"Fish oil supplements and probiotic yoghurts during pregnancy may decrease children's risk of developing allergies," reports The Independent.
Allergies – such as asthma, eczema and food allergies – have become common in the UK. Previous research has suggested that women's diets while pregnant and breastfeeding, and how long they breastfeed for, may affect the child's chances of developing allergies.
A new review looked at archives dating back to 1946 to get an overview of the research in this area. There were two notable findings.
Taking probiotic supplements, which contain so-called "healthy bacteria", may reduce the chances of children getting eczema by 22% – however, it's unclear whether the possible benefit came from women taking the supplement during pregnancy or breastfeeding, or from infants taking supplemented formula.
Taking fish oil supplements during pregnancy and breastfeeding may reduce the chance of children becoming sensitised to egg (a sign of a potential allergy) by 31% – it may possibly also reduce the chances of peanut allergy, but there was less evidence for this.
There was also some evidence that breastfeeding may reduce the risk of eczema and that probiotics may reduce the risk of becoming allergic to cows' milk, but these findings were based on lower-quality evidence.
The results may be used to inform future guidance around what to eat when pregnant or breastfeeding, or what to feed infants.
Fish oil with omega-3 supplements is considered safe in pregnancy, but mums-to-be should avoid taking any supplements that contain fish liver, such as cod liver oil.
There are no known risks from taking probiotics during pregnancy.
The study was carried out by researchers from Imperial College London, the University of Oxford and the University of Nottingham. It was funded by the UK Food Standards Agency and published in the peer-reviewed journal PLOS Medicine on an open-access basis, so it's free to read online.
The study was covered widely in the UK media, with the focus mainly on the fish oil findings. The reporting was generally accurate, although the evidence would appear to be stronger for probiotics than fish oils.
This study was a systematic review and meta-analysis. It included randomised controlled trials of interventions such as supplements, and observational studies of behaviour such as breastfeeding and general diet to see whether there were any links with children's allergies.
These types of study are the best way to get a good overview of the state of research on a topic, and a meta-analysis can be a useful way of pooling results from many different studies. However, the overall findings are only as reliable as the underlying studies.
Researchers searched for studies that looked at the effects of milk feeding (including breastfeeding) and diet of mothers and babies on children's allergies. They included observational studies from 1965 until July 2013 and interventional studies from 1965 until December 2017. The randomised controlled trials and observational studies were analysed separately.
They pooled figures from similar studies to calculate how interventions such as food supplements, or behaviour such as breastfeeding and general diet, affected the chances of children getting any type of allergy.
They checked the studies for potential bias and looked to see whether the pattern of results suggested that some studies with negative findings had not been published.
The researchers analysed 433 studies with a total of 1,506,815 participants – 260 of these studies covered milk feeding and 173 covered other maternal or infant diets.
Children who had been exposed to probiotic supplements, either directly through supplemented formula or via their mother's diet when pregnant or breastfeeding, were 22% less likely to get eczema, based on 19 trials (relative risk [RR] 0.78, 95% confidence interval [CI] 0.68 to 0.9). The researchers were moderately certain about these results, which equate to about 44 fewer cases per 1,000 children. It's unclear whether the trials mostly looked at supplementation during pregnancy and breastfeeding, or supplementation of the infant's diet.
Children born to women who took fish oil supplements during pregnancy and breastfeeding were 31% less likely to show a sensitivity to egg at age 1, based on 6 trials (RR 0.69, 95% CI 0.53 to 0.9). The researchers were moderately certain about these results, which equate to about 31 fewer cases per 1,000 children. These children were also 38% less likely to show a sensitivity to peanuts, but this was based on only 2 trials (RR 0.62, 95% CI 0.4 to 0.96).
Breastfeeding for longer was associated with a lower risk of the child getting recurrent wheeze (a sign of asthma), but the researchers said they had low certainty about these results, partly because these were observational studies that didn't completely take account of potential confounders.
Avoiding certain foods while pregnant or breastfeeding did not seem to reduce the risk of allergy. The researchers also found no convincing results for other types of supplements or for any specific type of diet, such as eating more vegetables.
They said tests of their results showed more certainty for the probiotic supplements than the fish oil supplements.
The researchers said they "found a relationship between maternal diet during pregnancy and lactation [breastfeeding] and eczema or allergic sensitisation to food during childhood" and that their findings "suggest that current infant-feeding guidance needs revision".
Because allergies are so common among children and can have a major effect on their lives, anything that helps us understand how to reduce the risk is very welcome. This study suggests certain aspects of women's diets during pregnancy and while breastfeeding, as well as infant-feeding practices, may have an effect on the development of allergies in children.
However, plenty of questions remain. The study doesn't clearly tell us which probiotic supplements were taken in studies, at what dose or by whom. There isn't enough clear evidence for us to know whether pregnant women, infants or both may benefit from taking supplements. That means recommendations can't be made from this study.
Also, while lots of people eat probiotic yoghurts, we don't know if these contain sufficient probiotic bacteria to be helpful or whether they are the right strains of probiotics.
Moreover, while fish oil supplements during pregnancy or breastfeeding were linked to a lowered chance of egg sensitisation when children were tested, that isn't the same as food allergy. Studies use egg-sensitisation tests to assess the risk of food allergy, but sensitisation does not necessarily mean an allergy will develop. We need to see longer-term studies that look at the effects of supplements on real-world food allergies.
There were some further limitations.
Many of the studies looking at the effects of diet in pregnancy differed in the way they were carried out and reported.
Study results were inconclusive or inconsistent, meaning the researchers couldn't be sure of any harms or benefits.
The 2013 cut-off for observational studies meant recent studies may have been missed.
The study didn't look at children's diet beyond age 1, which might have an effect on allergies.
We will need to await any future updates to guidance or policy around diet or supplementation during pregnancy and breastfeeding, or feeding infants. Find out more about allergies and how to manage them.
If you're pregnant, it's important to avoid any supplement, such as cod liver oil, that contains high levels of the retinol form of vitamin A. High doses of retinol can damage your baby.