Wednesday August 29 2012
Peanut allergies are one of the most common types of food allergies
The Daily Telegraph advises that ‘Eating nuts in pregnancy 'reduces chance of childhood allergy'.
The report is based on data collected as part of a larger study into the health and lifestyle of Danish women. Researchers asked more than 60,000 women halfway through their pregnancy about their diet, including information on how often they ate nuts.
The researchers then checked the health of the women’s babies after they gave birth, specifically looking at whether the child had been diagnosed with asthma by the time they were 18 months, or had symptoms of wheeze. This was followed by a second assessment taken when the child was 7 years old.
The main finding of the report was that maternal consumption of peanuts or tree nuts (as least once a week) was associated with a 20-25% decreased risk of the child being diagnosed with asthma at 18 months. There was no significant difference in risk when the children were 7 years old.
It is plausible that consumption of nuts during pregnancy would expose the developing baby to the compounds that are in nuts and so may decrease the likelihood that they would develop an allergy. Still, it is difficult to say why nut exposure should specifically influence the risk of asthma symptoms. There is also the possibility that the association could be due to other unmeasured factors, for example, women who eat nuts may have a general healthier lifestyle and diet.
Where did the story come from?
The study was carried out by researchers from the Harvard School of Public Health, Boston, and the Statens Serum Institut, Copenhagen, Denmark. The study received various sources of financial support, including the Danish Council for Strategic Research, and the Danish Council for Independent Research.
The study was published in the peer-reviewed Journal of Allergy and Clinical Immunology.
The media coverage is generally representative of this research.
What kind of research was this?
The Danish National Birth Cohort is a prospective cohort study examining the factors that influence the developing fetus during pregnancy, and how these may influence the early life and diseases of children. This particular study used information collected on nut consumption during pregnancy to see how this was associated with asthma diagnoses or asthma symptoms, such as wheezing.
The study has taken into account extensive possible confounding factors that could be associated with both maternal intake of nuts and a child’s risk of asthma. However, it is difficult to account for all possible factors that could be having an influence. Nut eating during pregnancy may reflect a general healthier lifestyle and diet, and women who follow a healthy lifestyle may instil such habits in their children, which may then decrease their risk of asthma.
What did the research involve?
Between 1996 and 2002, Danish women were enrolled to the cohort during their first antenatal visit. This study included 61,908 women who had a single baby and who had completed all questionnaires.
A 360-item food frequency questionnaire was given at around 25 weeks of pregnancy. This asked about snack consumption in the past month, separately assessing ‘peanut and pistachio’ intake and the intake of ‘nuts and almonds’ (the researchers assumed that most women consumed peanuts rather than pistachios in the former category). Four categories of consumption were generated:
- once a month
- one to three times a month
- one or more times a week
Women were questioned about childhood asthma when the child was 18 months and 7 years old.
At 18 months they were asked whether a diagnosis of childhood asthma had been confirmed by a doctor (doctor-diagnosed asthma), whether there were wheeze symptoms, and the number of wheeze episodes since birth.
At 7 years, asthma cases were defined as those who self-reported doctor-diagnosed asthma plus wheezing symptoms in the past 12 months. The presence of other allergies, such as hayfever, was also reported at 7 years. The researchers also had access to the Danish National Patient Registry, which collects data on admissions related to asthma, and the Register of Medicinal Product Statistics, which contains information on prescriptions.
The researchers then looked at the association between nut consumption and the development of asthma, wheezing, or other allergies.
When conducting their analyses they took into account numerous potential confounders including:
- parental education
- socioeconomic status
- dietary factors other than nut consumption
What were the basic results?
A total of 61% of women (37,323) reported no peanut and tree nut intake during pregnancy, 3% of women (1,639) consumed peanuts one or more times per week, and 9% consumed tree nuts one or more times per week.
The researchers found a general inverse relationship between peanut or tree nut consumption and asthma at 18 months.
- Compared to never consumption, consumption of peanuts one or more times per week was associated with a 21% reduced risk of doctor-diagnosed asthma at 18 months odds ratio 0.79, 95% Confidence interval (CI) 0.65 to 0.97).
- Compared to never consumption, consumption of tree nuts one or more times per week was associated with a 25% reduced risk of doctor-diagnosed asthma at 18 months (odds ratio 0.75, 95% CI, 0.67 to 0.84). However, there was no difference in risk of asthma diagnosis when the children were 7 years old.
- Compared with never consumption, children of mothers who ate peanuts one or more times per week were 34% less likely to have an asthma diagnosis recorded in the registry (odds ratio 0.66, 95% CI 0.44 to 0.98) and were 17% (borderline significance) less likely to have a prescription recorded for asthma medication (odds ratio 0.83, 95% CI 0.70 to 1.00).
- There was a trend for consumption of peanuts and tree nuts once monthly and two to three times monthly associated with significantly reduced risk of asthma compared to never consumption.
How did the researchers interpret the results?
The researchers conclude that their results ‘do not suggest that women should decrease peanut and tree nut intake during pregnancy’ and they say that consumption of peanuts and tree nuts during pregnancy ‘might even decrease the risk of allergic disease development in children’.
This study is well conducted and has:
- a large sample size
- taken into account extensive possible confounders that could be involved in the relationship
- asked for doctor-diagnosed asthma, rather than just parental and self-reported outcomes
- confirmed its findings using admissions related to asthma recorded in the Danish National Patient Registry, and prescriptions for asthma medication recorded in the Register of Medicinal Product Statistics
However, even though it has taken into account so many possible factors that could be having an influence on the association between nut-eating during pregnancy and child asthma, it is difficult to ensure that they have all been fully accounted for. Nut eating during pregnancy may reflect a general healthier lifestyle and diet, and such women may instil such habits in their children, which may decrease their risk of asthma.
It is difficult with a one-off food assessment to ensure that the responses are accurate and representative of longer term patterns. Also, with the four categories generated, ranging from no nuts to one or more times a week, it is difficult to gauge how many nuts were being eaten at a time (e.g. two or three, or a whole bag).
A further point of note is that, despite the large cohort size, 61% of the cohort reported no nut consumption at all during pregnancy, and the highest consumption category (one or more times a week) contained the fewest number of women. The calculations with the greatest statistical reliability are those involving the large sample sizes.
The idea of exposing an individual to low levels of an allergen in order to decrease their sensitivity to it is not a new one, and in fact this sort of therapy (immunotherapy) is already used in the treatment of certain allergies. Therefore, it is plausible that consumption of nuts during pregnancy would expose the developing baby to the compounds that are in nuts and so may decrease the likelihood that they would develop allergy as a child.
However, it is difficult to say why nut exposure should specifically influence risk of asthma symptoms.
Further research into this link is needed, but for now it perhaps best suggests that women (who do not have nut allergies themselves) do not need to stop eating nuts during pregnancy or decrease their consumption.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on twitter.