Selenium, Fish, and Mercury: Important Facts

Fish should be an important part of our diets. Professor Nick Ralston at the University of North Dakota has drawn our attention to the webpages entitled Fish, Mercury, and Nutrition: The Net Effects. The URL is http://net-effects.und.edu/factsheets.aspx

The Cover of the Fish, Mercury, and Nutrition brochure
The Fish, Mercury, and Nutrition webpages contain fact sheets, a documentary video, video clips, and links to informative websites.

On these webpages, Professor Ralston and his colleagues explain why the benefits of regularly eating fish, especially ocean fish, are often overlooked. Too many of us have cut back on our fish consumption because we have been worried about exposure to toxic mercury.

However, we have been receiving insufficient information about the relative benefits and risks of eating ocean fish. In fact, our avoiding fish meals may be having negative health consequences in some cases. read more

Selenium and Mercury Toxicity

A plate of salmon.
Eating fish will give pregnant women and children selenium and other nutrients that will promote the children’s growth and development. Eating fish may give adults heart health benefits. However, some ocean fish contain more mercury than selenium and should therefore be avoided. Consequently, the US Food and Drug Administration advises against eating meals from predatory whales, sharks, swordfish, king mackerel, marlin, orange roughy, tilefish, and big-eye tuna. Most other ocean fish will have more selenium than mercury in their tissues and should be safe, even advisable, to eat.

The selenium in our cells is the molecular “target” of toxic mercury. Inhibition of the normal biological activity of seleno-enzymes is the mechanism by which mercury damages our cells, most particularly our brain and nerve cells [Ralston & Raymond 2018].

Conceiving of selenium as the “target” of mercury leads to a better understanding of mercury toxicity than the old theory of selenium as the “tonic” that binds toxic mercury in a form that is no longer harmful [Ralston & Raymond 2018].

Professor Nicholas Ralston and consultant Lisa Raymond have done a review of the research literature about the characteristics of mercury toxicity to identify the selenium-dependent aspects of mercury’s biochemical mechanisms and effects. Their conclusions [Ralston & Raymond 2018]: read more

Selenium Intake and Status Related to Health

The quantity of selenium in foodstuffs may be inadequate in many parts of the world.  Sub-optimal selenium status is reported to be widespread throughout Europe, the UK, and the Middle East [Stoffaneller & Morse]. Coastal regions in the US tend to have selenium-poor soil. Vast regions in China, Korea, Siberia, Tibet, and New Zealand are low selenium regions. Low selenium status is associated with increased risk of cancer and cardiovascular disease and thyroid disorders [Tolonen].
The research evidence to date suggests that there is a U-shaped relationship between selenium intake and health.  According to a recent report by the long-time selenium researcher Professor Dr. Margaret P. Rayman, University of Surrey, UK, both selenium deficiency and selenium excess have been associated with adverse health effects.

Conditions Indicating a Need for Selenium Supplementation

Professor Rayman lists a number of conditions that have been associated in the research literature with selenium deficiency:

  • Keshan disease (a heart muscle disease caused by a selenium deficiency together with a strain of Coxsackie virus)
  • Kashin-Beck disease (a bone disease for which selenium deficiency is a factor)
  • Increased viral virulence
  • Increased mortality
  • Poorer immune function
  • Problematic fertility/reproduction
  • Thyroid autoimmune disease
  • Cognitive decline/dementia
  • Type-2 diabetes
  • Prostate cancer risk
  • Colo-rectal cancer risk (in women)
  • Increased risk of tuberculosis in HIV patients

Professor Rayman does not specify a plasma/serum selenium level for selenium deficiency

She does mention a US National Health and Nutrition Examination Survey that measured the serum selenium levels in 13,887 adult participants and then followed up for mortality for up to 18 years.  The mortality in that study showed a U-shaped association between serum selenium and death, with a serum selenium concentration of 135 micrograms per liter at the bottom of the U [Rayman 2019]. read more

Mercury’s neurotoxicity and disruption of selenium biochemistry

The brain is particularly vulnerable to oxidative damage in the absence of adequate antioxidant selenoprotein protection for a variety of reasons: the brain has limited antioxidant enzyme pathways, the brain has high iron content, and the brain contains many long-chain polyunsaturated fatty acids, which are vulnerable to lipid oxidation. Oxidative damage to the brain results in structural and functional damage to brain cells and tissues. A selenium yeast supplement has proven effective at reducing the levels of bio-markers of lipid peroxidation and oxidative damage to DNA. An exclusively selenomethionine supplement was not effective [Richie 2014].
Selenium containing antioxidant selenoproteins play an important role in the prevention and reversal of oxidative damage in the brain.  This role has generally been underestimated in studies of the toxicity of elemental mercury and methylmercury.  The common understanding has been that selenium helps to prevent mercury toxicity by binding with mercury and rendering the mercury inactive.

This chemical binding and inactivation of mercury does take place.  Mercury has a great affinity for selenium, estimated to be approximately one million times stronger than mercury’s affinity for sulfur.  So, selenium’s binding with mercury in the tissues does keep the mercury from getting into mischief in the brain and spinal cord, peripheral nervous system, and endocrine system. read more

Selenium and mercury and eating fish

Ocean fish – salmon, herring, mackerel, and sardines – are good sources of the omega-3 fatty acids EPA and DHA that have been associated with healthy fetal development, healthy cardiovascular function, and healthy ageing.  Whatever mercury there is in these fish has bound with the selenium in the fish.   This chemical binding has rendered the mercury harmless but has also depleted the amount of selenium available for absorption.

Too many of us are missing out on the health benefits of the omega-3 fatty acids available to us from eating certain types of fish a couple of times a week.  Why are we avoiding fish?  Because many of us are afraid of “eating mercury” in the fish.

It turns out, there is research to show that this is a misconception.  Professor Nick Ralston and his colleagues at the University of North Dakota’s Energy and Environmental Research Center have measured and evaluated the molar ratios of selenium in fish to the mercury in fish [Ralston 2007, 2016].

Their studies show that many of the edible ocean fish have an abundance of selenium in relation to mercury.  So, not only are we missing out on the omega-3 fatty acid benefits, we are also missing out on a good source of dietary selenium [Berry 2008]. read more

Selenium and heavy metals and childbirth

Studies show that higher concentrations of dangerous heavy metals — cadmium, mercury, and lead — in the blood of the mother are likely to be duplicated in the umbilical cord blood, in the blood of the fetus, in the breast milk, and in the blood of the newborn child, with potentially harmful long-term consequences for the child. There is evidence that selenium binds with cadmium and mercury and enables their elimination from the mothers’ bodies.

Early exposure to natural elements that are toxic – cadmium, mercury, and lead – can have long-term adverse health consequences.  Children exposed to these elements while still in the uterus and while breast-feeding may suffer lasting damage to the brain and nervous system and to the kidneys and liver.  The question is: to what extent can selenium supplementation reduce the risk of toxic damage?

Given the dangerous nature of these poisonous heavy metals – cadmium, mercury, and lead – both for the mother and for the fetus and the neonate, it is difficult to carry out randomized controlled studies.  Instead, the best evidence we have for the beneficial effects of selenium supplementation comes from studies that relate the degree of exposure and the selenium status of the mother and the child. read more

Selenium: protection against the accumulation of mercury in the body

Even relatively low exposure to mercury during pregnancy can impair the development of the baby’s brain and nervous system. Fortunately, supplementation with a high-selenium yeast preparation has been shown to reduce significantly the accumulation of mercury in the tissues in the body.

Mercury.  In the form of methylmercury, it is a very harmful biological toxin.  It is a threat to our brains and nervous systems and our livers and our kidneys.  Too much exposure to methylmercury is likely to cause brain damage and nerve damage.

Fortunately, selenium supplements can help.  And they do help.  The relationship of mercury and selenium is a story with an ironic twist.  To the extent that selenium binds with mercury in the body and de-toxifies the mercury – a very good thing for us – to that extent the body is robbed of selenium that could be used for the production of beneficial selenoproteins with other important biological functions.

Protecting us against the toxic effects of mercury means fewer selenoproteins to act as antioxidants neutralizing harmful free radicals, fewer selenoproteins to strengthen immune system function and thyroid function, and fewer selenoproteins to help reduce the risk of cancer. read more