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.
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.
In my mind, I keep coming back to the question of heart health and the role that selenium supplements play in heart health. That may surprise many of you because we tend to think of selenium supplements primarily for the prevention of cancer and for the prevention of thyroid disorders and for protection against the toxic effects of heavy metals like mercury and cadmium. We know that our immune system needs adequate intakes of selenium if it is to function optimally. That’s correct, isn’t it?
Selenium supplements and good heart health
But, what about the relationship between selenium intakes and status and heart health? Writing in the medical journal, The Lancet, Professor Margaret P. Rayman, University of Surrey, in Guildford, United Kingdom, has listed and documented the following ways that adequate selenium status and optimal levels of selenoproteins can potentially benefit the heart [Rayman 2012]:
Recently, I did a search of the Medline database on the subject of selenium supplementation and its effect or non-effect on glucose metabolism, insulin resistance, glycemic control, etc. I limited my search to reports of data from randomized, controlled trials.
Altogether, the Medline search yielded 48 hits. Reading through the abstracts of the 48 journal articles, I was able to eliminate 30 references from consideration. These 30 eliminated studies were studies that included all of my search terms but did not actually measure the effect of an intervention with selenium on some aspect of glucose metabolism.
18 good selenium supplementation studies
I was left with 18 good intervention studies that met my search criteria. I read these studies and separated them into three distinct categories related to the effect of selenium supplementation on glucose metabolism and insulin sensitivity:
In 2013, Dr. Margaret Rayman, University of Surrey, United Kingdom, reviewed the published reports of randomized controlled trials in which a selenium supplement had been used as a single-agent treatment option and in which there had been a follow-up or sub-group analysis of the effect of the selenium supplementation on the risk of type-2 diabetes.
Dr. Rayman found 5 such studies. I want to summarize Dr. Rayman’s review, and then I want to see what studies have been done since 2013.
Selenium and diabetes studies Nutritional Prevention of Cancer (NPC) study
In 2007, Dr. Saverio Stranges published the results of a post hoc analysis of the data from the Nutritional Prevention of Cancer study. In the NPC study, supplementation for an average of 4.5 years with 200 micrograms of a high selenium yeast preparation resulted in significant reductions in the risk of lung cancer, colon cancer, and prostate cancer [Clark 1996]. The age of most participants in the study was 63 years, plus or minus 10 years.
Selenium supplements – especially selenium supplements as a component of a multi-micronutrient cocktail – can help to delay the decline of the immune system and can reduce the risk of death in HIV-infected patients. Most of the data that we have comes from randomized controlled studies carried out in African countries, but the results are relevant to the United States and Europe. Moreover, the results from studies of HIV-infected patients speak to the issue of the anti-microbial protection and antioxidant protection that comes with adequate selenium status.
Selenium and HIV and CD4 counts
CD4 cells are white blood cells that are part of the immune system. Specifically, the CD4 cells fight infections in the body. The HIV virus kills CD4 cells. When a person has fewer CD4 cells, he or she is at greater risk of contracting an infection.
There is a clear relationship between a pregnant woman’s selenium status and her risk of pre-eclampsia. Observational studies show that low selenium status is associated with a greater risk of pre-eclampsia. Studies of selenium supplementation have shown reduced incidence of pre-eclampsia [Xu].
What is pre-eclampsia?
Pre-eclampsia is a complex condition characterized by the onset, around week 20 of the pregnancy, of the following symptoms:
abnormally high protein levels in the urine (indicating possible damage to the kidneys)
systolic blood pressure greater than 140 mmHg or diastolic blood pressure greater than 90 mmHg when measured twice with an interval of at least 4–6 hours and not more than 7 days apart [Uzan]
decreased platelet levels in the blood [Mayo Clinic]
Pre-eclampsia seems to be caused by an abnormal development of the new blood vessels that are needed to carry blood to the placenta. The new blood vessels are narrower than they should be, and they respond differently to hormonal signaling [Mayo Clinic].
Here is a little-known fact. Human testicles contain high concentrations of one of the important selenium containing antioxidant enzymes, the selenoprotein glutathione peroxidase number 4 (GPx4). Furthermore, the testicles have special receptors for another of the important selenoproteins, the selenoprotein P [Rayman 2012].
Adequate intakes of selenium for the synthesis of the selenoproteins are necessary to produce mature and viable sperm. The selenoprotein GPX4 seems to be indispensable:
to the development and maturation of the sperm cells
to the antioxidant protection of the sperm cells
to the structural cohesiveness of the sperm cells
to the motility and viability of the sperm cells [Foresta]
Selenium and semen quality
What are we talking about here? Let’s define some terms that are relevant to male fertility.
Our bodies produce semen by combining sperm cells from the testicles with various fluids from the seminal vesicles, the prostate, and the two small glands called Cowper’s glands. The semen needs to contain high counts of sperm cells – perhaps 200 million plus – and needs to contain sperm cells capable of good motility if one of the sperm cells is to succeed in reaching and fertilizing the mature female egg.
Iodine is an essential component of the thyroid hormones. After iodine, selenium is arguably the micronutrient most important to the thyroid gland. Proportionally, there is more selenium in the thyroid gland than there is selenium in any other organ in the body. There are good reasons for the presence of selenium in the thyroid gland. Iodine and selenium are both required for thyroid hormone synthesis and function.
Our bodies do not make selenium. We must get the selenium that we need from our food and from supplements. The selenium that we absorb is incorporated into the amino acid selenocysteine. Selenocysteine, then, is a necessary component of some 25 selenoproteins that are needed for various biological functions.
The proper daily dosage of selenium for normal people? Normal people? How many of us are approximately normal? 68 percent of us, perhaps? Yes, we humans are more the same in many ways than we are different. However, there is considerable biochemical variation amongst us humans. So, it is difficult to say who is average and normal and then suggest an ideal daily dosage of selenium.
What do the numbers from selenium studies say?
Let’s look at the numbers from published research and see what sense we can make of them. Remember: we humans need adequate plasma selenium concentrations for optimal antioxidant and anti-viral and anti-carcinogenic protection [Schrauzer 2009].
Putative beneficial range for plasma selenium status Hurst and Fairweather-Tait et al, researchers based at Norwich Medical School, University of East Anglia, in the United Kingdom have suggested that the “putative beneficial range” lies between 120 and 150 nanograms of selenium per milliliter of plasma [Hurst 2010].