Not to defend fast food, but does it really cause asthma?

Here's a case for confounding variables if there ever was one: a new study suggests that 'fast foods' cause asthma, rhinoconjunctivitis and eczema.  Indeed, the BBC tells us that teens who eat fast foods three or more times a week are 39% more likely to have severe asthma than kids who eat three or more servings of fruit each week. Sounds pretty clearly causal, doesn't it?  Well, maybe not.

The results are from a long-standing international study of asthma, begun in 1991 because of concern over increasing asthma rates.  Called ISAAC, the International Study of Asthma and Allergies in Childhood, the study is based in New Zealand but is associated with centers all over the world.  Asthma incidence began to rise in the mid 1980's and the trend has yet to be satisfactorily explained by environmental epidemiology.  And of course, many tens of millions of dollars have been spent looking for a genetic cause, without success.   

An ISAAC study published in 2004 reported that eating hamburgers was associated with increased risk.  Other studies have found some association with maternal diet during pregnancy, as well as childhood diet. 

Results from the latest paper relied on questionnaires translated into 53 languages, and included more than 500,000 children from 54 countries. Along with lifestyle questions, teenagers, and parents of younger children, were asked about symptoms of asthma, watery itchy eyes and eczema.  And, quoting from the paper, they were asked the following about their diet: ‘In the past 12 months, how often, on average, did you (did your child) eat or drink the following: meat; seafood; fruit; vegetables (green and root); pulses (peas, beans, lentils); cereal; pasta (including bread); rice; butter; margarine; nuts; potatoes; milk; eggs and fast food/burgers?’.  They could choose 'never or occasionally,' 'once or twice per week,' or ' ≥3 times per week.'

Covariates considered included exercise, television watching, maternal education, maternal smoking in the first year of life and current maternal smoking.

While the BBC story makes a case for fast food being the cause of asthma, rhinoconjunctivitis and eczema, the story is much less clear if you look at the details actually presented in the paper. In teenagers, fruits are 'protective' if eaten once or twice, or 3 or more times a week,

[m]ilk was inversely associated with current wheeze once or twice per week, severe asthma ≥3 times per week, VQ [current wheeze], current and severe rhinoconjunctivitis once or twice per week and current and severe eczema once or twice per week as well as ≥3 times per week for current eczema. Vegetable consumption was also inversely associated with current wheeze ≥3 times per week and VQ once or twice per week and ≥3 times per week.

So, milk was protective if consumed once or twice per week, but not if consumed three or more times per week.

'Risk factor foods' were butter, fast food, margarine and pasta ≥3 times per week.  And seafood was positively associated with 'severe asthma, current and severe rhinoconjunctivitis and current and severe eczema. Butter, margarine, nuts, pasta and pulses [legumes] were positively associated with one or more conditions.'  If there is some consistent causal factor -- dairy fat, say, why butter but not milk?  Why not eggs of meat, if it's just fat that's the risk factor?

In younger children (6-7),

For all centres combined, eggs, fruit, meat and milk ≥3 times per week were inversely associated with all three conditions, current and severe. Cereal ≥3 times per week was inversely associated with severe asthma. Vegetables once or twice per week and ≥3 times per week were inversely associated with current and severe wheeze as well as current and severe rhinoconjunctivitis and ≥3 times per week with current eczema.

So, eggs and meat are protective, whereas they aren't for older children (aged 13-14).  Indeed, risk factor food for younger children is fast food only.  And, the only food category both age groups have in common is fast food.  Which, of course, could include a large variety of things. 

To add to the difficulty making sense of these data, "there is considerable variation for some foods between centres within countries."  Indeed, take a look at the odds ratios (ORs), the measure of the strength of association between variables and outcome, for fruits and fast foods by age and country.  They are, by and large, all over the place.  So, the odds of having severe eczema if you eat fruit in Mexico, say, vs. never eating fruit are 0.15, or 2.0.  That is, risk ranges from much lower than never eating fruit, to twice as high. The odds of having current wheeze if you eat fruit in Spain are 0.50 or 2.0.  The overall data may well suggest a trend, but it's not one that is reliably evident center by center, or even within each country. 

So, apart from the question of why this story has been picked up by the media (well, we know why -- anything that indicts fast food has the potential to be sensational), how is it that these results can be all over the place like this?

For one thing, and we've said this numerous times in numerous posts, food questionnaires are notoriously unreliable.  Do you know how many times per week you consumed pasta in the past 12 months?  Or fruit? 

And, who is likely to eat more fast food?  Kids in cities.  Higher risk of asthma in cities has been documented over and over again.  It isn't clear why that is, though allergy to cockroaches has been suggested, or proximity to asthmagenic industry, although this contradicts the other widely held hypothesis that too much cleanliness is causal (this is the Hygiene Hypothesis, which we blogged about here), and kids on farms have frequently been found to be at lower risk of asthma, for whatever reason.  Ok, it's possible that fast food is the reason, but it's also possible (and we'd say likely) that it's a confounding variable, a factor that stands in for an actual but unknown and unmeasured risk factor that kids in cities are more exposed to than kids in rural areas.  We aren't saying that's so, just suggesting it's a possibility; whether this study shows this rural/urban disparity is not clear. 

The asthma epidemic is relatively recent.  Having fresh fruits and vegetables year-round is also relatively recent, so most grand and great-grandparents were disease-free for some other reason.  Unless, of course, fruit and veg are cancelling out the effect of fast foods in kids who eat both, which we can't tell from the report, but that does seem to be a rather contorted explanation, if so.

It is not our purpose to defend fast food here, but really, how unhealthy is it, relatively speaking?  Yes, it packs a lot of calories, and for a lot of people that's not a good thing, but what else might be wrong with fast food?  And we are talking about fast food as a whole -- Big Macs, fries, fish sandwiches, milk shakes; everything lumped into one.  The authors write, 'Biologically plausible mechanisms for the relationship between fast food consumption and asthma and allergic disease could be related to higher saturated fatty acids, trans fatty acids, sodium, carbohydrates and sugar levels of fast food and possibly preservatives.'  Maybe, but if so, what have fast foods got that bacon, beef, any desserts, and so on don't?

And, the authors suggest that fruits and vegetables are loaded with antioxidants, which are said to be protective -- against almost everything.  Except that multiple large double blind studies have found that they aren't.  So, again, it's possible that fruit and vegetable consumption is a proxy for some other unidentified protective factor.  Or not. 

Given that the data used in this study are recall data, the diseases self-reported, the causal reasoning purely speculative, and that there's a possible problem with multiple confounding variables, we're going to go out on a limb here and suggest that, based on this study, the diet/asthma-eczema-rhinoconjunctivitis connection is a tenuous one at best.