Mercury levels considered safe pose a risk to the heart.
Yaginuma-Sakurai, K, K Murata, M Shimada, N Kunihiko, N Kurokawa, S Kameo and H Satoh. Intervention study on cardiac autonomic nervous system effects of methylmercury from seafood. Neurotoxicology and Teratology doi.10.1016/j.ntt.2009.08.009.
Researchers in Japan have found that eating fish tainted with methylmercury – at levels currently considered safe for human exposure – can lead to a slightly unstable resting heart rate. Over time, these types of changes are associated with an elevated risk of developing coronary heart disease.
The fish also contain high levels of fish oils – including omega 3 fatty acids – that are generally considered to be protective for the heart. But, in this study, eating more fish oil in tandem with the methylmercury did not prevent the heart anomalies.
The results emphasize that the health benefits and risks of fish consumption must be considered collectively when making dietary recommendations.
Even though higher methylmercury exposure increased the amount of variability in the resting heart rate, the changes were not permanent. The abnormalities improved as fish consumption – and the exposure to the methylmercury – decreased.
The data from this study are particularly significant because direct experimental studies on humans are relatively rare. The study provides compelling information not readily obtained from epidemiology studies, which can only look for associations between exposure and the effect of interest.
Resting heart rate is regulated by the autonomic nervous system and should be steady. A disruption to this type of nervous system control is called a sympathovagel imbalance. It can produce subtle – but significant – changes to the resting heart rate. The slightly varied beating is associated with an increased risk of developing cardiac disease.
Methylmercury is a potent neurologic toxicant. Mercury released from industries, such as mining and coal burning power plants, settles in the environment. There, it can be transformed into the more harmful methylmercury. Many species of fish are contaminated with the pollutant.
To keep people's exposure within a range that is considered safe, governments issue recommendations about how much fish to eat. Fatty fish – especially long-lived predators such as tuna, swordfish and tilefish – generally have the highest levels of methylmercury. Exposure in adults can cause a range of neurologic problems including trouble speaking, difficulty maintaining balance, memory impairment and loss of coordinated movement. Exposure to the unborn can lead to even more severe and permanent effects; therefore pregnant woman should minimize their risk of exposure.
Researchers divided 54 healthy volunteers into two groups. One group ate tuna and swordfish for 14 weeks at levels high enough to reach the weekly methylmercury levels deemed tolerable by the Japanese government (3.4 micrograms per kilogram (µg/kg) of body weight per week).
Mercury exposure levels were monitored throughout the study using hair samples. Baseline methylmercury levels were approximately the same in both the control and treatment group prior to the study and within the range typically seen in the Japanese population.
Methylmercury levels in the hair samples rose steadily across the 14-week treatment period. Average levels started at 2.3 µg/g and peaked at around 8 µg/g just after the exposure period ended. Levels slowly declined back to near baseline – 4.9 µg/g – during the next 15 weeks. No other health effects – including changes in body weight or overall heart rate – were observed in the subjects.