Does fish oil during pregnancy help prevent asthma in kids?

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Study adds fuel to the fire

This study strengthens the case for fish oil supplementation during pregnancy to reduce the risk of asthma in offspring, despite the recent Cochrane review that showed no benefit.1,7 The Palmer study used a much lower amount of omega-3s (900 mg/d fish oil vs 2400 mg/d in the current trial).1,8 Olsen et al supplemented with a greater amount of omega-3s (2700 mg/d) and did find a benefit.9 The NNT from the Olsen study (19.6) is consistent with that of the current investigation, suggesting that a higher dosage may be necessary to prevent the onset of asthma.

This study strengthens the case for fish oil supplementation during pregnancy to reduce the risk of asthma in children.

Additionally, this study followed children for a longer period than did the Palmer study, which may have led to more accurate diagnoses of asthma.1,8 Lastly, the diagnosis of asthma in the Palmer study was based on parent survey data and use of daily asthma medicine rather than on daily diary cards, which are often more accurate.

Consider fish consumption. Both this study and the Olsen trial were performed in Denmark.1,9 While Denmark and the United States have had a relatively similar level of fish consumption since the 1990s, women in Denmark may eat a higher proportion of oily fish than women in the United States, given the more common inclusion of mackerel and herring in their diet.10 Thus, the effect of supplementation may be more pronounced in women in the United States.


Questions remain: Ideal dose and which women to treat?

The US Food and Drug Administration currently recommends 8 to 12 ounces of fish per week for pregnant women, but there are no guidelines on the ideal amount of fish oil to be consumed.11 The Palmer study,8 using 900 mg/d fish oil, did not show a benefit, whereas there did appear to be benefit in this study (2400 mg/d)1 and the Olsen study (2700 mg/d).9 Further research is needed to determine the optimal dosage.

Only women whose blood levels of EPA and DHA are low to begin with will likely benefit from this intervention.

The decreased risk of persistent wheeze or asthma was seen only in the children of the women whose EPA and DHA blood levels were in the lowest third of the study population. Thus, only women whose blood levels are low to begin with will likely benefit from this intervention. Currently, EPA and DHA levels are not routinely checked, but there may be some benefit to doing so.

One proxy for blood levels is maternal intake of fish at baseline. The investigators found that there was an association between dietary intake of fish and blood levels of EPA and DHA (r=0.32; P<.001).1 Therefore, additional screening questions to determine fish consumption would be useful for identifying women most likely to benefit from supplementation.


Multiple pills and additional cost

Since omega-3 fatty acids are relatively safe and the NNT in the general population is low, it may be worth supplementing all pregnant women, even without a commercially-available blood test for EPA or DHA. Nevertheless, some women may find it challenging to take up to an additional 4 pills/d for 13 or more weeks. Also, there is an associated cost with these supplements, although it is low.


The PURLs Surveillance System was supported in part by Grant Number UL1RR024999 from the National Center For Research Resources, a Clinical Translational Science Award to the University of Chicago. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Center For Research Resources or the National Institutes of Health.

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