In this study we found some limited support for our primary hypothesis linking relatively low levels of maternal n-3 compared to n-6 PUFAs with later wheeze and atopy. Higher maternal EPA, DHA, and total n-3 PUFAs at 34 weeks' gestation were associated with a lower risk of persistent/late wheeze at age of 6 years. This association was only significant in nonatopic children, and there was no association between total n-3 PUFAs, total n-6 PUFAs or the ratio of n-3 to n-6 PUFAs, and childhood atopy as measured by skin sensitisation. However, maternal AA, a metabolic product of the essential n-6 precursor LA, was significantly positively associated with FENO, a marker of airways inflammation. Associations may reflect a direct action of intrauterine PUFA supply upon fetal development. Alternatively associations may occur as a consequence of dietary patterns or metabolic disturbance shared by mothers and their children. Little support was found for the secondary hypothesis, although the ratio of ALA to its unsaturated metabolic products was positively associated with persistent/late wheeze in nonatopic children and the ratio of LA to its unsaturated metabolic products was inversely associated with skin sensitisation at 6 years. However, these findings cannot easily be interpreted as evidence for decreased precursor conversion in atopic disease.
The inverse associations between total maternal EPA, DHA, and total n
-3 PUFAs and childhood persistent/late wheeze in the group as a whole, but particularly in those without atopy, are consistent with the idea that highly unsaturated n
-3 fatty acids might protect against childhood wheeze, as suggested by Black and Sharpe [10
]. However, only the associations with nonatopic persistent/late wheeze were significant, and no significant association was found between maternal n
-3 PUFAs and skin sensitisation or atopic persistent/late wheeze. There was no evidence that higher maternal n
-6 PUFAs increased the offspring's risk of wheeze or skin sensitisation. Whilst these results do not support competing effects of n
-3 and n
-6 PUFAs upon allergic skin sensitisation or the original Black and Sharp hypothesis [10
], the positive association between maternal AA concentration and FENO suggests that eosinophilic airways inflammation is higher in the children of mothers with high AA status.
A previous study found no association between maternal or umbilical cord AA and atopy, although the study included less than 300 mother-child pairs and did not consider objective measures of atopy such as skin sensitization [38
]. The weak inverse association between AA and persistent/late wheeze in nonatopic children found in the current study, however, contradicts the hypothesised association between high levels of n
-6 PUFAs and asthma. Although this finding was unexpected and may have occurred by chance, inverse associations have previously been found between AA and risk of atopy in adults [24
] and in infants [20
]. This finding is also comparable to an inverse association found previously between the maternal ratio of n
-6 to n
-3 long-chain PUFAs and childhood eczema [25
The ratio of ALA to its unsaturated metabolic products was significantly positively associated with nonatopic persistent/late wheeze and weakly with transient wheeze. These results are compatible with the suggestion of altered fatty acid metabolism in mothers of children who later develop allergic disease [21
], and also with genetic data linking polymorphisms in the gene clusters coding for fatty acid desaturases and the prevalence of allergic rhinitis and atopic eczema [40
]. However, given that no associations were found between this ratio and skin sensitisation or persistent/late wheeze with atopy, there was no evidence that inefficiency in ALA metabolism is associated with childhood wheeze as a consequence of allergic sensitisation. However, the inverse association between the ratio of LA to its unsaturated metabolic products and skin sensitisation might indicate altered LA metabolism in the mothers of children who later develop atopy compared to mothers of nonatopic children. The direction of this association does not appear to support a link between decreased fatty acid desaturase activity and childhood atopy, although definitive assertions about flux through pathways cannot be made without stable isotope studies.
The current study has some limitations. Although a priori hypotheses were examined, due to the number of associations tested, unwarranted biological significance may be attributed to false positive results. Response bias cannot be excluded as the entire original cohort was not followed up; this would only invalidate the study's conclusions, however, if the relationship between maternal PUFA status and childhood outcomes differed between study participants and those with missing data. A broad range of potential confounders were considered but residual confounding cannot be excluded. We were unable to correct for maternal PUFA status earlier in pregnancy, for breast milk fatty acid composition, or for the children's PUFA intake and status in postnatal life, which are all likely to be factors contributing to immune development and allergy risk in childhood. It is also unclear whether sufficient variation in exposure exists within this population to meaningfully affect outcomes; less than 1% of the mothers in this study reported an intake of at least one fatty fish meal per week, the intake providing the n
-3 PUFA dose administered in a successful fish oil intervention trial [15
]. Epidemiological studies are further complicated by the multiple asthma phenotypes thought to exist, as these may show different patterns of association with early life influences. Heterogeneity within study outcomes is likely, and this may decrease the study's sensitivity. A final limitation is that we measured the fatty acid composition of maternal plasma PC, the major phospholipid in the bloodstream. Although this has the advantage that plasma PC fatty acids are influenced by dietary intakes and that the context of the Black and Sharp hypothesis is an effect of maternal diet on later risk of allergy and asthma [10
], plasma phospholipids are not the principal vehicle for supply of fatty acids from the maternal circulation to the fetus [41
]. For this reason, the fatty acid composition of maternal plasma triacylglycerols and nonesterified fatty acids would also be useful to measure, although the PUFAs in the former correlate strongly with those in plasma PC, as would the fatty acid composition of cord blood plasma lipids.
In summary, measurement of maternal PUFAs in a large population-based cohort enabled us to investigate hypotheses linking PUFA exposure during late-pregnancy to childhood wheeze and atopy. An inverse association was found for maternal EPA, DHA, and total n-3 PUFAs and the relative risk of persistent/late nonatopic wheeze whilst high maternal AA was associated with increased FENO, a marker of airways inflammation. Together with a number of weaker associations, albeit with small effect sizes, these results provide some support for a protective link between the maternal n-3 PUFAs and wheeze and atopy in childhood. Support for the suggested role of n-6 PUFAs was minimal.