“We didn't see any evidence that higher levels of mercury were linked to higher cardiovascular harm," says researcher Dariush Mozaffarian, MD, DrPH, an associate professor of medicine and epidemiology at Brigham and Women's Hospital and Harvard Medical School, Boston.
''At exposure levels commonly seen in the U.S., we didn't see any evidence of harm," he tells WebMD. Mozaffarian cautions that his study, which followed nearly 7,000 men and women, was in adults only and that caveats about limiting intake of high-mercury fish still hold for pregnant women, nursing mothers, and children.
However, the new research is good news for others who eat seafood in moderation, another expert says. "It's reassuring that mercury [from fish] does not appear to be a major cause of cardiovascular disease, based on this major study," says Gina Solomon, MD, senior scientist at the Natural Resources Defense Council. The environmental action group has studied mercury in fish and posts a guide on its web site detailing how to eat fish more safely.
The study findings are reported in The New England Journal of Medicine.
Mercury Exposure and Heart Disease: Background
But fish is also a major source of mercury exposure, and chronic low levels have been linked with brain development delays in infants. That has triggered advice that intake of fish high in mercury be avoided in women who are pregnant or nursing and in children.
But in adults who aren't pregnant or nursing, the main concern has been a potential for cardiovascular toxicity.
Previous research has produced conflicting findings, Mozaffarian tells WebMD, so he conducted the new study.
Mercury Exposure and Heart Disease: Study Details
Mozaffarian and colleagues evaluated data from two large studies that included more than 51,000 men from the Health Professionals Follow-up Study (begun in 1986) and more than 121,000 women enrolled in the Nurses' Health Study (begun in 1976).
Every two years, the participants answered questions about their medical history, risk factors, disease, and lifestyle.
The researchers zeroed in on 3,427 participants who did not develop heart disease during the follow-up and another 3,427 who did. Mercury levels were evaluated from toenail clippings the participants provided. Toenail clippings are an excellent biomarker for mercury, Mozaffarian says, because mercury binds tightly to the protein in the toenail.
They also evaluated levels of selenium, a trace nutrient some think protects against mercury toxicity, in the clippings.
The median follow-up (half were longer, half less) from time of sampling to the time of an event was 11.3 years. Men's average age at the study start was 61; women, 53.
Those with higher levels of mercury concentration did not have a higher risk of cardiovascular events. Levels of selenium, whether high or low, weren't linked with adverse effects.
When the researchers compared those who had the highest mercury levels with the lowest, they found a trend toward lower cardiovascular disease risk with the higher mercury levels. They speculate that is because of the other beneficial effects of eating fish.
Before they adjusted for such factors as age, the researchers found higher mercury levels linked with high cholesterol. But Mozafarian says that finding could simply be because of age or that those with high cholesterol were eating more fish to get healthier.
"If you are not pregnant, nursing, or trying to become pregnant, there is no reason to be concerned about mercury levels in fish," Mozaffarian tells WebMD. "Fish is part of a healthy diet."
He does suggest that people who eat fish very often -- say, five times a week or more -- eat a variety and not just fish that have higher mercury levels. Among those varieties with higher mercury levels are shark, swordfish, king mackerel, and tile fish.
Mozaffarian reports funding from GlaxoSmithKline, Sigma-Tau Pharmaceuticals, and Pronova BioPharma (which makes omega-3 derived pharmaceutical products).
Mercury Exposure and Heart Disease: Perspective
The study shows that "the levels of mercury in the fish most Americans are eating are not high enough to offset the positive effects,'' says Solomon, who is also an associate clinical professor of medicine at the University of California, San Francisco.
It is possible, Solomon and Mozaffarian say, to eat so much fish that side effects from the mercury can develop, such as numbness of the fingers and toes and muscle weakness. But those side effects aren't typical with moderate consumption.
Of the study findings, Solomon says, "it doesn't change my advice to my patients, which is to consume seafood with attention and in moderation."
The Dietary Guidelines for Americans, 2010, suggest adults increase their relatively low intake of seafood, noting that about 8 ounces a week has been linked in some research with reductions in cardiac deaths in healthy people. According to the guidelines, pregnant and breastfeeding women can eat 8 to 12 ounces of seafood per week from a variety of seafood types. They should limit white (albacore) tuna to 6 ounces per week and avoid eating fish with higher mercury levels.
Low-mercury fish and seafood includes salmon, sardines, scallops, and shrimp, among others, Solomon says.