Dr. Nick Ralston speaks to me this week about the mercury concerns in fish. In his research, he has found that the selenium content in fish protects us from the toxic effects of mercury. It’s safe to eat the fish!
Click here to view the transcript for #86 It is Safe to Eat Mercury-laden Fish? with Dr. Nick Ralston.
If a fish has more selenium that it does mercury, this aids to detox the mercury from the body. I feel mercury fears in fish are overblown. That’s why I decided to have one of the top mercury researchers on the show. There is no need to avoid fish – a very healthy food – simply due to mercury concerns. I’m not saying to eat fish every day. I just think it should be part of a health diet a few times a week. Note that selenium will not detox other toxins in fish like other heavy metals or PCB’s, etc. This must still be taken into account if you decide to consume fish.
About Nick Ralston
Dr. Nick Ralston leads research programs involving human and environmental health. His group’s current investigations involve evaluating human health effects and risks associated with environmental contaminant exposures and effective remediation and other approaches to reduce environmental health risks. His Ph.D. in Biomedical Research Biochemistry is from the Mayo Medical Center in Rochester, Minnesota, and his B.S. degree in Biology is from Mayville State University in Mayville, North Dakota.
Dr. Ralston’s principal areas of interest and expertise include the full range of mercury toxicology, with a primary focus on its effects on selenium interactions and how these influence quantitative assessments of potential pathological effects of mercury-selenium interactions at all levels, from molecular, to cellular, to physiological, to epidemiological, to ecological and especially in relation to global environmental health. His primary interests are in the pathophysiology of toxic trace element exposures as well as prevention, protection, and remediation strategies.
His current research interests include examinations of the molecular mechanism of methylmercury toxicity, selenium-dependent biochemical processes involved in the neuro- and cardiotoxicity of mercury as well as mercury phytoremediation and other means of diminishing bioaccumulation of mercury in freshwater fish. Other interests include continuation of his work on the biochemical mechanisms of pulmonary particulate pathologies and studies of the physiological basis for the beneficial effects of omega-3 fatty acids in cardiovascular and neurological health.
Mercury Vs. Selenium
It is well established that selenium can counteract mercury toxicity. This antidotal effect has been known since the 1960s and has been noted in all investigated species of mammals, birds, and fish. Until the importance of selenium was understood, this phenomenon was speculated to be the result of selenium binding to Hg, rendering it ineffectual at creating mayhem. It is now understood that selenium’s antidotal effect for counteracting methylmercury occurs by ensuring that normal selenoprotein synthesis is maintained. Therefore, this can be viewed as a matter of molar ratio: the number of selenium atoms versus mercury atoms present or consumed.
Mercury exposure through fish consumption
Human populations are exposed to mercury through fish consumption. Understandably, people are concerned about “eating mercury.” In reality, the health risks of fish consumption vary with fish type and location. Methylmercury exposure risks vary in response to the selenium:mercury molar ratios in fish (the higher the ratio of selenium to mercury, the more likely that selenoprotein synthesis will be undisturbed) as well as individual and regional differences in selenium intake. Environmental availability of selenium is highly variable, abundant in soils of one area and dangerously low in regions only miles away. Although the overall selenium status in the United States is good, differences in relative quantities and quality of food choices can result in individual differences in selenium status.
Oceans are rich in selenium. Thus most ocean fish contain more moles of selenium than of mercury (as presented below). Importantly, the detrimental effects from eating seafood were associated with maternal consumption of pilot whale, a marine mammal that contains much more mercury than selenium.
Although typical varieties of ocean fish are selenium-rich, the selenium in freshwater fish is more variable and may be limited in certain regions. The selenium levels in lake fish reflect the regional selenium levels in the soils. Methylmercury concentrations are higher in
fish living in lakes where selenium availability is limited. Likewise, the lakes that have higher selenium availability tend to have fish with low methylmercury levels. In studies of low selenium–high methylmercury lakes, adding selenium to the lakes resulted in mercury levels in fish diminishing by more than 75%.
Therefore, the risks of consuming freshwater fish will vary, depending on their mercury and selenium levels. So while ocean fish offer nutritional benefits, the benefits of freshwater fish can be regionally specific. Although selenium levels may not be known for most lakes, local consumption advisories will inform you of the mercury levels in fish, so look to these to determine meal frequency and portion size. Freshwater advisories may change as health officials better understand the local and regional selenium:mercury levels.