In a study published in JAMA Pediatrics, researchers looked at an impressive number of new mothers—more than 26,000—and found that among them, those who ate more fish while they were pregnant tended to have children with higher BMIs.
While at first glance the findings might implicate fish as yet another food choice that adds on pounds, it’s worth taking a closer look at the study before avoiding the fish section.
The data were collected from women in 10 European countries and one U.S. city who all gave birth between 1996 and 2011. The women answered questions about their diet, including how much fish they ate. The researchers also collected information on the mothers’ pre-pregnancy weight, age, smoking status, education and whether she breastfed her child.
The size of the study is one of its strengths; the larger the number of people, the stronger and more reliable the findings generally are. There are also studies documenting the potential harmful effects on growing fetuses of exposure to mercury, which can be found in many deep ocean fish like tuna. That prompted the U.S. Food and Drug Administration and Environmental Protection Agency to advise pregnant women to eat no more than three servings of fish a week, to limit their exposure to the heavy metal.
The current study shows that women who ate fish at least three times a week were 22% more likely to have children with rapid growth in their first two years, and 22% more likely to have children who were overweight or obese at six years, compared to women who ate fish less than three times a week. The researchers note that the hormone-disrupting effects of some contaminants found in fish, including mercury, could explain the effect on BMI.
But they also point out that the connection is still an association. The findings do not suggest that eating more fish causes pregnant women to have heavier children.
For one, the scientists did not distinguish whether the women were eating deep sea or river fish, which carry different amounts of pollutants. They also did not analyze how the fish was cooked, whether it was fried, which can contribute to overweight or obesity, or grilled or broiled.
But perhaps most importantly, the authors also did not take into account the overall exposure to organic pollutants that the women might have been exposed to, either in their environment or in their water. While it’s true that the result were consistent across many different countries, all of these countries are relatively industrialized and may have similar rates of environmental pollution.
Finally, they acknowledge that they did not have data on the mothers’ total diet during pregnancy, nor did they have information on her exercise habits. A mother’s weight gain during pregnancy can affect the child’s weight, and the researchers say that they used the mother’s gestational weight gain as a proxy for her diet and energy use. But if a mother’s diet included fatty foods, that could have a stronger effect on her child’s weight than her fish consumption. Finally, the scientists also did not have information on the children's diet and exercise habits; because they were followed until they were six years old, their own eating patterns and physical activity could have more influence on their weight than any foods they were exposed to in utero.
Most nutrition experts recommend that people eat more fish because of they contain high levels of healthy fats like omega-3s. Studies have shown that while there is a small risk of harm to developing fetuses from mercury in some fish, the benefits outweigh these risks. And these results, while worthy of more study, shouldn’t change that advice.