Thursday, 1 March 2012

Selenium

A few years back I had the pleasure of attending a fascinating presentation by Prof. Margaret Rayman of the University of Surrey, UK on the role of selenium on human health. Some UK viewers in particular might have heard about Prof. Rayman and her work on selenium being potentially tied into all sorts of things including the immune system and in particular, the various selenoproteins whose biological roles are pretty varied and widespread. The presentation I attended did have a slight autism research slant to it with the caveat that the very limited peer-reviewed literature looking at selenium and autism is pretty mixed at the moment (here and here) and some of the discussion was quite speculative about aspects potentially 'surrounding' some cases of autism.

Prof. Rayman has just published an updated review of selenium in relation to human health* in the Lancet which can be viewed here. I thought therefore it might be a good time to summarise some of the areas potentially pertinent to this blog where selenium might be useful, alongside the balance required between health and toxicity when looking at selenium intake.

Selenium comes from various dietary sources and is to quite a large degree dependent on the local geography (concentrations in soil) and rainfall. Selenium intake varies quite a lot from area to area with lower intake linked to things like the drop in atmospheric deposition from reductions in fossil fuel burning to advances in food technology increasing grain yields but at the same time reducing plant selenium concentrations. For quite a few years now, there has been some evidence of a steady decline in biological levels of selenium in certain parts of the world like the UK as per studies like this one.

Having read that and perhaps thinking 'should I be taking more?' I would caution that striking a balance in your selenium intake is important. Too little might be deficiency; too much might also cause some pretty severe issues as per this report** based on what happens when a supplement contains too much selenium. I should also at this point reiterate my caveat on not giving medical advice or endorsing selenium supplementation without the required medical input.

The thing that always stood out for me with regards to selenium was the wide and varied range of biological processes that show some relationship to it. So areas as diverse as fertility and reproduction to viral virulence to cancer have been linked to selenium and adequate consumption. Granted, the evidence has not been all one-way, as per the results of the SELECT trial which looked at selenium + vitamin E supplementation for preventing prostate cancer, but still there is at least some evidence for assuring that we all have adequate levels of selenium in our biology to keep healthy. It is important also to remember that selenium is the only trace mineral specified in the genetic code (as selenocysteine). Cysteine..mmm, now where have I read about that recently?

Going back to those selenoproteins identified in humans, 25 of them at the time of writing, these are coded for in the human genome and carry various functions. The obvious target for this blog would be the glutathione peroxidases (GPXx) whose important antioxidant functions have been mentioned in autism research circles alongside the potentially important role of glutathione (GSH) to some cases of autism. To try and succinctly summarise what the GPXx do: its all about respiration and oxygen forming free radicals and reactive oxygen species which in conjunction with lipids/fats tie into hydroperoxides which can damage membrane function and increase things like inflammation; but before getting this far GPXx sets in to catalyse the removal of said hydroperoxides to form alcohols and water. Phew.

The inner workings of the immune system also tie into selenium although studies on humans are fewer than animal models. Selenium deficiency would normally be followed by a loss of immunocompetence; so things like B cell function has been noted to show some relationship, as has a possible link with immunotolerance. There is even some suggestion that selenium deficiency might (might!) also tie into malabsorptive states such as that seen in coeliac disease with onward 'effects' to some other interesting functions covered previously on this blog. Speculative but interesting nevertheless.

One final area that I just want to mention is that of methylation and selenium in light of my new found interest in all things folate cycle related. Selenium has the propensity to affect methylation specifically with the selenium-homocysteine relationship in mind and the inverse relationship suggested between the two parameters***. Make of that what you will.

All in all, selenium is an interesting compound. I will perhaps end with a rather long quote from Prof. Rayman: "The crucial factor that needs to be emphasised is the inextricable U-shaped link with selenium status: additional selenium intake (eg, from food fortification or supplements) may well benefit people with low status. However, people of adequate or high status could be affected adversely and should not take selenium supplements". 'Nuff said.

To finish, a tribute to Davy Jones. Rest in peace.

* Rayman M. Selenium and human health. Lancet. February 2012.
DOI: 10.1016/S0140-6736(11)61452-9

** MacFarquhar JK. et al. Acute selenium toxicity associated with a dietary supplement. Archives of Internal Medicine. 2010; 170: 256-261.
DOI: 10.1001/archinternmed.2009.495

*** Combs GF Jr. et al. Determinants of selenium status in healthy adults. Nutrition Journal. 2011; 10: 75.
DOI: 10.1186/1475-2891-10-75