Thursday September 25 2008
“Babies given fish to eat within the first nine months of their lives are less likely to develop eczema”, The Independent reported. It said that a Swedish study of almost 5,000 families found that introducing fish to a baby’s diet cuts the risk that they will develop the skin condition by 24%. It did not matter if this was lean and white fish or oily fish such as mackerel or fresh tuna, the paper says. The study also found that keeping birds in the home reduced the chance of developing eczema by 65%, while a family history of eczema increased the risk.
In this study, nearly 5,000 families completed questionnaires on their child’s environment, health, diet and allergies at six months and one year after the child was born. It was a large study and by benefit of its size probably gives a reasonably accurate estimate that 20% of Swedish children have eczema. However, this was an observational study and therefore cannot prove causation that, for instance, “fish fights eczema”. In addition, the study’s limitations bring the reliability of some of the links into question. It would be sensible to wait for further research, preferably using typical UK diets, before fish is advocated as a baby food for preventing eczema.
Where did the story come from?
Dr Bernt Alm from the Department of Paediatrics at the University of Gothenburg, Queen Silvia Children’s Hospital and colleagues from other clinics and hospitals around Sweden carried out the research. The study was supported by various research foundations including the Sahlgrenska Academy and the Research Foundation of the Swedish Asthma and Allergy Association. The study was published in the peer-reviewed medical journal: Archives of Disease in Childhood.
What kind of scientific study was this?
The researchers say that the incidence of eczema in infants has increased in western societies. It is known that heredity is a major influence, and food and food introduction may also affect its onset and severity. The researchers wanted to investigate the current prevalence of eczema in Sweden and its relationship with various risk factors and the introduction to food in one-year-olds.
The data was obtained from an ongoing Swedish study called the ‘Infants of Western Sweden’. This is a prospective cohort study and a secondary analysis of the data. It was carried out in a population of mixed urban, rural and coastal areas containing 1.5 million inhabitants, 500,000 of whom live in Gothenburg. Of 16,682 infants born in 2003, just under half (8,176 families) were randomly selected to be asked to participate.
The families that agreed to take part were asked to complete a questionnaire six months after their baby’s birth and another questionnaire when the children were 12 months old. The two questionnaires sent to these families contained questions about the family, environment, events around birth, smoking, breastfeeding, food introduction and any diseases in the first year, with special reference to allergic diseases. From the selected families, 68.5% responded to the first questionnaire. Out of these 5,605 families, 4,941 also completed the second questionnaire (88.2% of those who answered the first). Medical birth register data were available for 4,921 (60.2% of the originally selected population).
Both six and twelve-month questionnaires asked if the child had had eczema. They also asked if the child had been diagnosed with a food allergy, and if so, which type of allergy. What sort of food the child ate and the frequency it was eaten was also asked at six and twelve months. Specific questions were also asked about how often they ate yoghurt and fermented vegetables, as sources of lactic acid, and fish (possible answers were: three or more times a week, one to three times a week, one to three times a month, a few times a year or never). The types of fish surveyed were lean fish (cod or haddock), salmon, flatfish, mackerel or herring.
The researchers analysed the prevalence (the rate of eczema) in the population of 4,953 children and used a statistical analysis known as binary logistic regression to estimate the effect of a number of risk factors.
What were the results of the study?
At one year of age, one in five (20.9%) of the infants had previous or current eczema. The average age at which eczema appeared was four months. Statistical analysis showed that a familial occurrence of eczema, especially in brothers and sisters, was the strongest risk factor, with the odds ratio suggesting an 87% increase in risk (OR 1.87, 95% CI 1.50 to 2.33). Eczema reported for the mother was also a significant risk factor with about a 40% increase in risk (OR 1.4, 95% CI 1.30 to 1.84).
Introducing fish before nine months of age was linked to a 24% reduction in risk of eczema (OR 0.76, 95% CI 0.62 to 0.94) and having a bird in the home with a 65% reduction (OR 0.35, 95% CI 0.17 to 0.75) suggesting that these actions were beneficial.
All four of these were shown to be independent risk factors, that is, the link remained after other factors were adjusted for (taken into account).
What interpretations did the researchers draw from these results?
The researchers say that, “one in five infants suffers from eczema during the first year of life. Familial eczema increased the risk, while early fish introduction and bird keeping decreased it. Breastfeeding and the time of milk and egg introduction did not affect the risk.”
What does the NHS Knowledge Service make of this study?
This study has shown some strong links between certain risk factors and the rates of eczema. As the data has come from a large cohort study, the estimate that around 20% of children at one year of age have or have had eczema is likely to be reasonably accurate for Sweden.
The authors acknowledge that there are a few limitations:
- The prevalence of eczema was estimated using questionnaires sent to the parents, and as this did not require a doctor’s diagnosis there may be some uncertainty about the exact rate of eczema. The authors believe that their results are valid since they broadly agree with rates found in other studies.
- There is a possibility of recall (memory) bias, where bias could have been introduced by parents of children with eczema recalling food habits differently to parents of children untroubled by the condition. This is always a limitation in questionnaire studies, and may have been minimised by the relatively short intervals between questionnaires.
- There is also the possibility of reverse causation. This occurs when, for example, the protective effect of having a bird in the home is really the result of non-allergic families keeping more birds at home than allergic families. This is perhaps the most intriguing part to this study and the researchers offer a possible explanation in that birds are almost always kept inside, providing continuous indoor exposure to a toxin, which they suggest might act in bolstering the immune system
This study was observational and, as such, cannot prove causation. It is also important to consider how well the feeding habits of parents and infants in Sweden reflect the typical feeding practices in the UK. However, the strength of the links found and the fact that eczema, as the newspapers confirm, is on the increase, suggests that this is an important area for study. More research, preferably a randomised trial design, could provide a clearer estimate of the degree of protection, if any, provided by eating fish or keeping birds. At present, it would be advisable to wait for further research, preferably using typical UK diets, before fish is advocated as a baby food for preventing eczema.