There is an important association between eating fish and seafood with higher levels of mercury and being at a higher risk of developing amyotrophic lateral sclerosis (ALS), according to a preliminary study. Results of the study will be shared during the 69th annual meeting of the American Academy of Neurology.

“For most people, eating fish is part of a healthy diet,” said study author Elijah Stommel, MD, PhD, a professor of neurology at Dartmouth’s Geisel School of Medicine, a neurologist at Dartmouth–Hitchcock Medical Center, and a fellow of the American Academy of Neurology. “But questions remain about the possible impact of mercury in fish.”

While a growing body of evidence from previous studies has pointed to mercury as a risk factor for ALS – in the U.S., primarily through eating fish contaminated with the neurotoxic metal – the exact cause of the disease is still unknown.

In the case–control study, researchers used questionnaires to assess fish and seafood consumption among 518 study participants (294 who had ALS and 224 who did not) and determine estimated annual exposure to mercury. Participants reported on the amount and type of fish and seafood they consumed, and whether they were bought in stores or caught by fishing. Researchers also measured mercury levels found in toenail samples obtained from ALS patients and compared them to those from participants without ALS.

The results of the study showed that among participants who consumed fish and seafood on a regular basis, those in the top 25 percent of estimated annual mercury levels, based on fish–related intake or toenail clippings, were at a two–fold higher risk of ALS.

“It’s important to emphasize that these results don’t negate the fact that eating fish provides many health benefits,” says Stommel. “However, they do suggest that people may want to choose species that are known to have a lower mercury content and avoid consuming fish caught in waters where mercury contamination is well–documented. More research needs to be done before fish–consumption guidelines for neurodegenerative illness can be made.”

Consumers are encouraged to consult current U.S. Food and Drug Administration health recommendations on fish and seafood consumption, and to check water body specific fish advisories before eating fish caught by angling.