Many people think of fish and seafood as being healthy. However, new research suggests eating certain species that tend to have high levels of mercury may be linked to a greater risk of developing amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease.

Questions remain about the possible impact of mercury in fish, according to a preliminary study released Monday that will be presented at the American Academy of Neurology's 69th annual meeting in Boston in April.

Fish and seafood consumption as a regular part of the diet was not associated with ALS, the study said.

"For most people, eating fish is part of a healthy diet," said study author Elijah Stommel of Dartmouth College in Hanover, N.H., and a fellow of the academy.

In addition, the authors said their study does not negate the fact that eating fish provides many health benefits.

Instead, it suggests people may want to choose species that are known to have a lower mercury content, and avoid consuming fish caught in waters where there is mercury contamination.

The researchers stressed that more research is needed before fish consumption guidelines for neurodegenerative illness can be made.

Previous studies point to risk factor

While the exact cause of ALS is not known, some previous studies have suggested the neurotoxic metal to be a risk factor for ALS, a progressive neurological disease.

For the study, researchers surveyed 518 people — which included 294 who had ALS and 224 who didn't. They were asked how much fish and seafood they ate. Participants reported the types of fish they ate, and whether they were purchased from stores or caught when they were fishing.

The study found that among participants who ate fish and seafood regularly, those in the top 25 per cent for estimated annual mercury intake were at double the risk for ALS compared to those with lower levels.

A total of 61 per cent of people with ALS were in the top 25 per cent of estimated mercury intake, compared to 44 per cent of people who did not have ALS.

Mercury binds tightly to the proteins in fish tissue. Most fish will contain trace amounts of mercury, depending on the level of mercury in their environment and their place in the food chain.

Larger fish generally contain more mercury

The bigger the fish and the higher up the food chain it is, the more mercury it will tend to contain. Large predatory fish species such as swordfish, shark and certain kinds of tuna tend to have higher levels than non-predatory fish or species farther down the food chain.

Health Canada advises Canadians to limit consumption of fresh or frozen tuna, shark, swordfish, escolar, marlin, and orange roughy. In general, you can eat up to 150 grams per week of these fish species combined. For tuna, that's roughly the amount sold in one can.

Women who are or may become pregnant and breastfeeding mothers are advised to limit consumption to 150 grams per month. Young children between five and 11 years of age can eat up to 125 grams per month. Very young children between one and four years of age should eat no more than 75 gram per month of these fish species.

The study was supported by the Diamond Endowment Fund, the ALS Association, the Centres 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.