5 mi, 1 mi), and 3) does the number or density of them influence the amount of consumption (i.e., is it worse to have 4 quick service restaurants within a .5 mile than it is to have 2 within a .5 mile)?

Researchers have also investigated whether there is an association between exposure to quick service restaurants and other health outcomes, like obesity and increased body fat.  Most of the studies6 that investigate exposure to restaurants and frequency of fast food consumption, weight differences and markers of obesity, use cross sectional (observational) data.  This means that they measure the ‘risk’ (exposure to restaurants) and ‘outcome’ (consumption/patronage, weight, BMI, obesity) at the same time.   

There are a few studies that follow people over time to see if increasing fast food consumption increases weight or BMI,7 but they too suffer the limitations of an observational study.  

Observational studies on fast food exposure and outcomes measure the number of quick service restaurants a person is exposed to, usually within .5, 1 and 5 miles of their home or school and then compare something they think should be different (weight, obesity rate, body fat, weight gain) between people with varying levels of exposure.  Good research will include measures of other things that could ‘cause’ the outcome of interest, so that the only difference between the groups is their level of exposure to that causal factor.  True equivalence between groups is only possible in randomized experiments and even then, equivalence is often approached more than actualized. 

In the ideal, a researcher would conduct an experimental study instead of an observational study.  He or she would randomly assign some people to live near a bunch of quick service restaurants (e.g., McDonalds, Burger King, Church’s, Taco Bell, Pizza Hut, Long John Silvers) and other people not to live near them. These two groups would be alike in the outcomes that the researcher planned to measure – e.g., the average meals of fast food per week or the average BMI at the start of the study. 

Such a study is not possible, and observational cross sectional studies lack the sine qua non of causality – temporality.  Before we can say that X caused Y, e.g., that exposure to restaurants leads to obesity, we have to show that the ‘exposure’ came before the outcome.  Since the researchers in fast food studies have not created the environments (they did not pay McDonalds to open stores within 5 miles of your house ) nor randomly assign normal weight people to live in environments with varying levels of quick service restaurants, they cannot make the ‘living near a quick service restaurant leads to obesity’ conclusion.  Without being able to randomize to environments or introduce the risk (create temporality), researchers cannot rule out other possibilities for their findings, but even with other possible causes, the hypothesis that being near to quick service restaurants at home, work or school leads to more frequent quick service restaurant visits, over consumption of calories and the adverse health events that follow, makes sense. 

In the most recent fast food study, Burgoine and colleagues8 used data from a national UK study to examine several outcomes related to exposure to quick service restaurants.  Over 5000 persons (N = 5442) answered questions (and completed a dietary recall) as part of a UK Health study; the people were also measured by study staff such that their BMIs could be calculated from objective data.  The participants provided their home and work addresses so that the researchers, using GIS, could map the quick service restaurants and supermarkets that were within about a mile of their homes, their jobs and even along their routes to work!  The researchers created 4 levels of exposure and each participant was placed into the relevant group for analysis (there were 4 exposure settings: home, work, on the commute and all combined).  NOTE: In the UK, quick service restaurants are called take away food outlets and the food sold there is take away food – but the definition is the same as the quick service restaurant definition I gave above.

The number of take away outlets per level was different for each of the 4 exposure settings, but level 1 usually had 3 or less outlets and level 4 had 15 or more. 

The researchers compared groups (based on their level of exposure: level 1 thru level 4) on dietary intake, BMI and the odds of being overweight or obese.  The researchers compared each group of exposure level 2, 3 and 4 to the group of people with level 1 exposure (the least exposed group) in all of the settings (home, work, commute, combined).  With regard to the amount of take away food consumed (by self-report in the dietary recall), the most exposed group (level 4) did consume more take away food than the least exposed group (level1) but no differences that could be attributed to exposure were found in the other groups.  In regards to consumption and exposure in the work environment, groups in level 3 and 4 ate more take away foods than those in level 1 and the amount of extra consumption was greater in the level 4 group than the level 3 group.  Groups in level 2 and 1 were not found to be different from each other in the amount of take away foods consumed.

With regard to the risk of increased consumption of take away foods related to the amount of exposure on the commute, there was only one difference and it was between the least exposed and the next to least exposed (i.e., level 2).  In this case, the least exposed group had 1 or fewer take away outlets on their route and the level 2 group had 1 to 5.  The more exposed group ate LESS take away meals than the 0 -1 outlet group.  No other differences in consumption were related to exposure on the commute.   The last category, the combination home, work, and commute, was similar in finding to the work results.  Both groups with level 3 exposure and level 4 exposure ate more take away food than those in the level 1 group and the amount consumed by level 4 group members was higher than that consumed by the level 3 group members. 

Similar differences were seen with the BMI and the odds of being obese, but exposure to take away outlets did not influence the odds of being overweight.  More exposure (level 3 and 4) was associated with higher BMI and odds of being obese. The most substantial difference in the odds of being obese was for the home environment.  Here the most exposed group (level 4 with 15-47 outlets near their home) was more than twice as likely to be obese than those in the no exposure group.

The Burgoine study is a cross sectional study so one cannot conclude that the increased exposure to take away food outlets led to the higher BMIs or obesity or even the greater amount of fast food eaten, but it supports the hypothesis.  I agree with the authors of the study that policy to limit exposure (perhaps through zoning laws) is a good idea.

(By the way, one of the factors that the researchers controlled for in their statistical model was the number of supermarkets in each of the food environments.  This allows them to make the assertion that regardless of access to fresh foods in a grocery store, the presence of take away outlets is still negative.)

References Cited

No comments: