“Three burgers a week could be enough to bring on asthma,” The Daily Express claimed. Several other newspapers have also reported the results of an international study that looked at asthma risk and eating habits in over 50,000 children in 20 countries.
The study behind these news reports is flawed due to several problems noted by the researchers. For example, they did not measure important risk factors, such as the children’s weight, or fully account for their social and economic background. Some interesting trends in the results were mentioned by the Express, for example that a diet high in fish is linked to less asthma symptoms in rich countries, while youngsters in poor countries had fewer symptoms if they ate a diet rich in cooked vegetables.
This study will need to be followed by others that look at, and adjust for, all known risk factors and follow participants over time. This is necessary to establish whether diet directly affects asthma risk, or whether diet is an indicator of social factors that influence the condition.
Where did the story come from?
The study was carried out by researchers from Germany, Spain and London, along with a large supporting study group that supplied its data. The co-ordination and analysis were funded by the Fifth Framework Programme of the European Commission, whereas the field work and data collection were funded locally.
The study was published in Thorax, a peer-reviewed medical journal.
The other papers and news sources reporting on this study discussed its limitations in varying amounts of detail. For example, the BBC picked up on the fact that the link to burgers was only shown in wealthy countries, while The Daily Telegraph commented on a theoretical biological basis for the findings. The Telegraph pointed out that foods such as fruit and vegetables are rich in vitamin C or antioxidants, which may explain the link between diet and asthma. Most of the stories implied that the study is yet another reason to follow a healthy diet.
What kind of research was this?
The aim of this research was to investigate how different foods, particularly the “Mediterranean-type diet”, are linked to asthma symptoms and diagnosis and allergies in children. The authors point out that previous reports in this area had not used objective measurements of allergy (also known as atopy), such as skin prick testing, so they hoped to use an improved study design to investigate the association between diet and increasing rates of asthma and atopy in Western countries.
The study was large and the researchers catalogued an impressive list of food types, scoring them on how closely they match the idealised Mediterranean dietary pattern. The objective measurement of atopy, through skin prick testing, was a strength of the study, although no food items were subsequently found to be linked to allergic sensitisation. As the study did not include measurements of the participants’ weight or energy intake, it is not possible to gauge the extent to which any link could be related to being overweight or obese, which are thought to be associated with asthma.
What did the research involve?
The International Study on Allergies and Asthma in Childhood (ISAAC) is a long-running study in 29 centres in 20 countries around the world. Overall, it reports important differences in the rates of asthma between countries and aims to improve the understanding of the possible causes of these differences.
This piece of research was a sub-study using data from ISAAC. To gather their population, the researchers picked a random sample of at least 10 schools in a defined geographical area of each country and chose more than 1,000 children from each of these centres. Overall, about 63,000 (76.4% of those eligible) schoolchildren between 8 and 12 years old were invited to take part. Between 1995 and 2005, parents provided 50,004 complete questionnaires on their children’s diet
These food questionnaires asked about allergic symptoms and diseases. The researchers also tested 29,579 (59%) of them with skin prick testing to see if they had allergic sensitivities to the common allergens. They then used standard statistical techniques to test the strength of any links found in a cross-sectional analysis. They adjusted the analysis in several ways for age, sex, environmental tobacco smoke, parental atopy (allergy), exercise, number of siblings and maternal education.
The parental questionnaires asked for information on those foods already thought to be linked to asthma or allergies. This was based on theories of the contributory or preventative role of antioxidants and oily fish or saturated fat intake in these conditions. The questionnaire asked about meat, fish, fresh fruit, raw green vegetables, cooked green vegetables, burgers, fruit juice and fizzy drink. The researchers scored food intake according to how well it matched the Mediterranean dietary pattern, characterised by high fruit and vegetable intake and low consumption of saturated fatty acids from animal sources.
The researchers also collected data on age, sex and some of the other factors thought to be linked to asthma, such as current exposure to environmental tobacco smoke, number of brothers or sisters, parents’ history of asthma, hay fever or eczema and exercise. Most centres invited all children in that region for skin prick testing, where the researchers tested for six common airborne allergens, including house dust mites, cat hair, and tree and grass pollen. Importantly, the children were not weighed at this visit.
So that they could test any differences between the countries, the researchers also classified the study centres into two broad categories based on gross national income (GNI) per capita. They defined “non-affluent” countries as those where GNI was less than $9,200 a year per capita.
What were the basic results?
After the researchers’ statistical adjustments, fruit intake was associated with a lower rate of symptoms of wheezing in both affluent and non-affluent countries. Consumption of fish in affluent countries and cooked green vegetables in non-affluent countries were associated with a lower rate of symptoms of wheezing.
In all countries, more frequent consumption of fish and cooked or raw vegetables was associated with lower reported rates of asthma diagnosis. Burger consumption was associated with higher lifetime rates of asthma diagnosis.
Further analysis across all countries showed that, compared with no or occasional consumption, the consumption of burgers more than three times a week was associated with about a 40% increased risk of asthma diagnosis (adjusted odds ratio 1.42, 95% confidence interval 1.08 to 1.87).
On the skin prick testing, none of the food items was associated with allergic sensitisation.
Food considered to be part of the Mediterranean diet was associated with a lower prevalence of current wheeze and asthma.
How did the researchers interpret the results?
The researchers say that diet is associated with wheeze and asthma but not with allergic sensitisation in children. They claim that their results provide further evidence that eating a Mediterranean diet may provide some protection against wheeze and asthma in childhood.
This large, carefully conducted study set out to provide new international data regarding the different rates of asthma and allergies between countries. The researchers tested theories of how food might be linked to the causes of these conditions. Several limitations are mentioned by the authors:
- The dietary questionnaire asked about past eating habits in children. These may not have been accurately remembered or reported by parents. This could have reduced or increased the strength of the associations reported.
- The researchers selected the foods that are part of either Western or Mediterranean dietary patterns and, as mentioned by several newspapers, were unable to adjust for total energy intake and body mass index, which were not asked about or measured. This is an important limitation as asthma is thought to be linked to being overweight.
- The researchers performed numerous statistical comparisons, which increases the likelihood of finding a significant result simply due to chance.
Overall, this study has improved the understanding of how asthma rates may differ across the world, but has raised further questions about the link between diet and asthma. Ideally, the potential link will be explored through studies that control for all the other factors thought to increase the risk of asthma.
There are many good reasons to eat healthily. We do not yet know whether avoiding asthma is one of them, but it’s best to follow a balanced diet regardless.