This study investigated associations between maternal intake of different food groups during pregnancy and symptoms of asthma and atopy in children. There was no evidence of associations between asthma, respiratory or atopic outcomes in 5‐year‐old children and maternal intakes of total fruit, citrus/kiwi fruit, total vegetables, green leafy vegetables, fruit juice, whole grain products, fat from dairy products or butter versus margarine/low fat spread use. However, we have shown beneficial associations between maternal apple intake and childhood wheeze and asthma, and between maternal fish intake and childhood eczema and hay fever. There are some reports of beneficial effects of maternal fish consumption and maternal fish oil supplementation during pregnancy on childhood asthma and neonatal cord blood mononuclear cell responses20,21
but, to our knowledge, our finding of the protective effects of maternal apple consumption during pregnancy on childhood wheeze and asthma is new.
In this cohort we have reported beneficial associations between maternal vitamin E intake during pregnancy and cord blood mononuclear cell responses at birth,3
wheeze at age 2 years4
and wheeze and asthma at age 5 years.2
We have also found beneficial associations between maternal zinc intake during pregnancy and asthma and eczema in children at the age of 5 years, and between maternal vitamin D intake and wheeze in children at age 5 years.2,5
One of the aims of this study was to investigate whether these associations with maternal nutrient intakes could be a consequence of associations with individual foods rich in one or several of these nutrients, with obvious implications for a potential dietary intervention during pregnancy. It would seem that the associations reported here with maternal intake of apples and fish are insufficient to account for the associations with vitamin E, vitamin D and zinc because, in the UK, apples and fish provide less than 10% of dietary vitamin E and zinc intakes in women of this age group. In addition, the pattern of associations between vitamin E, vitamin D, zinc and childhood respiratory and allergic outcomes differed from those in the present study. In the UK there is no single major dietary source of vitamin E in women aged 25–49 years, with intake being evenly distributed between fat spreads (15%), cereals/cereal products (10%), potatoes/potato snacks (12–14%), vegetables (16–17%) and meat/meat products (10%).22
In the present study the absence of any association between the usual dietary sources of vitamin E and respiratory outcomes suggests that the associations with vitamin E in a previous report2
were unlikely to represent associations with other nutrients commonly found in foods containing vitamin E.
The present study suggests beneficial associations between maternal apple intake during pregnancy and wheeze and asthma at age 5 years. The evidence from other observational studies on children's diet and respiratory and atopic symptoms is relatively consistent, showing beneficial effects of fruit and vegetable intake on indicators of asthma.8,9,10,11
However, it is not clear whether these effects can be attributed to specific nutrients or that a high intake of fruit and vegetables is an indicator of a healthier lifestyle. The specific association found with apples in this study—and not with total fruit, citrus, fruit juice or vegetable consumption—suggests an effect specific to apples, possibly because of their phytochemical content such as flavonoids. Flavonoids are polyphenolic compounds with powerful antioxidant capacities and are associated with reduced risks of several diseases including asthma and chronic obstructive pulmonary disease.23,24,25
Intake of apples as a significant source of flavonoids and other polyphenols has been beneficially associated with asthma, bronchial hypersensitivity and lung function in adults.24,26,27,28
These effects are usually ascribed to the strong antioxidant capacities of apples, although there is also evidence that some polyphenolic compounds can influence cytokine gene expression by Th cells, promoting the secretion of the Th1 cytokine interferon γ and inhibiting secretion of the Th2 cytokine interleukin‐4.29
However, there is a lack of epidemiological evidence on the relation between the intake of flavonoids or specific flavonoid‐rich foods and asthma or allergy in children. Although the consumption of total fresh fruit has increased in recent years, apple consumption in the UK fell from 207 g/person/day in 1974 to 173 g/person/day in 2004/5.30
It has also been suggested that the mineral content of fruit and vegetables declined between 1940 and 1991.31
This could be the consequence of changes in cultivation, the use of fertilisers and the choice of fruit species that can be more easily harvested or stored.
The observation of beneficial associations between maternal total and oily fish consumption and current eczema and ever hay fever at age 5 years, respectively, is consistent with earlier observations.20,21
Dunstan et al21
examined the effect of fish oil supplementation during pregnancy on early developing immune responses and clinical outcomes in infants predisposed to allergic disease. Neonates born to mothers supplemented with fish oil tended to have lower cord blood mononuclear cell cytokine responses to allergens and, at 1 year of age, significantly less severe disease if they had atopic dermatitis. Salam et al20
studied the association between maternal fish consumption during pregnancy and childhood asthma. They found that maternal oily fish consumption at least monthly was significantly protective for persistent asthma in 5‐year‐old children. Other epidemiological evidence on the effect of fish intake or fish oil supplementation on asthma or allergic diseases provided by observational and intervention studies in children is inconsistent.32,33
It is therefore more likely that the time window for n‐3 polyunsaturated fatty acids to have an effect on immune regulation and subsequent asthma and atopic disease is indeed in fetal life, and that effects are limited once allergic immune responses are established.21
Originally, the study population of 2000 pregnant women was demographically very similar to the local obstetric population.4
In this study there was some evidence of response bias due to the loss to follow‐up with time. Participating mothers were of higher socioeconomic status and had slightly higher consumption of fruit, green leafy vegetables, whole grain products and fish and had fewer respiratory symptoms.2
An analysis of the wheezing symptoms of the children whose mothers responded at 2 years but not at 5 years indicated that the children with no data at 5 years were more likely to have wheezed at 2 years (not shown). This type of response bias often plays a role in cohort studies because it is known that subjects with poorer socioeconomic status and lifestyle (lower educational level, poorer diet, smoking, etc) are more difficult to trace, and that people who suffer poor health during the follow‐up period are prone to attrition.34
However, due to this type of bias, it is more likely that the observed associations in this study are underestimated than overestimated; for instance, improved ascertainment at 5 years would have resulted in a larger proportion of wheezy children with low maternal apple consumption, which would make the observed associations between maternal apple consumption and childhood asthma/wheezing symptoms stronger (in this case odds ratios closer to zero). A limitation of FFQ‐derived estimates is that they are susceptible to dietary misreporting which leads to dietary misclassification of intake and/or portion sizes. Usually this misclassification is random and it also weakens rather than augments the associations. To avoid multiple hypothesis testing we chose a restricted number of food groups based on our previous findings in this cohort and earlier reported associations,13,14,27
the lipid hypothesis35
and antioxidant hypothesis.36
It is possible that the associations reported could be a consequence of the number of analyses performed. However, we consider this is unlikely because some of the associations were highly significant and the associations were clustered with food groups that have previously been associated with similar outcomes in children and adults.
The predominance of associations between maternal food intakes and doctor‐diagnosed outcomes raises the possibility of ascertainment bias, whereby mothers more conscious of health issues were both more likely to follow dietary advice to eat healthily and more likely to take their unwell children to the doctor to receive a formal diagnosis. Such ascertainment bias seems unlikely because it predicts that maternal apple consumption should be adversely associated with childhood doctor‐diagnosed conditions—the opposite to what we report.
The observed associations with maternal food intake during pregnancy were independent of the childhood diet because inclusion of children's apple and fish consumption in the models did not change the results, despite maternal and childhood diet being weakly correlated.
Published cross‐sectional surveys of children have reported associations between the dietary intake of citrus, kiwi fruit and vegetables and indicators of asthma, 8,9,10
but the present study failed to show any consistent associations between the food intake of children aged 5 years and respiratory and atopic symptoms. The children in the present study were younger than those participating in previous studies (6–11 years), and this may account for the disparity between this and previous studies. In the present study it would appear that, until at least the age of 5 years, maternal diet during pregnancy is more influential on respiratory health than childhood diet. Further follow‐up of this birth cohort will be required to determine whether the associations with maternal diet decline in older children, and whether maternal and childhood diets interact in older children.
The associations between maternal apple consumption and asthma and symptoms could represent effects on airway and immune development, while the associations between maternal (oily) fish consumption and eczema and hay fever suggest effects on Th cell differentiation,37
yet no associations were found with lung function measures, exhaled nitric oxide and atopic sensitisation. This could reflect a loss of power due to the smaller number of children who underwent spirometric or skin prick tests.
The results of this cohort study indicate that there were no consistent linear associations between maternal intake of total fruit, citrus/kiwi fruit, total vegetables, green leafy vegetables, fruit juice, whole grain products, fat from dairy products or butter versus margarine/low fat spread use during pregnancy and asthma, respiratory and atopic outcomes in 5‐year‐old children. We did, however, find some evidence for protective effects of maternal apple and fish consumption. Thus, in addition to maternal intake of vitamin E, vitamin D and zinc during pregnancy,2,3,4,5
maternal consumption of apples and fish during pregnancy may reduce the risk of children developing asthma or atopic disease. If these results are confirmed, recommendations on dietary modification during pregnancy may help to prevent childhood asthma and allergy.