The negative impact of high amounts of methylmercury in seafood on blood pressure may outweigh the protective effects of fish nutrients, researchers report in Hypertension: Journal of the American Heart Association.

Researchers found that even when blood pressure was within the normal range and numerous other factors, including omega-3 fatty acids and selenium were carefully controlled for, the environmental mercury was associated with higher blood pressure and pulse pressure among Nunavik Inuit adults in a recent study.

Researchers conducted a survey in the 14 Nunavik communities in northern Quebec, Canada, where the traditional diet is based mainly on fish and marine mammals, and thus, residents regularly ingest higher levels of environmental mercury. Individuals taking medication for high blood pressure during data collection were excluded. The survey featured two-stage stratified random sampling and data from 732 Inuit adults (319 men and 413 women), average age 34.
The average total mercury blood concentration in people in the United States is now 4 nmol/L (NHANES study) compared to 50 nmol/L in the Inuit population.

Researchers found a 10 percent increase in blood mercury was associated with an increase of 0.2 mm Hg in systolic blood pressure after controlling for other factors.

"The fish nutrients did not modify the relationship between mercury and blood pressure," said Eric Dewailly, M.D., Ph.D., lead author of the study, and professor in the Department of Preventive Medicine at Laval University and at the National Institute of Public Health, in Quebec, Canada.

"It's known that chronic exposure to low concentrations of environmental mercury increases oxidative stress, which leads to a reduction in nitric oxide bioavailability, endothelial dysfunction and a decrease in smooth muscle relaxation," Dewailly said. "Mercury exposure also results in decreased acetylcholine levels in rats. And a decrease in these levels could also lead to decreased relaxation of the smooth muscle, indirectly increasing blood pressure."

Researchers collected information on age and gender as well as possible confounding factors such as smoking, alcohol, physical activity and socioeconomic status. They collected and analyzed blood samples to determine levels of mercury, lead, selenium, cholesterol, triglycerides and insulin sensitivity. They also measured waist circumference and waist-hip ratio, to assess the effect of obesity. Researchers did not control for sodium intake.

Furthermore, researchers recorded each participant's average systolic blood pressure and diastolic blood pressure and calculated pulse pressure. They incorporated population weights into all statistical analyses so results could be generalized to the entire population of Nunavik.

Omega-3 fatty acids are well known for their heart health benefits. Researchers said this may be the first study to evaluate the influence of omega-3 and selenium on the association between mercury and blood pressure. The team plans to use the findings as a baseline in a future study in the same population to confirm if mercury exposure caused the development of high blood pressure.

"Our results suggest that eating large quantities of species containing a high mercury content and low omega-3 such as big predator fish (big tuna, swordfish, marlin, sharks, etc.) is not a good idea for our health," Dewailly said.

The American Heart Association recommends two 4-6 ounce fish meals a week, with an emphasis on oily fish, such as sardines, herring, trout and salmon, which are high in omega-3 fatty acids. Diets rich in omega-3 fatty acids have been associated with a reduced risk of both sudden cardiac death and death from coronary artery disease.

Contamination of certain fish with methylmercury and other contaminants is a potential concern, so children and nursing women are advised by the U.S. Food and Drug Administration (FDA) to avoid eating those fish with the potential for the highest level of contamination - shark, swordfish, king mackerel or tilefish. For middle-aged and older men, as well as post-menopausal women, the cardiovascular benefits of eating fish far outweigh the risks posed by mercury contamination, according to the FDA.

Co-authors are Beatrice Valera, Ph.D. candidate and Paul Poirier, M.D., Ph.D. Author disclosures are on the study.

This study was funded by the Quebec Ministry of Health, Nunavik Regional Board of Health and Social Services, the Canadian Institutes for Health Research, the Fonds en Recherche en Sante du Quebec, the Northern Contaminants Program and the ArcticNet network.