Fish are high in several beneficial nutrients, including some that are related to healthy brain development. But several organizations, including the U.S. Food and Drug Administration (FDA), recommend that women who are pregnant limit the amount of fish they eat.
The reason is that most fish and shellfish contain low-levels of methylmercury.
Almost all people have at least trace amounts of methylmercury in their tissues, reflecting the organic compounds widespread presence in the environment. Fetuses, infants and children are the most vulnerable to the possible adverse effects of mercury exposure.
One of the major concerns of the medical community and mothers-to-be is the possibility of a link between eating fish that contains mercury, and their child developing autism.
A new study addresses that concern and says that children exposed to low levels of mercury in the womb because their mothers ate large amounts of fish, dont appear to be at an increased risk for autism.
The new findings from more than 30 years of research in the Republic of Seychelles -- a group of islands in the western Indian Ocean -- found no such link, the study authors said.
"This study shows no evidence of a correlation between low level mercury exposure and autism spectrum-like behaviors among children whose mothers ate, on average, up to 12 meals of fish each week during pregnancy," study lead author Edwin van Wijngaarden, associate professor in the public health sciences department at the University of Rochester Medical Center in New York, said in a medical center news release.
"These findings contribute to the growing body of literature that suggest that exposure to the chemical does not play an important role in the onset of these behaviors," he added.
One autism expert added a note of caution, however.
"The study found no link between high mercury levels and later autism spectrum disorder behaviors. However, this should not be taken to mean that high levels of mercury are safe to ingest," said Alycia Hallday, senior director of environmental and clinical science at the advocacy group Autism Speaks.
"Other studies comparing this [Seychelles] cohort to those in other parts of the world indicate that this cohort may be spared from many adverse effects because it is consumed with nutrient-rich ocean fish," she explained.
For the study, the researchers initially determined the level of prenatal mercury exposure by analyzing the mothers' hair samples. Then the researchers used two questionnaires -- one given to parents, the other to the children's teachers -- to see if the children showed signs of autism spectrum-like behaviors. The tests included questions on language skills, communication skills and repetitive behaviors. While the tests don't give a definitive diagnosis, they are used widely in the United States as an initial screening tool and may indicate the need for additional testing, the researchers said.
The study also noted the concerns of and limitations recommended by the FDA and other organizations.
"This study shows no consistent association in children with mothers with mercury levels that were six to 10 times higher than those found in the U.S. and Europe. This is a sentinel population and if (the association between low-level mercury exposure and autism) does not exist here than it probably does not exist," Philip Davidson, principal investigator of the Seychelles Child Development Study and professor emeritus in pediatrics at the University of Rochester Medical Center, said in the news release.
The finding lends support to an emerging belief that the good may outweigh the possible bad when it comes to eating fish during pregnancy. Specifically, if the mercury did not harm brain development at the levels of exposure experienced by the children in this study, then the benefits of the nutrients in fish may counteract or surpass the potential negative effects of mercury, the study authors said.
So, which fish have low or high mercury content? The American Pregnancy Association provides this list on their website.
- Orange roughy
- Mackerel (king)
- Tuna (bigeye, Ahi)
Eat no more than three 6-oz servings per month
- Sea Bass (Chilean)
- Mackerel (Spanish, Gulf)
- Tuna (canned, white albacore) See tuna chart below
- Tuna (Yellow fin)
Eat no more than six 6-oz servings per month
- Bass (Striped, Black)
- Cod (Alaskan)
- Croaker (White Pacific)
- Halibut ( Pacific and Atlantic) Jacksmelt ( Silverside)
- Mahi Mahi
- Perch (freshwater)
- Sea Trout (Weakfish)
- Tuna (canned, chunk light)
- Tuna (Skipjack)
Enjoy two 6-oz servings per week
- Crab (Domestic)
- Mackerel (N Atlantic, Chub)
- Perch (ocean)
- Salmon (Canned, Fresh)
- Shad (American)
- Squid (Calamari)
- Trout (freshwater)
The study was published online July 23 in the journal Epidemiology