Do you still eat & recommend fish as part of a healthy diet? I do. Of course these recommendations have now become species-specific and include other key criteria such as ‘wild not farmed’ and ‘local not imported’ in response to increasing concerns about contaminants especially organic methyl-mercury (MeHg) and the organohalogen pollutants (OHPs or POPs). Most of us understand about biomagnification of MeHg in the food chain which results in the highest levels in the largest fish but did you also know that as the MeHg content rises, the relative Selenium content drops? What are we worried about? Lots actually. A key mechanism behind MeHg toxicity that has come to light is due to its negative interaction with selenium. Now, as many of you know, I just love a good nutritional interaction and this is a great example of one!
It is now understood that MeHg is a highly specific, irreversible inhibitor of Se-dependent enzymes required to prevent and reverse oxidative damage throughout the body (i.e. glutathione peroxidase & thioredoxin reductase), particularly in the brain and neuroendocrine tissues. In fact inhibition of selenoenzyme activities in these vulnerable tissues appears to be the proximal cause of the pathological effects known to accompany MeHg toxicity.
MeHg binds tightly to Se rendering it unavailable for selenoenzyme activities and their synthesis. This makes sense in terms of the cardiovascular, thyroid and oxidative consequences associated with MeHg toxicity. So essentially what MeHg is doing in one sense is inducing a profound selenium deficiency which funnily enough shares a lot of the features of MeHg toxicity: thyroid dysregulation, immune system dysfunctions, & infertility! But wait there’s more…several pieces of research highlight the potency of this relationship showing that we can predict who will manifest Hg toxicity features following exposure by the individual’s Se status e.g. those that maintained reasonable Se levels in spite of the Hg exposure didn’t manifest the toxicity picture. One study on this topic looked specifically at Hg amalgams. Another study fed juvenile rats a diet of fish with a known content of MeHg, and a variable amount of natural or supplemental Se. They found that Se in blood, brain, & spinal cord was positively correlated (r between 0.69 and 0.90) with protection from neurological damage attributable to MeHg.
In response to these findings, there is some discussion about grading the health and safety of fish for human consumption according to their Se:Hg ratio (ideally >1), however, further research in this area suggests this is complicated with evidence that this is not simply species-specific but strongly influenced also by locality (Jones, Butler & Macleod 2013). While an American study found for both saltwater and freshwater fish, some species with ratios >1 had a significant proportion of individual fish with ratios < 1 (Burger & Gochfeld 2013). So what’s the upshot of all this and where do fish oil supplements fit in? Well MeHg levels in TGA approved supplements are extremely low so this is really a non-issue as long as your patients are not shopping for their fish oils overseas and with all our fish eating friends – keep an eye on their mercury levels (blood or hair) – we’ve picked up a few with extremely high levels from seafood intake alone and consider Se the logical first step in addressing high levels.
Want to read more? Check out… https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3760827/pdf/pone.0074695.pdf