Many people believe sushi is a nutritious meal and a smart choice for heart health, but a recent study suggests the Japanese delicacy could actually be bad for you.

Researchers at Rutgers University studied sushi consumption and its role in methylmercury exposure- a type of mercury. They found tuna sashimi, a popular ingredient for sushi, contains the highest levels of methylmercury in fish.

The effects of methylmercury exposure in humans as a result of excessive fish consumption can include neurodevelopmental deficits, poorer cognitive performance and increased rates of cardiovascular disease.

The consumption of sushi has increased greatly in the United States and other countries; however, there is very little data on the cuisine.

"In the [United States], most attention to the risks from contaminants in fish has been devoted to  either self-caught fish or to commercial fish available in fish markets and grocery stores," according to the researchers report. "There is a growing trend for the consumption of sushi [and] there is very little quantitative data on either consumption patterns of sushi or contaminants in sushi."

Researchers found higher levels of methylmercury can be detrimental to the positive effects of omega-3 fatty acids, which are known to reduce cholesterol levels, reduce the risk of some cancers and incidence of heart disease, blood pressure, stroke, and pre-term delivery. It does this by countering some of the "cardioprotective" effects of omega-3 fatty acids.

For the study, mercury levels in sushi samples from across the United States were examined and more than 1,200 people were interviewed about their consumption of sushi and other fish producers and mercury samples

They found that Large tuna, such as the Atlantic Bluefin or Bigeye, which are prized for sushi contain the highest mercury levels and that the demand for high-grade tuna for sushi has placed the species into jeopardy by overfishing.

Ninety-two percent of participants consumed an average of five fish and fish-sushi means per month and the top 10 percent of all participants across all ethnic groups exceeded the Center for Disease Control Minimal Risk Level and the World Health Organization Provisional Tolerable Weekly Intake of methylmercury, according to a press release.

Sushi made with eel, crab, salmon and kelp were found to have lower levels of methymercury.