Monday November 12 2007
The link between diet and allergy was only seen for dairy products
Drinking organic milk or eating organic products such as yoghurt or cheese can protect children against eczema and asthma, reported the Daily Mail on November 9 2007. Children “raised on organic dairy products are a third less likely to suffer allergies in their first two years than those fed conventional food”, the newspaper said.
The stories are based on a study in pregnant women that examined the link between their child’s diet and any eczema or wheeze. The study is reasonable, but there are limitations to its interpretation, as no overall link between diet and allergy was seen, and it is difficult to separate out socio-economic factors that may also be effecting the results.
Where did the story come from?
Dr Ischa Kummeling and colleagues from Maastricht University and other academic centres in Holland conducted this research. There are no details about funding sources. It was published in the peer-reviewed British Journal of Nutrition.
What kind of scientific study was this?
The study is a prospective cohort study which enrolled pregnant women from two separate places. The majority of women (2834) were recruited through another study. They were all Dutch women who were 34 weeks pregnant, and the researchers called them the “conventional cohort”. A second smaller group of pregnant women (491) were recruited through midwives, under-five clinics, Steiner schools, posters and flyers in organic food shops. The researchers called this group the “alternative cohort”.
The researchers sent questionnaires to the mothers when their babies were three, seven, 12, and 24 months of age. These questionnaires assessed the health of the child and asked about symptoms of wheeze, eczema, etc. When the children were two years old, their organic food consumption was measured using a questionnaire that asked the parents what food their toddler ate and whether it was produced conventionally or organically. The researchers specifically asked about the consumption of seven different food groups (including vegetables, eggs, dairy products, meat, and bread) and calculated how much organic food was being consumed. Based on the percentage of organic food consumed, the children were assigned to one of the following dietary categories:
- conventional diet: in which less than 50% of food eaten is organic
- moderately organic: in which organic food is eaten 50-90% of the time
- strictly organic: in which organic food is consumed more than 90% of the time
A similar food questionnaire was given to the mothers before they gave birth (34 weeks gestation).
When the children were two years old, blood samples were taken (816 mothers agreed to this – 65% of the study) and antibodies in the blood that would indicate allergic responses to eggs, cows’ milk, peanuts, pollen, cats and dogs and house dust mites were measured. The researchers then assessed whether there was any relationship between dietary category and experience of eczema. They took into account other factors which might be increasing eczema risk, such as the child’s BMI, history of allergy in parents or siblings, breast feeding, pets, exposure to tobacco smoke etc.
What were the results of the study?
The researchers found that in the first two years of life, the majority of infants (2,308) consumed a conventional diet. Two hundred and eighty three children (10%) consumed a “moderately organic” diet and only 175 (six per cent) consumed a strictly organic diet. They found that an organic diet (moderate or strictly organic) had no effect on the risk of eczema or wheeze when compared with a conventional diet. There was also no protective effect of an organic diet on the development of an allergy to a particular substance over time (as determined through blood antibodies) – called sensitisation.
The researchers then analysed the effects of particular food groups. The only significant result they found was that a strictly organic consumption of dairy products (i.e. organic dairy products consumed more than 90% of the time) reduced risk of eczema compared with levels of consumption of conventional dairy products. There was no protective effect of any other food group or of organic dairy products on wheeze. They also did not find any difference in child’s eczema or wheeze between the “conventional” mothers and the “alternative” mothers.
What interpretations did the researchers draw from these results?
The researchers say that the consumption of organic dairy products in the context of an organic diet is associated with reduced risk of eczema. They call for “further studies to substantiate these results using more detailed and quantitative information”. Their caveats to the interpretation of their findings are based on some study weaknesses, which are discussed below.
What does the NHS Knowledge Service make of this study?
This was a reasonable study; however all the results of the study and some potential methodological limitations must be considered when interpreting the findings. Without these considerations, and in light of some of the inflated headlines, the results may be read out of context.
- There was no statistically significantly protective effect of organic intake in any other food category aside from dairy, or of a moderate or strictly organic diet overall. There was no evidence of any link to wheeze, and therefore the Daily Mail headline that ‘Organic milk, cheese and yoghurt “protects children against asthma”’ is a misinterpretation of the study.
- The researchers also say that symptoms of wheeze and eczema are quite non-specific and may have been misclassified by parents when they self-reported through the questionnaires. The definitions that the researchers used to classify children as having eczema or asthma are vague and this may also have led to error. For example, eczema was “an itchy rash that was coming and going in the past months”; rashes are very common in children and these may be allergic but are also commonly related to viral infections. Without any further clarification, it cannot be determined if all of these children actually had eczema. Likewise, asthma cannot be reliably diagnosed in a child from “recurrent” or “prolonged” wheeze. These non-specific definitions may have increased the numbers of children “diagnosed” with these conditions.
- The researchers highlight the following point: their study does not allow them to determine whether the lower risk of eczema in children using organic dairy products was “actually due to a high consumption of organic dairy products by the mother, conferring protection already starting in the intra-uterine period and during lactation”.
- When the researchers analysed the consumption of the different food groups, they were testing specific groupings of participants in the whole study. There are potential problems with such “subgroup analyses” and positive results are more likely to occur by chance. This finding needs to be corroborated by other studies.
- The study only asked mothers about food consumption during the second year of life. It therefore does not take into account dietary changes (in terms of organic content) that may have occurred prior to this time point. It also relies on mothers to recall how much of the various foodstuffs they had given their child over the course of the year. This may be subject to some recall problems.
The researchers highlight the fact that their study considered organic dairy products in the context of an organic diet and not as an isolated product within a conventional diet. They say that because of this, “it is uncertain whether these findings represents a true association, and should be interpreted with some caution until it can be confirmed”.
Sir Muir Gray adds...
Another association but is it causation? This study does not make it clear. Many peple prefer organic foods and it does not look as though that causes any problem for pregnant women and, in fact, it may do some good.