Justification: fish is the main source of n-3 LCPUFAs. Multiple clinical trials have been performed to assess the effect of supplementation with n-3 LCPUFAs in pregnant women on gestation and the offspring. When it comes to its association with asthma prevention, research has found variable results.1,2
Validity or scientific rigour: when it came to internal validity, the clinical question was clearly defined. Random allocation was performed with a concealed sequence and a full followup was carried out. Participants and researchers were both blinded. There were some limitations, such as an adherence to treatment of less than 80% in 29% of participants, or the presence of various co-interventions, such as the administration of vitamin D. Although the results remained unchanged in the adjusted analysis, the stratified analysis showed that the effect of n-3 LCPUFAs was only observed when the control group did not receive vitamin D supplementation. Considering that vitamin D is used for asthma prevention and control,3 it cannot be ruled out that the protective effect of co-intervention (vitamin D) affected the results. The Bayesian model found factors with a stronger association with asthma development, such as maternal smoking and preterm birth, than supplementation with n-3 LCPUFAs.
The main limitation in its external validity is that the sample consisted of Caucasian women with a daily intake of fish that exceeded the consumption reported in other countries, with a high prevalence of self-reported asthma, eczema or allergic rhinitis (30%, 21% and 38%) and of high socioeconomic status.
Clinical relevance: this study supports the protective effect of n-3 LCPUFAs supplementation against the development of asthma in children, with a number needed to treat (NNT) of 14.6 and a larger effect size in pregnant women with lower baseline levels of EPA and DHA. In a randomised controlled trial (RCT) published by Olsen in 2008,1 the HR of the group that received a fish oil supplement also suggested a protective effect against allergic asthma (HR, 0.13; 95 CI, 0.03 to 0.60). When it came to fish intake, there was a significant association between low intake and asthma and atopic dermatitis (HR, 0.10; 95 CI, 0.01 to 0.87). A 2015 Cochrane review2 did not find differences in the development of asthma or wheezing in children aged more than 36 months based on supplementation with n-3 LCPUFAs or fish oil during pregnancy, breastfeeding or both (relative risk [RR], 1.10; 95 CI, 0.34 to 3.58). In terms of safety, there was no increase in maternal postpartum haemorrhage nor a higher incidence of fever or infection in children. Best et al4 reviewed observational and experimental studies with inconsistent results. In the former, low fish intake was associated with asthma development, while the rest did not find an association between low fish intake or supplementation with n-3 LCPUFAs and the development of asthma or wheezing. A RCT conducted by the same author5 that has been reviewed in this journal6 did not find differences in the risk of developing asthma or IgE-mediated wheezing in the six-year followup.
Applicability to clinical practice: the findings of this study do not suffice to recommend supplementation with n-3 LCPUFAs during pregnancy for the purpose of preventing asthma. To reach more conclusive findings, additional well-designed RCTs need to be conducted to confirm whether this protective effect does in fact exist, and assess potential differences between subgroups with a lower intake of fish.
Conflicts of interest: the authors of the commentary have no conflicts of interest to declare.