Fish oils and asthma

Thursday July 10 2008

“Fish oil supplements may reduce risk of asthma” reported The Daily Telegraph , saying that children born to mothers who took fish oil in the later stages of their pregnancy were about 60% less likely to develop asthma than other children.

The trial behind this story gave women fish oil supplements during their third trimester and compared the effects on their child’s health with olive oil capsules or no supplements. It found small numbers of children with asthma overall. These small numbers mean that the results -suggesting a protective effect of fish oils-  may have occurred by chance. Larger trials are needed to determine the real effects of fish oils on asthma in offspring.

Where did the story come from?

Dr Sjurdur Olsen and colleagues from Statens Serum Institut in Denmark, Harvard School of Public Health, Aarhus University Hospital in Denmark, the University of Aarhus and the University of Copenhagen carried out this study. The research was funded by the European Union FP6 consortium, Early Nutrition Programming Project, the Danish Strategic Research Council, the Lundbeck Foundation and the Danish Medical Research Council. The study was published in the peer-reviewed medical journal: the American Journal of Clinical Nutrition .

What kind of scientific study was this?

This publication was of long-term follow-up data of women who were enrolled in a randomised controlled trial in 1990 to investigate the effects of fish oil supplementation on various outcomes. Pregnant women attending the main midwife clinic in Aarhus in Denmark between November 1989 and July 1990 for assessment at their 30th week were invited to participate. Those with placental abruption in a previous pregnancy, or with serious bleeding in the current pregnancy, were excluded. Also excluded were women with multiple pregnancies, allergies to fish, regular use of fish oil or drugs that might inhibit the action of fish oils. The 533 women who agreed to participate were interviewed about their lifestyle factors and were given a food frequency questionnaire to determine the intake of fish in their diet (high, medium and low intake). Women were then randomised to one of three groups. The first received four daily fish oil capsules (Pikasol fish oil), the second received identical-looking capsules containing olive oil, while the third received no supplement at all.

All citizens of Denmark have a unique identification number, which links them to their children. The researchers linked these numbers to the national hospital discharge registry (which records diagnoses from hopitalisations, GP or specialist visits and emergency admissions in Denmark). Using the records, the researchers recorded any diagnosis of asthma (different types), allergic rhinitis (allergies) and eczema in the children.

The main purpose of the study was to compare diagnoses of asthma (any type) between children of mothers who took fish oil compared with children of mothers who took olive oil. They also examined the rates of asthma in children of mothers who took no supplements.

What were the results of the study?

Of the 533 mothers (and children) randomised in the study, 522 were still alive and could be identified through the databases in August 2006. Eight children (of 263) in the fish oil group developed asthma compared with 11 children (of 136) in the olive oil group. This meant that compared with children of mothers who took olive oil, the children of mothers taking fish oil supplements were about 60% less likely to have asthma 16 years after the study began.

When the researchers divided the women according to how much fish they ate, they found the risk-reducing effect was greatest in women with low dietary fish intake (though this was only just statistically significant). The effect was less strong in women with high fish intake, and weakest in women with medium fish intake. There was no effect on asthma rates in children when the women given fish oil supplements were compared with those given no supplements. Children from the no-supplement group performed better (i.e. had fewer cases of asthma, eczema or allergic rhinitis) than children from those given olive oil.

What interpretations did the researchers draw from these results?

The researchers conclude that their results suggest that increased intake of fish oils in the third trimester “may confer protection against asthma in the offspring”. They say that “clearly there is a need for both large randomised controlled trials with long follow-ups…to examine this further”.

What does the NHS Knowledge Service make of this study?

There are several points to bear in mind when interpreting this study:

  • Firstly, the absolute numbers of asthma cases identified during the 16 years of follow-up were very small. For example, when the researchers analysed the women according to how much dietary fish they had, there were only five cases of asthma in the low dietary fish group (four to mothers taking olive oil and one to mothers taking fish oil). In the high dietary fish group, there were three in each group. These very small numbers should be noted, as relative measures of risk reduction (i.e. by saying that fish oils reduced risk by 60%) can be misleading. Importantly, the reliability of results based on such small sample sizes is questionable.
  • The way that the researchers identified cases (through medical diagnoses) may have underestimated the total number of cases by not recording the less severe cases that don’t present in this way. 
  • The researchers report that 48% of the women guessed that they were receiving olive oil as opposed to fish oil supplements, while 85% of women receiving fish oil guessed this. Both of these factors may have affected the way the women behaved – i.e. some may have supplemented their intake of fish oil.
  • To explain why children of women given no supplements at all had similar rates of asthma to children of mothers given fish oils, the researchers suggest contamination bias – i.e. that women in the supplement group suspected that oils were good for them (given the nature of the study) and supplemented their intake.
  • It is unclear from this study whether the women included were balanced at the start of the study for other factors that may be linked to a reduction in risk of asthma. These include parental smoking, child’s diet, family history of allergies or asthma, gender, low birth weight etc.

Overall, this study does not offer convincing evidence that fish oils are responsible for the reduction in asthma seen in these children. As the researchers themselves say, more research is needed, both through large randomised controlled trials to clarify whether this reduction in asthma risk is real, and through studies that investigate potential biological mechanisms behind any risk reductions.

Analysis by Bazian
Edited by NHS Choices