Eating, asthma and allergies

Eating fish and "fruity vegetables", such as tomatoes and aubergines helps cut the risk of developing asthma and allergies, newspapers reported. A six-year study of children in Spain found that those who ate more of these foods were less likely to suffer from asthma and allergies.

The Daily Express opened with "Child victims of asthma or allergies can reduce their suffering". However, this first paragraph is misleading, and, as the rest of the report accurately conveys, the research these stories were based on was a study of risk, and did not investigate how to alleviate these conditions.

The Guardian reported that the study had also found that fruit was beneficial, however, other than tomatoes, aubergines and courgettes, fruit in general was not examined by the study. The Guardian also reported that this study monitored children for the first six years of their lives, and that the mothers completed questionnaires “every year until their children were aged six and a half”. In fact, the children’s mothers were periodically questioned about their breastfeeding habits within the first two years of the child’s life, and then completed a questionnaire about their children at age six and a half, in effect taking a snapshot of the children’s diet, wheezing and allergies over the past 12 months.

The research paper looked at the proportions of different food groups consumed by children aged six and a half years old and whether they had developed asthma or any allergies at the time of the study.

The study cannot establish whether specific dietary behaviours lead to allergic conditions, and further research would be needed to make any conclusions on this subject.

It is always a good idea for children to eat a healthy balanced diet including fish and vegetables.

Where did the story come from?

Dr Leda Chatzi of University of Crete and colleagues from Spanish universities and research institutions carried out this research. Funding was provided by Instituto de Salud Carlos III and an EU grant. The study was published in the peer-reviewed medical journal Pediatric Allergy and Immunology .

What kind of scientific study was this?

This was a cross sectional study where mothers were questioned about their child’s eating habits and breathing problems to investigate whether there was any relationship between the two. The children were also given skin prick tests for allergies.

The researchers recruited expectant mothers at antenatal clinics in 1998, and their 468 children (232 boys and 228 girls) were subsequently followed up when they were six and a half years old. The mothers answered a questionnaire on whether their child had been wheezing over the past 12 months or, whether they had been diagnosed as having an atopic wheeze, a wheeze caused by allergies,such as asthma.

The mothers also completed questions on other medical health, lifestyle, and a food analysis that looked at the amounts of 96 different food types that were eaten by the child over the past 12 months. Statistical methods were then used to look at the relationships between foods eaten and asthma symptoms. Parental asthma, parental smoking, or whether the child was breast fed were also taken into account in the analysis.

What were the results of the study?

The researchers found that at six and a half years , 8.7% of children currently had wheeze; 5.8% had atopic wheeze; and 17.0% showed an allergic response to common allergens such as pollen in skin-prick testing.

They found that children who ate over 60 grams fish per day were less likely to show an allergic response on skin prick testing than those who ate 39 grams or less. Children who ate more than 40 grams of ‘fruity vegetables’ per day (e.g. tomatoes, courgette, aubergine) were less likely to have wheeze (including atopic wheeze)  than those who ate 17 grams or less. These results remained significant when adjustments were made for other potential contributing factors such as parental smoking or asthma, diet during pregnancy, and total energy intake of the child. No other links between dietary intake (including other types of fruit and vegetable) and wheeze or atopy were seen.

What interpretations did the researchers draw from these results?

The researchers conclude that intakes of ‘fruity vegetables’ and fish have a beneficial effect in reducing wheeze and atopy, an allergic reaction to allergens which aren’t in direct contact with that part of the body, respectively. They suggest that this may be in part explained by the high concentrations of antioxidants found in such vegetables; however the reason for the reduced risk with fish intake is uncertain.

What does the NHS Knowledge Service make of this study?

Although this study reports significant findings, these results may be due to chance. There is not enough evidence here to suggest that a poor diet is a cause of allergy and asthma, or that if you eat more ‘fruity vegetables’ and fish you are going to be in any way protected. Some important points to consider:

• This small cross sectional study is not a reliable study design for establishing whether any aspect of the diet leads to wheeze or atopy. This is because this type of study cannot establish whether the children had been eating these particular foods, before the onset of these conditions. The study can only give us a rough indication of the eating habits in these children at a single point in time.
• The study relied on the mother’s self-reporting of the child’s eating patterns over the past 12 months. This may have led to a lot of inaccuracies and differences in reporting. It relies upon accurate recall, and it is unlikely that exact portion sizes could have been accurately remembered for that period of time.
• We do not know anything about what the children were eating in the preceding 5 years of their lives.
• The researchers conducted many statistical tests which makes it more likely that they may find an association purely by chance, rather than there being a true link.
• The findings of increased vegetable and fish intake among those without these symptoms may be related to other factors, such as these children having a more healthy and active lifestyle in general.
• The study was carried out in Menorca, Spain; therefore we cannot so easily generalise the findings to other countries around the world where genetic risk, lifestyle and environmental risk factors may all be very different.

Asthma and allergies are fairly common in children and have multiple causes, including family history. Much further research is needed before any link between what we eat and risk of allergy or asthma can be made.

Sir Muir Gray adds…

The study does not say much about what the children ate less of, which may also explain the difference. Although we can become allergic to natural as well as manufactured substances, this evidence supports the common sense principle that foods directly from nature are more likely to be beneficial, and will help the children keep their body weight, cholesterol and blood pressure lower.

The message is clear, don’t tell the little blighters these foods are good for them and make them as tasty as possible.