“Fast food link to childhood asthma and eczema” hit the headlines yesterday and, whilst there were variations on this theme, I was glad to see that the word ‘link’ was there across the headlines. All too often, research which could only show an association between one thing and another is reported in news items as though a cause and effect had been established. You know the kind of thing, “red meat kills!” and all that (I blogged about this here last year). So, what’s the story this time?
Researchers conducting the International Study of Asthma and Allergies in Childhood (niftily named ISAAC) explored whether fast foods have a role in the development of asthma, eczema and rhinoconjunctivitis (runny or blocked nose and itchy eyes when not suffering from a cold), conditions on the rise, and have published their findings in the peer reviewed journal Thorax. ISAAC is a multicentre cross-sectional study; this kind of study can provide lots of information about a popuation, including lifestyle factors and how many have certain health conditions, for instance. It can’t establish causation.
In this part of the study, questionnaires were were completed by over 300,000 adolescents and parents of nearly 200,000 younger children, in 51 countries, gathering information on symptoms and diet. The researchers made statistical adjustments for factors (such as exercise and region of the world) which might contribute to any association seen (‘confounders’). There’s a wealth of detail in the paper but the key findings are:
- Fast food intake (three or more servings a week) was consistently positively associated with current and severe prevalence of wheeze, rhinoconjunctivitis and eczema
- Fruit consumption showed a consistent inverse (protective) association for current and severe wheeze and rhinitis in adolescents and all three conditions in children
- For adolescents, milk and vegetables were also identified as protective foods, while for children milk, vegetables, meat, eggs and fruit were associated wih a reduced risk of these conditions and cereals with a reduced risk of asthma
Fast food consumption may be contributing to the increasing prevalence of asthma, rhinoconjunctivitis and eczema in adolescents and children. For other foods, the picture…is less clear. However, in concordance with international dietary recommendations, diets that have a regular consumption of fruit and vegetables are likely to protect against asthma, allergic disease and other non-communicable diseases.
The researchers themselves point out some potential weaknesses of the study, including the reliance on self-completed questionnaires which can be subject to misclassification and the errors that come with relying on our fallible powers of recall. Also, the cross-sectional design means that there isn’t information on the people studied over time, which leaves open the possibility that symptoms led to changes in diet, rather than the other way round for example. However, this is an enormous study of over half a million children and teens from a large number of countries and the associations it suggests surely warrant further exploration.
I’m not sure now, though, that the headline writers’ use of the word ‘link’ will have done much to caution readers against thinking ’cause’. After all, the research team themselves have used the ‘c’ word in the title of their paper, even though this is a question the study can’t hope to answer. But they’ve given us plenty of food for thought haven’t they? I certainly won’t hesitate to flourish this paper at my young elves as I heap an extra portion of broccoli onto the plates.
Ellwood P, Asher MI, García-Marcos L, Williams H, Keil U, Robertson C, Nagel G, the ISAAC Phase III Study Group. Do fast foods cause asthma, rhinoconjunctivitis and eczema? Global findings from the International Study of Asthma and Allergies in Childhood (ISAAC) Phase Three. Thorax doi:10.1136/thoraxjnl-2012-202285