Pregnant women who eat fish more than three times a week give birth to babies who grow more quickly in the first years of life, and have a higher risk of being overweight as preschoolers, according to international research in which many Dutch families participated, and which was published in the scientific journal JAMA Pediatrics.

Fish is a known source of persistent organic pollutants in our diet, and persistent organic pollutants do not biodegrade or do so poorly. These substances accumulate in the food chain and ultimately in humans. Some of these substances have endocrine-disrupting effects that may contribute to the development of overweight.

To limit the intake of these and other contaminated substances (e.g. mercury), the US government recommends that people eat no more than three portions of fish per week. The Netherlands Nutrition Centre Foundation (Voedingscentrum) recommends that people eat fish twice a week. Little is known about the amount and sort of fish that pregnant women should eat to optimize the growth and development of their children.

Consumption varied

The average intake of fish by pregnant women varied between research regions: it ranged from a half portion per week in Belgium to four and a half portions per week in Spain. High fish intake was defined as more than three servings per week, low fish intake as one serving or less, and moderate fish intake between those extremes. There were large differences between the cohorts in the four participating Dutch studies, but most pregnant women in the Netherlands were still on the low or moderate side. Almost a third of all the studied babies were fast growers in the first two years of life, while 19% of the children were overweight at age 4 and 15% were at age 6.

Pregnant women with a high fish intake (more than three times a week) had children whose body mass index (BMI) at 2, 4 and 6 years of age was higher than children born to women with a lower fish intake. High fish intake was also associated with a higher probability of rapid growth in the first two years of life, and a higher chance of being overweight at 4 and 6 years of age, compared to the children of women with a low fish intake. These differences were more pronounced in girls than in boys.

Reasons why remain unclear

Environmental contamination of fish would offer an explanation for the relationship between high fish intake during pregnancy and the risk of overweight in children, according to the authors of the article. There were no obvious differences found between effects of oily fish, lean fish and other seafood (squid, crustaceans and shellfish). The data were not detailed enough to draw conclusions about differences between freshwater or saltwater fish, specific types of fish, or storage and preparation methods.

In addition, there is a lack of detailed information on the levels of persistent organic pollutants in fish in the research populations. Therefore, as the authors write, it is unclear whether the suspect pollutants actually play a role in the observed relationship between fish intake and overweight.

The authors concluded that their findings confirm the current US health guidelines for eating fish. The Dutch recommendation from the Netherlands Nutrition Centre Foundation about the consumption of fish by pregnant women stays two servings per week.

About the study

Dr Leda Chatzi from the University of Crete, Greece, and her co-authors brought together data from long-term studies conducted in 10 European countries and the United States. These studies followed the growth of children born to more than 26,000 women until the age of 6 years.

The Dutch birth cohort studies that contributed to this research were KOALA (Maastricht University), Generation R (Erasmus University Medical Center Rotterdam), ABCD (Academic Medical Center, Amsterdam) and PIAMA (Utrecht University and the National Institute for Public Health and the Environment). The first author, Nikos Stratakis, is a PhD student in the department of Complex Genetics at Maastricht University.