Caroline Cassels

May 03, 2017

BOSTON — Mercury in fish and seafood has been linked to an increased risk for amyotrophic lateral sclerosis (ALS), new research shows.

Findings from a preliminary study showed that among persons who ate fish and seafood regularly, those in the top 25th percentile for an estimated annual mercury intake had a more than two-fold increased risk for ALS compared with those with lower levels.

"For most people, eating fish is part of a healthy diet. But questions remain about the possible impact of mercury in fish," study investigator Elijah Stommel, MD, PhD, said in a press release.

The findings were presented here at the American Academy of Neurology 2017 Annual Meeting (AAN) by coinvestigator Angeline S. Andrew, PhD.

Species/Location-Dependent Effect?

The exact cause of ALS is unknown. However, the investigators note that previous research suggests mercury may be a risk factor for the disease. In the United States, the primary source of mercury exposure is consumption of contaminated fish.

Dr Andrew cited a 1990 study by Dean G. Sienko, MD, and colleagues at the University of Wisconsin, Madison, which showed an increased risk for ALS in a cluster of patients who ate fish frequently.

The risk for ALS in this small study was particularly high among those who engaged in subsistence fishing, wherein more than 50% of the fish they consumed was caught fresh from Lake Michigan. This risk, said Dr Andrew, was attributed to various factors but also potentially to the methylmercury in the fish.

On the other hand, Dr Andrew noted that some large prospective dietary studies suggest that omega 3 polyunsaturated fatty acids (which are present in some species of fish, including salmon and sardines, but not others) may significantly reduce ALS risk.

Taken together, she said, these research findings suggest there "may be a species- and location-dependent effect involved."

To investigate the potential link between mercury and ALS, the investigators survey 518 individuals — 294 with ALS and 224 without the disease — to determine how much fish they ate, as well as the types of fish and whether they were purchased from stores or fresh caught by participants.

Toenails Tell the Tale

Using inductively coupled plasma mass spectrometry, the researchers measured participants' mercury levels in toenail samples from study participants with ALS and those without neurodegenerative disease.

Toenails were used as biomarkers of mercury exposure because they have a high sulfur keratin content, bind to methylmercury, and reflect a long-term exposure period of 12 to 15 months.

Dr Andrew also noted that autopsy studies have shown that methylmercury levels in the brain are highly correlated with levels in toenails.

After adjusting for age and sex, the researchers found those in the top quartile for estimated annual methylmercury intake based on self-reported consumption and toenail samples had a two-fold increased risk for ALS compared to those with lower levels.

A total of 61% of participants with ALS were in the top 25% of estimated mercury intake vs 44% of those without the disease.

The study also showed an ALS risk was associated with high levels of mercury in toenails and estimated annual mercury consumption from fish that appeared to be related to subsistence fishing (odds ratio, 2.2; 95% confidence interval, 1.1 - 5.0).

While these data are compelling, Dr Andrew noted that the findings need to be replicated before fish-consumption guidelines for neurodegenerative illness can be made.

Although the study's findings do not negate the many health benefits of fish, individuals should consider choosing species with lower mercury content, such as salmon and sardines, the investigators note.

In addition, Dr Andrew said the public should heed local water advisories warning of mercury and other contaminants, such as polychlorinated biphenyl, before consuming fish caught from local lakes and rivers.

The researchers also note the US Food and Drug Administration recommend women of childbearing age and children eat two to three weekly meals of species, such as salmon or sardines, that are low in mercury but high in omega-3 fatty acids.

The FDA recommends avoiding such species as shark and swordfish, which have some of the highest mercury levels.

The study was supported by the Diamond Endowment Fund, the ALS Association, the Centers for Disease Control and Prevention, the Agency for Toxic Substances and Disease Registry, the Dartmouth SYNERGY Clinical and Translational Science Institute, and donor funds from the French and Scheuer Families. The investigators have disclosed no relevant financial relationships.

American Academy of Neurology 2017 Annual Meeting (AAN). Abstract S15.  Presented April 24, 2017.

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