Sushi is often believed to be a low-calorie food, high in the omega-3 fatty acids that reportedly increase life expectancy for consumers. However, regularly divulging in a tray of sushi rolls may lead to overexposure of mercury, and an increased risk of heart disease, according to a recent study.

The raw fish dish can provide several potential health hazards for consumers due to the mercury levels in some fish. Mercury is a pollutant that is toxic to the nervous system, and it can pose a danger if ingested regularly. Once it enters a waterway, naturally occurring bacteria absorb it and turn into methylmercury — a form absorbed by humans easily, leaving them vulnerable to its effects.

Fish with lower mercury levels contain less than 0.29 parts per million (ppm), while fish with the highest mercury levels have more than 0.3 ppm. Larger fish, such as large tuna, swordfish, shark, and mackerel are known to have the highest levels of mercury because they prey on smaller contaminated fish.

Human consumption of these fish can adversely affect fertility and blood pressure regulation, and can cause memory loss, tremors, vision loss, and numbness of the fingers and toes, says the Natural Resource Defense Council. Increased exposure to mercury may also lead to heart disease.

A team of researchers from Rutgers University, analyzed total mercury intake — realizing on average that about 90 percent of the mercury in fish is methylmercury — and used that percentage to calculate total mercury levels as a means to determine methylmercury intake and risk in a large cohort. More than 1,200 people were interviewed, with a balanced sample in which half were college-aged students and the other half were older people.

The participants were interviewed in places where sushi was consumed or purchase, such as restaurants, sushi bars, and supermarkets. They were approached by researchers who had no prior knowledge of their fish consumption. The researchers conducted 10-minute surveys that asked about the number of fish meals a month, number of fish-sushi and non-fish-sushi meals a month, and the number of sushi pieces and fish-sushi pieces that the participants consumed at a typical sushi meal.

In the study, the researchers distinguished three different types of meals: Fish meals were all meals that have fish, regardless of the source or type; fish sushi meals were sushi meals that contained fish; and fish sushi pieces were the number of sushi pieces consumed that contained fish, according to Medical Xpress.

The respondents were also asked to estimate their intake of sashimi, nigiri, and sushi rolls, but this produced confusion because these forms of sushi could be mixed in a single order, and not many participants knew the distinction. They were also allowed to comment and ask questions upon completion of the interview. Most of the participants believed eating sushi was healthy based on the belief that the Japanese have high longevity, wrote the researchers. Also, the participants were in disbelief that the mercury levels in their sushi was high enough to cause harm.

Over 90 percent of the survey respondents reported consuming an average of five fish and fish-sushi meals per month, with the top 10 percent of respondents exceeding the Center for Disease Control Minimal Risk Level and the World Health Organization Provisional Tolerable Weekly Intake of methylmercury. 

High levels of mercury have been linked to heart disease through the activation of an enzyme known to trigger the process that leads to plaque buildup in blood vessel walls. The activation of mercury typically involves a series of events in the cell membranes that are responsible for releasing phosphatidic acid, which is known to damage cells in the vessel lining — endothelial cells — and is believed to contribute to vascular disorders, according to the Ohio State University Wexner Medical Center.  

Large tuna, such as the Atlantic Bluefin or Bigeye, are known to be the biggest culprits with the highest mercury levels, while sushi made with eel, crab, and salmon were found to have safe levels of mercury.

Despite sushi’s high omega-3 fatty acid content, the high levels of mercury counteract any potential health benefits, such as lower cholesterol levels, incidence of heart disease, blood pressure, stroke, and pre-term delivery, wrote the researchers. Omega-3 supplements may provide a safe and healthy alternative for those who do not wish to consume fish.

The U.S. Food and Drug Administration says adults should eat at least two servings — a serving size is about three ounces — of fish with omega-3 fatty acids each week. Women who are pregnant or are planning to become pregnant, and children under the age of 12, are advised to limit the amount of fish they eat because they are most vulnerable to the potential effects of toxins in fish, such as harming the nervous system.

To learn more about safe fish consumption, click here.

Source: Burger J, Donio M, Gochfeld M, et al. Sushi Consumption Rates and Mercury Levels in Sushi: Ethnic and Demographic Differences in Exposure. Journal of Risk Research. 2013.