Ayotte, P, A Carrier, N Ouellet, V Boiteau, B Abdous, EAL Sidi, ML Château-Degat and E Dewailly. 2011. Relation between methylmercury exposure and plasma paraoxonase activity in Inuit adults from Nunavik. Environmental Health Perspectives

A new study finds a reason behind why mercury exposure increases heart disease risk.

High amounts of methylmercury in a person's blood can inhibit an enzyme that helps prevent atherosclerosis, researchers report in the journal Environmental Health Perspectives.

Atherosclerosis is the buildup of plaque deposits along artery walls that can lead to vessel hardening and eventual blockage. This, in turn, can lead to heart attacks and other cardiac problems. The discovery helps explain how eating excessive amounts of mercury-containing seafood can increase the risk of cardiovascular disease.

Fish are generally considered a necessary part of a heart-healthy diet. However, research shows a link between eating large amounts of seafood - particularly items high in methylmercury - and a higher risk for heart disease. Such a link has been found in populations - including certain Native American populations - that consume above average amounts of seafood.

Methylmercury is a pollutant that persists in the body and the environment. Certain fish species have high mercury levels and continue to be a large source of exposure for people. Elevated levels in the body can lead to well-known neurological and developmental problems, such as memory and attention troubles.

Less well known are its effects on the heart and circulatory system. For example, methylmercury can alter patterns in heart rate and affect blood pressure. These effects may counteract the health benefits of a seafood diet when methylmercury exposure is relatively high.

However, scientists don't know exactly how methylmercury affects cardiovascular health. A study of Inuit adults in Canada provides new evidence about the underlying process in humans, since fish and other seafood dominate the Inuit diet, and therefore, the population is highly exposed to methylmercury.

Scientists measured and compared blood mercury concentrations and activity levels of an enzyme called paraoxonase (PON1). Lab studies show mercury hinders PON1 activity. PON1 is thought to protect against arterial disease by breaking down harmful oxidized lipid fats that, in turn, act on the low-density (LDL) and high-density (HDL) lipid components of cholesterol.

The scientists also measured blood concentrations of selenium, an essential element commonly found in seafood that is thought to protect against mercury toxicity.

As expected, blood mercury levels were linked to a decrease in PON1 enzyme activity, and thus, decreased protection against heart disease. In contrast, selenium levels were associated with increased enzyme activity.

These findings are among the first to present evidence of a process by which methylmercury can lead to a higher risk of heart disease, and one way in which selenium may counteract this process.