Harvard researchers find developmental edge among children of mothers who ate more fish than average; authors recommend fatty, low-mercury fish like Salmon and Sardines
by Craig Weatherby
You’d think that by now, the benefits of fish and their omega-3s to child development would be widely known.
Sadly, as we reported last April parents remain woefully unaware of the key role that omega-3s play in brain and eye development.
And they’re also confused about where to get omega-3s. (The preferred sources, by far, are fish and/or fish oil: see “Omega-3s: Fish fat provides the best form”, sidebar, below.)
A very special omega-3 fatty acid called DHA plays major functional and structural roles human brain and eye cells.
Yet, our bodies cannot make DHA and the only food source is seafood (fish, fish oil, shellfish, and algae).
Now a joint American-Danish study links higher fish consumption to better physical and mental development.
Joint US-Danish study affirms that omega-3s enhance development
Researchers from Harvard Medical School and Copenhagen’s Statens Serum Institute interviewed more than 25,000 Danish mothers (Oken E, Osterdal ML, Gillman MW, et al. 2008).
The mothers’ prenatal diets—including the amounts and types of fish they consumed weekly—were ascertained through a detailed questionnaire administered when they were six months pregnant.
Mothers were also asked about their breastfeeding habits at 6 months after their children’s’ births.
During the interviews mothers were asked about specific physical and cognitive developmental milestones such as whether the child at six months could hold up his/her head, sit with a straight back, sit unsupported, respond to sound or voices, imitate sounds, or crawl.
At 18 months, the mothers were asked about more advanced milestones such as whether the child could climb stairs, remove his/her socks, drink from a cup, write or draw, use word-like sounds and put words together, and whether they could walk unassisted.
The children whose mothers ate the most fish during pregnancy were more likely to have better motor and cognitive skills:
- Compared with women who ate the least fish, the children of women with the highest fish intake—about 2 ounces
(60 grams) per day—were 25 percent more likely to have higher developmental scores at 6 months and 29 percent more likely to have higher scores at 18 months.
- Among mothers who ate the least fish, 5.7 percent of their children had the lowest developmental scores at 18 months, compared with only 3.7 percent of children whose mothers had the highest fish intake.
In this case—as in virtually every other such study—the benefits of fish-borne omega-3s outweighed any harm from mercury.
Breast milk contains omega-3 fatty acids naturally, and children breastfed longer showed better infant development
Children who were breastfed for 10 months or more were 28 percent more likely to have higher developmental scores, compared with children who were breastfed for one month or less.
And the child-development benefits of higher maternal fish consumption were similar whether infants were breastfed for shorter or longer periods.
This suggests that the developmental boost provided to nursing infants by breast milk that has more omega-3s than average exceeds the developmental boost provided when an infant gets average breast milk for a longer time.
Mothers should breast feed as long as is possible and comfortable, but eating fish or fish oil appears to boost the developmental power of breast milk substantially, making every nursing session especially impactful.
Authors urge mothers to eat more (low-mercury) fish
Citing another study conducted by her team, lead author Emily Oken, M.D., MPH, recommended that mothers choose lower-mercury species like salmon, sardines, herring, and sablefish:
“In previous work in a population of U.S. women, we similarly found that higher prenatal fish consumption was associated with an overall benefit for child cognitive development, but that higher mercury levels attenuated this benefit.” (Oken E, Radesky JS, Wright RO, et al. 2008)
And Dr. Oken went on to make the critical point: “Therefore, women should continue to eat fish—especially during pregnancy—but should choose fish types likely to be lower in mercury.”
Women in the U.S. have been advised by the FDA and EPA to limit their fish intake to two servings a week, to avoid five species with high traces of mercury and to choose salmon, sardines, shrimp, and other species low in mercury.
Unfortunately, these reasonable warnings have been widely misunderstood, according a study conducted by the lead author of the study we report here.
As Dr. Oken wrote five years ago, “After dissemination of federal recommendations, pregnant women… reported reduced consumption of fish… Because these fish may confer nutritional benefits to mother and infant, public health implications of these changes remain unclear” (Oken E et al. 2003).
While the Harvard-Danish team did not measure mercury levels in the mothers recruited for the current study, most women reported eating species of fish known to have low mercury levels, including cod, salmon, herring, and Atlantic mackerel. (Note: Mothers should avoid King mackerel, which averages 10 times more mercury than Atlantic mackerel.)
Information on mercury levels in commonly consumed fish is available online at the U.S. Food and Drug Administration.
And we post our own handy chart showing the average omega-3 and mercury content of the fish we offer.
- Omega-3 content is shown in blue under the middle column labeled “Total n-3”. (Chemists’ shorthand for omega-3 is “n-3”.)
- Mercury content is shown in red in the far right column, labeled “Avg Hg”. (Hg is the chemical symbol for mercury.)
This is far from the first study to show benefits from higher maternal or infant fish intake.
One of the best and most widely reported in major media outlets was a joint U.S. and U.K. study led by NIH psychiatrist Joseph Hibbeln, M.D.: see “Findings Verify Safety and Value of Higher Maternal Fish Intake”. (You’ll find an excerpt from his 2007 interview in Newsweek magazine in the sidebar on our Healthy Mom & Baby page.)
Dr. Hibbeln’s team compared the amount of fish eaten by pregnant mothers with the development and behavior of their offspring up to the age of eight, and came to two clear conclusions:
- “…we recorded no evidence to lend support to the warnings of the US [fish-and-mercury] advisory that pregnant women should limit their seafood consumption.
- “…children of mothers who ate small amounts of seafood were more likely to have suboptimum neuron-developmental outcomes than children of mothers who ate more seafood.”
To see more research related to this topic, search our newsletter archive for “children”.
These are some of the relevant articles you’ll see there:
Study unaffected by fishy funding
A 2006 study from Harvard that found greater rewards than risks from higher maternal fish intake was criticized for being funded by the canned tuna industry (HSPH 2006).
But this study, like most of its kind, was free of any commercial fish connections.
The Danish National Birth Cohort study—of which this study was one part—was financed by the Danish National Research Foundation, March of Dimes Birth Defects Foundation, Danish Ministry of Health, National Board of Health, Statens Serum Institut, BIOMED, Danish Heart Association, Danish Medical Research Council, Sygekassernes Helsefond, Danish Pharmaceutical Association, and the Early Nutrition Programming Project.
Dr. Oken was supported by a grant from the National Institutes of Health and a fellowship from the American Scandinavian Foundation, Inger and Jens Bruun Foundation.
- Harvard Medical School. Eating fish while pregnant, longer breastfeeding, lead to better infant development. September 9, 2008. Accessed online September 10, 2008 at http://www.eurekalert.org/pub_releases/2008-09/hms-efw090908.php
- Harvard School of Public Health (HSPH). New Study Shows the Benefits of Eating Fish Greatly Outweigh the Risks. October 17, 2006. Accessed online September 10, 2008 at http://www.hsph.harvard.edu/news/press-releases/2006-releases/press10172006.html.
- Oken E, Kleinman KP, Berland WE, Simon SR, Rich-Edwards JW, Gillman MW. Decline in fish consumption among pregnant women after a national mercury advisory. Obstet Gynecol. 2003;102(2):346-51.
- Oken E, Osterdal ML, Gillman MW, Knudsen VK, Halldorsson TI, Strøm M, Bellinger DC, Hadders-Algra M, Michaelsen KF, Olsen SF. Associations of maternal fish intake during pregnancy and breastfeeding duration with attainment of developmental milestones in early childhood: a study from the Danish National Birth Cohort. Am J Clin Nutr. 2008 Sep;88(3):789-96.
- Oken E, Radesky JS, Wright RO, Bellinger DC, Amarasiriwardena CJ, Kleinman KP, Hu H, Gillman MW. Maternal fish intake during pregnancy, blood mercury levels, and child cognition at age 3 years in a US cohort. Am J Epidemiol. 2008 May 15;167(10):1171-81. Epub 2008 Mar 18.