By Craig Weatherby
How much and what kind of fish should pregnant and nursing mothers eat?
The goal is to ensure optimal brain development, and minimize risk from mercury.
Attention-deficit/hyperactivity disorder (ADHD) affects as many as one in 10 children worldwide, but its causes remain unclear.
To date, research suggests that maternal and child diets high in seafood – ocean fish and shellfish – helps brain development.
The few exceptions are four species known to be high in mercury but low in protective selenium – shark, swordfish, tilefish, and king mackerel – which the U.S. EPA and FDA urge pregnant and nursing mothers to avoid.
Now, a study from Boston links low-level prenatal mercury exposure to a greater risk of ADHD-related behaviors, mostly in boys.
But the results also showed that higher maternal fish consumption during pregnancy can help reduce the risk of ADHD-related behaviors in children, by a healthy 60 percent.
As the authors concluded, “Low-level prenatal mercury exposure is associated with a greater risk of ADHD-related behaviors, and fish consumption during pregnancy is protective of [against] these behaviors.” (Sagiv SK et al. 2012)
Lead author Sharon Sagiv, Ph.D., of the Boston University School of Public Health (BUSPH), clarified what they meant: “Women need to know that nutrients in fish are really important for brain development, but they also need to be aware that high mercury levels in some fish pose a risk.” (BUSPH 2012)
Their findings echo those of a similar study published four years ago, which found, as the authors wrote, that “… higher fish intake was associated with better child cognitive test performance, and higher mercury levels with poorer test scores. Dietary recommendations for pregnant women should incorporate the nutritional benefits as well as the risks of fish intake.” (Oken E et al. 2002)
Higher fish intake curbed ADHD risk; higher mercury was harmful
The analysis involved 788 children born in New Bedford, Massachusetts between 1993 and 1998 (Sagiv SK et al. 2012).
Shortly after their mothers gave birth, researchers collected hair samples from the mothers and analyzed them for mercury.
They also gave the mothers a questionnaire to determine their fish consumption during pregnancy.
Eight years later, the children took standardized tests to detect any behaviors related to ADHD.
The results linked increased risk of childhood ADHD-related behaviors to increasing maternal hair mercury levels … but primarily in boys.
The mercury levels were lower than levels shown to be potentially harmful in most previous studies. “These are not very highly exposed populations,” Sagiv said. (BUSPH 2012)
At one microgram/per gram or more of mercury, the study found a 40 percent higher risk for mild to marked atypical inattentive behavior, and a 70 percent higher risk for impulsive/hyperactive behavior.
Conversely, the kids of mother who ate two or more servings of fish per week were 60 percent less likely to exhibit ADHD-related behaviors … despite the presence of trace mercury in all ocean fish.
Prior analyses have strongly linked protection from brain injury to the omega-3s in fish … but often overlooked the role of large amounts of protective selenium in ocean fish.
To learn why the selenium in seafood blunts or negate the effects of mercury, please visit our Purity page, which has links to our past coverage and to Fish, Mercury, and Nutrition) … a website that offers clear, unbiased information from leading experts in the field.
The researchers said further investigation is needed on the links between mercury and ADHD. “This is just one study,” said Sagiv. “More needs to be done.” (BUSPH 2012)
As the authors said, “The message is, ‘Eat fish, but don't eat fish high in mercury.'”
The study was supported by grants from the National Institute of Environmental Health Sciences of the National Institutes of Health.
Choose fatty, omega-3-rich fish that's low in mercury
The results show that it makes sense for mothers to frequently eat fish high in omega-3s but low in mercury.
These include salmon, sardines, mackerel, herring, sablefish, and younger, smaller, younger tuna.
(We only offer smaller albacore tuna weighing no more than 14 lbs., and smaller halibut that weigh 20 lbs. or fewer.)
As the press release from BUSPH noted, “Many fish have extremely low levels of mercury, so it is possible for a pregnant woman to eat nutritionally beneficial fish without exposure risks.” (BUSPH 2012)
The risks of dietary mercury to developing brains are clear … but the findings from prior epidemiological studies are inconsistent when it comes to ADHD risk.
Some studies have found statistical links between relatively high mercury exposure and higher risk of ADHD-related behaviors, while others saw no links.
This inconsistency may be partly attributable to the protective role of selenium in seafood.