Some 50 years ago now, Chinese researchers began to understand the health risks associated with low selenium status. Cross-sectional studies showed an association between low selenium concentrations in cereal grains, the low selenium status of local citizens, and the incidence of Keshan disease, a heart disease with high death rates. The administration of selenium supplements in intervention studies resulted in significant reductions in the incidence of Keshan disease [Chen 2012]. Selenium status is one of the main factors contributing to the development of Keshan disease.
Selenium and Kashin-Beck disease
Not long afterwards, Chinese researchers realized that Kashin-Beck disease, a disease of the bone, is prevalent in regions of China and Tibet that are poor in selenium. The researchers saw that a deficiency of selenium and iodine was a common factor Kashin-Beck disease regions [Yao 2011].
The Nutrition Intervention Trials conducted in the Linxian province in China yielded some of the first promising results linking selenium supplementation to the reduction of cancer incidence and mortality. The Linxian province at the time was characterized by a selenium-poor diet.
The treatment group that received selenium supplements was the group that showed significant health benefits of the supplementation:
Significantly lower total mortality
Significantly lower cancer mortality
Significantly lower stomach cancer mortality
The reduced mortality rates began to become apparent already after 1 – 2 years of supplementation. The patterns for reductions in cancer incidence generally approximated the patterns for cancer mortality [Blot].
The Linxian study results were exciting because they were published in the Journal of the National Cancer Institute and because they appeared before the results of the Nutritional Prevention of Cancer study in the United States [Clark].
Selenium is a trace element. It exists only in rare quantities in the world. It is produced primarily as a by-product of the process of mining copper. It is not recyclable. It is very unevenly distributed in the soils of the earth.
Consequently, the availability of selenium in grasses and grains and, at the next stage of the food chain, in animals, varies considerably from region to region in the world. The human dietary intakes of selenium vary accordingly around the world.
Selenium a vital nutrient for humans Selenium is a necessary micronutrient that our bodies do not produce. We get our selenium primarily from our diets. Selenium is important for good immune system function, good thyroid function, good reproductive function, and good protection of our cells’ DNA.
In many countries, breast cancer is the most commonly diagnosed and treated form of cancer. A 2014 meta-analysis of 16 studies has shown that there is a statistically significant association between serum selenium status and risk of breast cancer. The lower the serum selenium concentration, the greater the risk of breast cancer [Babaknejad].
What do we know about selenium and breast cancer?
Breast cancer is a frustrating topic for the selenium researcher. There is not enough evidence to permit definitive statements about the effects of selenium supplementation on the prevention of breast cancer.
For example, the Nutritional Prevention of Cancer (NPC) study – a study that showed significant associations between selenium supplementation and reduced risk of colorectal, lung, prostate, and total cancer – did not enroll enough women for the effect of selenium supplementation on breast cancer to be studied [Clark].
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].
There have been some recent research results relating to selenium supplementation and prostate gland tissue. Researchers in The Netherlands have published results showing that a five-week daily intervention with a high-selenium yeast supplement, 300 micrograms daily, is associated with a down-regulation of genes that are involved with cellular growth and proliferation, with cellular immune response, and with inflammation processes. Also down-regulated by the selenium supplementation is the activity of genes involved with wound-healing [Kok 2017].
The Dutch study was a randomized, double-blinded, placebo-controlled study. Interestingly, the researchers saw the opposite effect in the placebo group. In the placebo group, there was an up-regulation of the genes involved in cellular immune response [Kok 2017].
Selenium supplementation and the risk of prostate cancer? What do we know? We need to be careful in interpreting the research results that we have (and we need more research), but, yes, there is evidence for an inverse association between prostate cancer risk and selenium status [Hurst 2012].
As of this writing (April 2017), the protective effect of selenium supplementation against prostate cancer seems to be found in a relatively narrow range of plasma selenium status [Hurst 2012]. Furthermore, there seems to be a U-shaped relationship between selenium status and protection against prostate cancer.
If the concentrations of selenium in the plasma are too low, there is increased risk of prostate cancer. This is a serious concern in many regions of the world.
Authors of a recent meta-analysis of 69 studies of selenium exposure and cancer risk have concluded that high selenium exposure can reduce cancer risk, especially high selenium exposure that is reflected in high plasma or serum selenium status and/or in high toenail concentrations [Cai 2016]. Admittedly, higher selenium intakes (as compared to lower selenium intakes) can affect different forms of cancer differently. We still need more research to sort out which forms and dosages of dietary and supplemental selenium are most effective at reducing cancer risk. In this article, I want to summarize the findings of Dr. Xianlei Cai and colleagues.
What is a meta-analysis of selenium exposure and cancer risk?
A meta-analysis is a research method of combining the data from several selected research studies to reach conclusions that have greater statistical power. In the present case, Cai et al selected 69 studies that met their inclusion criteria. Each one of the 69 selected studies had the following characteristics:
How much selenium from food and supplements do we need on a daily basis? Which bio-markers of optimal selenium status seem to be most useful to answer this question? Dr. Rachel Hurst and Dr. Susan J. Fairweather-Tait, Norwich Medical School, United Kingdom, and their colleagues set out to find answers. The design of their study is very interesting.
They enrolled 119 healthy British men and women aged 50 – 64 years in a randomized, double-blind, placebo-controlled study that lasted 12 weeks [Hurst]. They excluded the following persons from the study:
people with already high plasma selenium status
people with long-term illnesses
people on various medications
people unwilling to discontinue taking vitamins and herbal remedies at least one month prior to the start of the study
selenium-enriched yeast tablets containing 50, 100, or 200 micrograms of a patented organic selenium (SelenoPrecise® preparation delivered by Pharma Nord, Denmark)
selenium-enriched onion meals that provided the equivalent of 50 micrograms of selenium per day
unenriched onion meals that provided the equivalent of less than 4 micrograms of selenium per day
Measurements of selenium
Remember: Selenium is a trace element. We measure its intake in micrograms per day, not milligrams. We measure selenium status in plasma and serum in terms of micrograms per liter (or equivalently, in nanograms per milliliter). Selenium in toenails or hair, then, we measure in micrograms per gram.
The trace element selenium is an essential component of the selenium-dependent enzymes called selenoproteins. We need these selenoproteins for the optimal biological functioning of our cells. Specifically, we need adequate daily intakes of selenium for the protection of cellular DNA, for successful reproduction, for proper thyroid gland function, for protection against infections, for anti-oxidative protection against the damage caused by free radicals, and for the prevention of cancer and heart disease.
What form of selenium is best?
Many of us cannot get enough selenium in our diets, especially if we do not regularly eat Brazil nuts (very high in selenium) or if we eat relatively little meat and fish. However, there are various supplemental forms of selenium available to us as consumers.