The issue will be highlighted at meeting late April in Boston of the American Academy of Neurology. A paper will presented to the meeting’s attendees that will link the potential of eating fish species that are known to be high in mercury to an increasing risk of developing ALS, or amyotrophic lateral sclerosis, a wasting disease of the central nervous system. The paper did go on to say that in general fish consumption was not a risk per se if a variety of fish was eaten.
"For most people, eating fish is part of a healthy diet," said study author Elijah Stommel, MD, PhD, of Dartmouth College in Hanover, N.H., and a Fellow of the American Academy of Neurology. "But questions remain about the possible impact of mercury in fish."
Bioaccumulation still not fully understood
According to a US government cooperative effort known as the National Atmospheric Deposition Program, mercury accumulates in biological tissue through in a process known as bioaccumulation. This is a set of complex reactions many of which are still unknown. What is known is that several bacteria incorporate environmental inorganic mercury into their bodies through chemical conversion to several organic mercury compounds, collectively called methyl mercury (Me-Hg). This Me-Hg form is more toxic and more difficult to remove from bacterial systems than inorganic mercury. Any higher-level organisms that consume these bacteria also consume the Me-Hg.
This bioaccumulation is most commonly observed in oceanic food chains. Among the fish species high in mercury that are mentioned in the paper, which was accepted for presentation at the meeting but has not yet gone through peer review for publication, are shark and swordfish, neither of which form a major part of US seafood consumption.
Health authorities in the US and elsewhere have downplayed the risk that seafood consumption poses if some commonsense precautions are taken. US dietary recommendations call for consumers to eat a variety of fish to avoid higher mercury exposures.
Yet the study that will be presented at the neurology meeting found among participants who ate fish and seafood regularly, those in the top 25% for estimated annual mercury intake were at double the risk for ALS compared to those with lower levels. The study was of a survey of 518 people, 294 of whom had ALS, and 224 of whom didn't, on how much fish and seafood they ate. Mercury levels were measured from toenail samples, and mercury exposure was estimated by referring to tables of mercury content in various fish species.
Levels overall are dropping
Those dietary recommendations on how best to consume fish to lessen mercury risk seem to be having an effect. For example, according to the United Nations Environmental Programme, in a study of a representative group of about 1700 women in the US (aged 16-49 years) for the years 1999-2000, about 8% of the women had mercury concentrations in blood and hair exceeding the levels corresponding to the US EPA’s reference dose (an estimate of a safe dose). But a more recent study conducted by Oregon State University found that mean concentrations were lower in 2009-2010 compared to the 1999-2000 time frame, even though overall US women were consuming more fish. But it also noted that among the consumers who ate the most fish, women living in coastal regions, mercury levels were higher than among inland populations.
Supplements could benefit
Adam Ismail, executive director of the Global Organization for EPA and DHA Omega-3s (GOED), said that as a rule his organization does not comment on research that has yet to go through peer review. His organization includes members from the dietary supplement and seafood realms, so he’s naturally loathe to pit one camp against another.
“I wholeheartedly agree with what they are saying in the recent guidelines,” Ismail told NutraIngredients-USA. “To eat a diversity of fish is sound guidance for consumers.
“We do have some issues with the FDA recommendations because it is ranking of fish by mercury content. It originally was going to address the risk/benefit ratio of consuming fish high in omega-3s, but in the final guidance they basically did away with all the work they had done on omega-3s,”he said.
Salmon, for instance, is commonly consumed in the US, is generally high in omega-3s and low in mercury. But questions have been raised about the exact omega-3s content of the various types of salmon from farmed and wild-caught sources. For consumers concerned about mercury, one trouble-free way to consume a defined quantity of omega-3s is via supplements, Ismail said.
“The one thing I can say with certainty is the mercury levels in omega-3s supplements are virtually undetectable and that is this is the result of the refining process taking out these contaminants. And even for the unrefined, ‘wild’ oils on the market (salmon oil would be an example) mercury levels tend to be very low,” he said.