Food and diet

Nut products and asthma

“Eating too many nuts in pregnancy could put babies at risk of asthma”, The Guardian reports. A study has followed more than 4,000 expectant mothers and their children over eight years, and found that children who developed asthma-like symptoms such as wheezing “were significantly more likely to be born to mothers who ate nut products, such as peanut butter, each day of their pregnancy, than women who ate nuts only rarely”, the newspaper says.

With asthma being highly prevalent among children in the UK, this research will be of interest to the medical community and the population in general. Links were found between asthma risk and daily consumption of nut products, but the study has some limitations. Therefore it cannot state conclusively that eating nuts during pregnancy causes asthma. As the researchers say, findings need to be replicated in other studies before advice can be given to pregnant women. While it is premature to advise pregnant women to avoid nuts altogether, recommending a healthy balanced diet and avoiding excessive consumption of one particular food is sensible.

Where did the story come from?

Saskia M Willers of the Institute for Risk Assessment Sciences, Utrecht University, The Netherlands, and colleagues of the National Institute for Public Health and the Environment, University Medical Centre Utrecht, University Medical Centre Groningen, Wilhemina Children’s Hospital and Erasmus Medical Centre, The Netherlands, carried out this research. The study was published in the peer-reviewed medical journal: American Journal of Respiratory Critical Care Medicine . Funding was provided by the Netherlands Organisation for Health Research and Development; the Netherlands Ministry of Spatial Planning, Housing and the Environment; Netherlands Asthma Fund; Netherlands Organisation for Scientific Research; and the Netherlands Ministry of Health, Welfare and Sport.

What kind of scientific study was this?

This was a cohort study in which the researchers investigated the role of maternal food consumption during pregnancy on childhood asthma.

This study was called The Prevention and Incidence of Asthma and Mite Allergy (PIAMA). In 1996, 10,232 pregnant women completed a screening questionnaire during a routine clinic assessment. Of these, 4,146 women agreed to participate in the study. During their 30th to 36th week of pregnancy, women were asked about the frequency of their consumption of various food substances (e.g. fruit, vegetables, fish, milk, eggs, nuts), and they chose from responses ranging from “never” through to “several times per day”. The responses were then categorised into three broad groups of “rarely”, “regularly” and “daily”.

Children were followed up at three months of age and then annually from years one to eight. Questionnaires were administered to parents, asking about the child’s diet and how frequently they had consumed various food substances over the past month. Other validated questionnaires were also given, asking about asthma, rhinitis (runny nose/cold symptoms) and eczema. In particular, the researchers were interested in wheezing, shortness of breath, prescription of inhaled steroids for asthma, and general “asthma symptoms” – parental report of one or more attacks of wheezing, shortness of breath or prescription of inhaled steroids in the past 12 months. They also looked at whether the child had had a cold in conjunction with asthma symptoms and “doctor-diagnosed asthma” in the past 12 months. Blood levels of the antibody IgE, which is involved in hypersensitivity allergic reactions, were specifically examined in a group of children.

Statistical methods were used to examine associations between maternal diet during pregnancy and various asthma symptoms in the child up to age eight. They compared women with rare consumption of certain foods with those with daily or regular consumption. Other factors that may be linked to diet or asthma were taken into account in the analysis. These included: child sex, birth weight, maternal smoking during pregnancy, smoking in the child’s home, maternal education, breastfeeding, parental allergy, older siblings in the home, maternal weight, and supplement use during pregnancy.

What were the results of the study?

Data was collected for 95.6% of the sample of women. By final assessment at eight years 80% of their children were assessed. Compared with those who dropped out or who had missing assessments, participants with complete data were more likely to have favourable food intake during pregnancy, to have breastfed, to have better educational levels, and were less likely to have lived in a smoking household or to suffer from allergies themselves. During pregnancy, 76.3% ate fruit daily, 55.9% ate vegetables daily and 84.2% consumed dairy products daily. The food substances ate least frequently among pregnant women were fish (daily 0.0%; rarely 74.4%), eggs (daily 0.2%; rarely 32.7%), nuts (daily 1.4 %; rarely 65.3%), and nut products (daily 6.1 %; rarely 55.9%).

In the children, the prevalence of any “asthma symptoms” over the past 12 months declined over the eight years from 23% at three years to 13% at eight years. In children with complete data, 61.3% had never wheezed, 24.0% were early transient wheezers, 4.3% were late onset wheezers, and 10.3% had persistent wheezing. When they looked at relationships between asthma and maternal diet, they found no associations between maternal consumption of vegetables, fruit, fish, eggs, dairy or nuts (or nut products) and childhood wheeze or other symptoms of asthma. However, researchers split the data when analysing nut products. They found no links between nut product consumption and asthma when they compared regular with rare maternal consumption. But they found significantly increased risk of wheezing (by 42%), shortness of breath (by 58%), steroid use (by 62%) and asthma symptoms (all three combined; 47%) when they compared daily versus rare consumption.

What interpretations did the researchers draw from these results?

The researchers found no associations between child asthma risk up to age eight and their mother’s consumption of a range of food substances during pregnancy (with the exception of daily compared with rare consumption of nut products). They say that findings need to be replicated in further studies before advice can be given to pregnant women.

What does the NHS Knowledge Service make of this study?

This is a well-conducted study which has followed a large number of children and their parents throughout early childhood. However, definite conclusions cannot be drawn on the effects of eating nut products during pregnancy. A few points to consider:

  • The news headlines that asthma risk is increased with nut consumption during pregnancy are inaccurate. There were no significant links found between nut consumption and child asthma, only the wider category of "nut products" (which included products such as peanut butter).
  • The increased risk of asthma symptoms in the child were only found when comparing daily consumption with rare nut product consumption, but not with regular versus rare consumption. The number of women eating daily nut products was relatively small (243) compared with regular (1,452) and rare consumption (2,216), which makes it more difficult to give accurate risk estimations than if sample sizes were larger in the daily category. 
  • The individual asthma symptoms examined do not represent a definitive diagnosis of asthma. If food associations had been examined for “doctor-diagnosed asthma", for example, results may have appeared very different.
  • Although many potential confounders for asthma have been considered, the condition has a wide number of potential causes, such as environmental factors and infections, which have not been considered. 
  • Frequency of food intake has been considered but portion size has not. This could vary considerably from one person to another.
  • Many children develop asthma symptoms when they are young, but the prevalence decreases with age, and it is not possible to tell how many cases with symptoms would persist into later adolescence and adulthood.
  • The potential mechanism of how maternal intake of nut products such as peanut butter may affect the developing foetus are unclear.

As the researchers say, these findings need to be replicated in other studies before advice can be given to pregnant women. At the current time, it may be best to advise expectant mothers that, although peanuts are not one of the food substances that need to be avoided during pregnancy, eating a healthy balanced diet is the ideal. Excessive consumption of one food in particular should be avoided where possible.

NHS Attribution