Mercury cancels brain benefits of fish oil, study confirms.

Jan 03, 2011

Lynch, ML, L-S Huang, C Cox, JJ Strain, GJ Myers, MP Bonham, CF Shamlaye, A Stokes-Riner, JMW Wallance, EM Duffy, TW Clarkson and PW Davidson. 2010. Varying coefficient function models to explore interactions between maternal nutritional status and prenatal methylmercury toxicity in the Seychelles Child Development Nutrition Study. Environmental Research http://dx.doi.org/10.1016/j.envres.2010.09.005.

Synopsis by Jennifer F. Nyland

A long-term dietary study untangles the effects on brain development of two well-known but contrary exposures - beneficial oil and toxic methyl mercury - that accompany a fish-rich diet.

Prenatal mercury exposure from a mother's fish-rich diet can reduce the beneficial effects fish oil has on brain development, report an international group of researchers. The babies exposed in the womb to higher methyl mercury levels scored lower on skills tests as infants and toddlers than those exposed to lower levels of the pollutant.

Of five nutrients tested, only the benefits of the fish oil DHA were affected by the mercury. DHA is one type of healthy oil found in fish.

Careful selection of which fish to eat during pregnancy is recommended following this recent analysis.

To what extent methyl mercury may interfere with fish oil's beneficial benefits on the brain is uncertain and still being investigated. In this case, researchers used computer models to understand the complex interactions of these co-exposures. The findings demonstrate that harmful and beneficial exposures can cancel each other out.

The beneficial effects of eating fish during pregnancy on a baby's brain development are relatively well accepted. However, some fish can contain high levels of mercury – a known neurotoxicant – in the form of methyl mercury.

Methyl mercury is transferred to people who eat the fish and is especially concerning for pregnant women whose children may be exposed while developing in the womb. Government agency advisories suggest women of childbearing years eat fish with low mercury levels as well as limit consumption of fish that contain high levels, such as the long lived sharks and swordfish and locally caught freshwater species.

The researchers used data collected through the Seychelles Child Development Nutrition Study (SCDNS). The study has followed mother-child pairs since the mid-1980s to determine the effects of nutrients, mercury and other exposures in children whose mothers eat a fish-rich diet.

In the new study, researchers examined the link between pregnant women's fish consumption and their children's brain development. The women recoreded what they ate. Methyl mercury measured in hair samples provided an estimate of their mercury burden. Two standard assessment tests – MDI and PDI – were used to determine the children's development at nine months and 30 months.

The mothers averaged nine fish meals per week. Their hair mercury was 5.7 parts per million (ppm) and levels ranged from 0.2 to 18.5 ppm.

Multiple exposures were measured by assessing mercury's influence on five different nutritional elements found in fish and known to be associated with neurological development. These included iron, iodine, choline, and long-chain polyunsaturated fatty acids of two different types (n-3 and n-6). N-3 fatty acid is also called DHA (docosahexaenoic acid).

Of the five nutritional components evaluated, only the beneficial effects of DHA – as measured by the children's test scores were negatively impacted by increased mercury exposure. The nutritional benefits of DHA were significantly reduced and then disappeared altogether at the higher exposures.

This decrease in performance was seen in both tests in both age groups, but it occurred differently as mercury levels rose. For instance, benefits seen in the 30-month PDI and the 9-month MDI decreased with any mercury exposure and disappeared when exposures reached 9 and 11 parts per million. The fish oil benefits measured with the 9-month PDI and the 30-month MDI increased slightly as mercury levels rose but then decreased or leveled off at exposures of 8 ppm.

Overall, the study is an attempt – using a sophisticated computer model – to better understand how one exposure might affect another if they both occur during development.

 

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