Mediterranean diet ‘prevents’ asthma

Pregnant women who eat a Mediterranean diet could protect their unborn child against asthma and other allergies later on, according to The Sun and other newspapers. A diet “high in vegetables and fish leads to fewer allergies once children are born”, the newspaper added. The Daily Mail said that “eating red meat more than three or four times a week appeared to increase the risks”.

The newspaper stories are based on a study in 468 Spanish women and their children which concluded that a Mediterranean diet in pregnancy reduced the risk of wheeze in children aged 6½. However, the study did not look at children who had a clinical diagnosis of asthma. Also, it relied on reports on diet from the mother six and a half years after the pregnancy; it is unlikely that this would have been remembered accurately. Asthma and allergies are common in children and have many causes, including family history. The study uses some weak methods and the newspapers have overstated the relationship between a mother’s diet during pregnancy and asthma-like symptoms in her children.

Another publication used data from this study which focuses on the child’s diet and risk of wheeze, and Behind the Headlines has previously identified its shortcomings – Behind the headlines: Eating, asthma and allergies. Much more research is needed before any causal link between what children or their mothers eat and the risk of allergy or asthma can be established.

Where did the story come from?

Dr Leda Chatzi from the University of Crete and colleagues from other medical and academic institutes in Spain and Mexico carried out this research. The study was funded by Instituto de Salud Carlos III red de Grupos Infancia y Media Ambiente, the Fundacio ‘‘La Caixa’’, the Instituto de Salud Carlos III red de Centros de Investigacion en Epidemiologia y Salud Publica and an EU grant. The study was published in the peer-reviewed medical journal: Thorax .

What kind of scientific study was this?

The study was a small cohort study of 507 pregnant women, recruited between 1997 and 1998, and the children that were born, when they presented for antenatal care at general practices in Menorca, Spain. They were followed until their children were 6½-years-old. Four hundred and sixty-eight mother-child pairs that had complete data available at the end of the study were included in the analysis.

Each year, the parents were asked (through interview and questionnaire) about any medical events the child had experienced in the preceding 12 months. At the six and a half year follow up stage, the researchers determined whether the child had any asthma like symptoms (either currently or over the past 12 months or in preceding years) or allergies (using a skin prick test). Parents also filled in a food frequency questionnaire that would provide details of their child’s diet at 6½. A shorter food frequency questionnaire about the mother’s diet during her pregnancy was also completed. From these questionnaires, the researchers assigned scores to diet during pregnancy and the child’s diet that represented how carefully a Mediterranean diet was being followed (this was based on the intake of foodstuffs such as vegetables, legumes, fish, nuts, etc).

Details on education, socioeconomic class, marital status, maternal disease, child’s exposure to cigarettes, breastfeeding, use of supplements, child’s respiratory infections at one year old and other information were collected at a questionnaire during pregnancy and again at the end. Weight and height data were also collected from children at the six and a half year follow up. All of this information was used to adjust the analyses.

What were the results of the study?

The researchers found that the child’s diet at 6½ years had little effect on the risk of persistent wheeze (one or more episodes of “whistling or wheezing from the chest” in the past year and in any preceding year), current atopic wheeze (wheeze associated with allergies) or current allergies alone (based on skin prick test).

The children of women who had a high adherence to a Mediterranean diet during pregnancy were less likely to have persistent wheeze, atopic wheeze or atopy at 6½ when compared with children of mothers who had low adherence scores. The results took into account gender, maternal and paternal asthma, maternal social class and education, body mass index and total energy intake at age 6½.

What interpretations did the researchers draw from these results?

The researchers conclude that high adherence to a Mediterranean diet during pregnancy reduces the risk of wheeze and atopy in children at 6½.

What does the NHS Knowledge Service make of this study?

This cohort study provides some evidence that a mother’s Mediterranean diet during pregnancy reduces the risk of allergies and asthma-like symptoms in their children. However, the following should be considered alongside these results:

• Although the exact method is unclear, it seems that mother’s diet during pregnancy was only assessed six and a half years later at the same time that the study was assessing the child’s diet. It is unlikely that mothers remembered exactly what they had eaten during their pregnancy, particularly when information was being collected on such a large number of food items. The results could be inaccurate if there were errors in mothers recollections of their diet six and a half years ago. The fact that they determined child’s problems and mother’s diet during pregnancy at the same time means that this is essentially a cross-sectional analysis.
• When the researchers took into account how well the both the mothers and their children stuck to a Mediterranean diet, they found that the only significant effect was on the risk of persistent wheeze and this was only in mothers who stuck tightly to the diet, who had children who did not. Although the researchers report that there was also a reduction in risk of wheeze in mothers and children who both who stuck to the diet  this result was not statistically significant. There appeared to be no effect on risk of atopic wheeze when both mother and child’s diet were taken into account. 
• The definitions used for allergy symptoms of “persistent wheeze”, “atopic wheeze” (defined as “wheeze and atopy”) and “atopy” (based on skin prick response) are broad and unclear. It is not certain what criteria have been used to diagnose atopic asthma in particular, and whether this is a clinician’s diagnosis or not. This may introduce error to the associations. 
• It is not certain that the findings of a mother’s Mediterranean diet and a reduced likelihood of asthma may not be related to other confounding factors. These children and mothers may have a more healthy and active lifestyle in general, for example.
• The way that the researchers collected information on child’s diet and experience of wheeze or atopy cannot establish a causal relationship between the two. A cross sectional study (which this part of the study essentially is) cannot determine whether the children were eating these types of foods before the onset of the conditions), i.e. it cannot establish causation. 
• Behind the Headlines has identified the shortcomings of this study before. See Behind the headlines: Eating, asthma and allergies for this discussion.

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

Sir Muir Gray adds...

Olive oil? Wonderful, not just for pregnant women and it tastes terrific too.