Selenium is an essential trace mineral and functions as part of the antioxidant enzyme
glutathione peroxidase. It is bonded to the essential sulfur amino acid,
methionine, in a naturally occurring form as found in the diet. Selenium is
found most abundantly in Brazil Nuts, organ meats, seafood, and wheat
Someone recently told me that taking selenium supplements is
beneficial. He made it sound like it could reduce the risk of some
cancers by 50% or more, which sounds too good to be true. Is it?
It may be true. A study published in the Journal of the American Medical
Association in 1996 found that people taking 200 micrograms of selenium a
day for over four years had a 46% lower risk of lung cancer, 63% lower risk
of prostate cancer, and 58% reduction in colorectal cancer. Overall, the
cancer risk dropped by 37% compared to people taking a placebo.
If a pharmaceutical company came out with a drug that could reduce the risk
of cancer to this degree, then just about every doctor in the country would
prescribe it. You would likely find full-page ads in newspapers and
magazines: "Ask your doctor for this new drug." Sometimes, when I lecture,
I'll ask people how many were prescribed selenium by their physicians, and
very few raise their hands.
Any time a new therapy comes along, the most important questions to ask
are, "What is the scientific evidence that it is effective? What are the
potential side effects? How much does it cost?" Physicians sometimes refer
to this as the "risk/benefit ratio." In the case of selenium, the potential
benefit is great, the cost is very low, and the side-effects and risk are
very low. Although this study needs to be replicated, the potential
benefits are so great and the risks and costs are so low, I now recommend
selenium for most people. Selenium was first linked with reduced cancer
risk in the 1960s. Theories about how the trace element could work to
prevent tumors include the following: it may act as an antioxidant; it may
be able to alter how the body processes carcinogens; it may have an effect
on how proteins are made, or selenium may play a role in how the immune
From the FDA's website:
"Selenium may reduce the risk of certain cancers. Some scientific
evidence suggests that consumption of selenium may reduce the risk of
certain forms of cancer. However, FDA has determined that this evidence
is limited and should be revisited."
High antioxidant intake, including selenium and tocopherol, has been shown
to protect against chronic diseases.
The research showed that nearly 40 percent of the sample population failed
to reach the desired levels of serum selenium concentrations, (less than
1.1 micromoles per liter), despite eating healthily.
The study followed the diet of 178 women with an average age of 63 over a
three-day period. Blood samples were taken to determine the serum levels of
The results show that low serum alpha-tocopherol and selenium
concentrations are highly prevalent even among well-educated and
well-nourished German women,” said the scientists.
“This result indicates that...women with low selenium antioxidant levels
would possibly benefit from the additional intake of selenium,” recommended
It is believed that selenium and alpha-tocopherol work together to inhibit
lipid peroxidation. “Insufficient status of one of these nutrients may
elevate the risk of oxidative stress and possibly for associated diseases,”
explained the researchers.
Results published in the Archives of Internal Medicine (2004, Vol. 164, pp.
2335-2342) indicated that supplementation of the diet with antioxidants may
lower the risk of cancers. Another study, published in the American Journal
of Clinical Nutrition (Vol. 80, pp. 154-162), reported that increased
selenium intake could boost the immune system.
These reports add to the growing body of evidence for the health benefits
of selenium intake. There have even been recommendations to enrich soil and
fertilizers with selenium to boost public consumption.
Selenium and Health
This section focuses on four diseases and disorders in which selenium might
play a role: cancer, cardiovascular disease, cognitive decline, and thyroid
Because of its effects on DNA repair, apoptosis, and the endocrine and
immune systems as well as other mechanisms, including its antioxidant
properties, selenium might play a role in the prevention of cancer.
Epidemiological studies have suggested an inverse association between
selenium status and the risk of colorectal, prostate, lung, bladder, skin,
esophageal, and gastric cancers. In a Cochrane review of selenium and
cancer prevention studies, compared with the the lowest category of
selenium intake, the highest intake category had a 31% lower cancer risk
and 45% lower cancer mortality risk as well as a 33% lower risk of bladder
cancer and, in men, 22% lower risk of prostate cancer. The authors found no
association between selenium intake and risk of breast cancer. A
meta-analysis of 20 epidemiologic studies showed a potential inverse
association between toenail, serum, and plasma selenium levels and prostate
Randomized controlled trials of selenium supplementation for cancer
prevention have yielded conflicting results. The authors of a Cochrane
review concluded, based on nine randomized clinical trials, that selenium
might help prevent gastrointestinal cancers but noted that these results
need to be confirmed in more appropriately designed randomized clinical
trials. A secondary analysis of the double-blind, randomized, controlled
Nutritional Prevention of Cancer Trial in 1,312 U.S. adults with a history
of basal cell or squamous cell carcinomas of the skin found that 200
mcg/day selenium as high-selenium baker’s yeast for 6 years was associated
with a 52% to 65% lower risk of prostate cancer. This effect was strongest
in men in the lowest tertile of selenium concentrations who had a baseline
prostate-specific antigen (PSA) level of 4 ng/mL or lower. The Selenium and
Vitamin E Cancer Prevention Trial (SELECT), a randomized, a controlled
trial in 35,533 men aged 50 years or older from the United States, Canada,
and Puerto Rico was discontinued after 5.5 years when analyses showed no
association between supplementation with 200 mcg/day selenium with or
without 400 international units (IU)/day vitamin E and prostate cancer
risk. An additional 1.5 years of follow-up data on participants after they
stopped taking the study supplements confirmed the lack of a significant
association between selenium supplementation and prostate cancer risk.
In 2003, the FDA allowed a qualified health claim on foods and dietary
supplements containing selenium to state that while "some scientific
evidence suggests that consumption of selenium may reduce the risk of
certain forms of cancer... FDA has determined that this evidence is limited
and not conclusive". More research is needed to confirm the relationship
between selenium concentrations and cancer risk and to determine whether
selenium supplements can help prevent any form of cancer.
Selenoproteins help prevent the oxidative modification of lipids, reducing
inflammation and preventing platelets from aggregating. For these reasons,
experts have suggested that selenium supplements could reduce the risk of
cardiovascular disease or deaths associated with cardiovascular disease.*
The epidemiological data on the role of selenium in cardiovascular disease
have yielded conflicting conclusions. Some observational studies have found
an inverse association between serum selenium concentrations and the risk
of hypertension or coronary heart disease. A meta-analysis of 25
observational studies found that people with lower selenium concentrations
had a higher risk of coronary heart disease. However, other observational
studies failed to find statistically significant links between selenium
concentrations and risk of heart disease or cardiac death, or they found
that higher selenium concentrations are associated with an
increased risk of cardiovascular disease.
Several clinical trials have examined whether selenium supplementation
reduces the risk of cardiovascular disease. In one randomized,
placebo-controlled study, for example, 474 healthy adults aged 60 to 74
years with a mean baseline plasma selenium concentration of 9.12 mcg/dL
were supplemented with 100, 200, or 300 mcg selenium per day or placebo for
6 months. The supplements lowered levels of total plasma cholesterol and
non–high-density-lipoprotein (HDL) plasma cholesterol (total cholesterol
levels minus HDL levels) compared with the placebo group, whereas the 300
mcg/day dose significantly increased HDL levels. Other trials have provided
evidence that selenium supplementation (200 mcg/day) or supplementation
with a multivitamin / multimineral pill containing selenium (100 mcg/day)
does not reduce the risk of cardiovascular disease or cardiac death. A
review of trials of selenium-only supplementation for the primary
prevention of cardiovascular disease found no statistically significant
effects of selenium on fatal and nonfatal cardiovascular events. Almost all
of the subjects in these clinical trials were well-nourished male adults in
the United States.
The limited clinical-trial evidence to date does not support the use of
selenium supplements for preventing heart disease, particularly in healthy
people who already obtain sufficient selenium from food. Additional
clinical trials are needed to better understand the contributions of
selenium from food and dietary supplements to cardiovascular health.
Serum selenium concentrations decline with age. Marginal or deficient
selenium concentrations might be associated with age-related declines in
brain function, possibly due to decreases in selenium’s antioxidant
The results of observational studies are mixed. In two large studies,
participants with lower plasma selenium levels at baseline were more likely
to experience cognitive decline over time, although whether the
participants in these studies were selenium deficient is not clear. An
analysis of NHANES data on 4,809 elderly people in the United States found
no association between serum selenium levels (which ranged from lower than
11.3 to higher than 13.5 mcg/dL) and memory test scores.
Researchers have evaluated whether taking an antioxidant supplement
containing selenium reduces the risk of cognitive impairment in elderly
people. An analysis of data from the Supplémentation en Vitamines et
Minéraux Antioxidants (SU.VI.MAX) study on 4,447 participants aged 45 to 60
years in France found that, compared with placebo, daily supplementation
with 120 mg ascorbic acid, 30 mg vitamin E, 6 mg beta-carotene, 100 mcg
selenium, and 20 mg zinc for 8 years was associated with higher episodic
memory and semantic fluency test scores 6 years after the study ended.
However, selenium’s independent contribution to the observed effects in
this study cannot be determined. The authors of a systematic review that
included nine placebo-controlled studies concluded that the available
clinical evidence is insufficient to determine whether selenium supplements
can prevent Alzheimer’s disease.
More evidence is required to determine whether selenium supplements might
help prevent or treat cognitive decline in elderly people.
Selenium concentration is higher in the thyroid gland than in any other
organ in the body, and, like iodine, selenium has important functions in
thyroid hormone synthesis and metabolism.
Epidemiological evidence supporting a relationship between selenium levels
and thyroid gland function includes an analysis of data on 1,900
participants in the SU.VI.MAX study indicating an inverse relationship
between serum selenium concentrations and thyroid volume, risk of goiter,
and risk of thyroid tissue damage in people with mild iodine deficiency.
However, these results were statistically significant only in women. A
cross-sectional study in 805 adults with mild iodine deficiency in Denmark
also found a significant inverse association between serum selenium
concentration and thyroid volume in women.
Randomized, controlled trials of selenium supplementation in patients with
thyroid disease have had varied results. In one randomized, double-blind,
placebo-controlled trial, 100, 200, or 300 mcg/day selenium for 6 months in
368 healthy adults aged 60 to 74 years had no effect on thyroid function,
even though plasma selenium levels increased significantly. Another
randomized, double-blind, placebo-controlled trial compared the effects of
200 mcg/day selenium (as sodium selenite), 1,200 mg/day pentoxifylline (an
anti-inflammatory agent), or placebo for 6 months in 159 patients with mild
Graves’ orbitopathy. Compared with patients treated with placebo, those
treated with selenium but not pentoxifylline reported a higher quality of
life. Furthermore, ophthalmic outcomes improved in 61% of patients in the
selenium group compared with 36% of those in the placebo group, and only 7%
of the selenium group had a mild progression of the disease, compared with
26% of those in the placebo group.
Women with thyroid peroxidase antibodies tend to develop hypothyroxinemia
while they are pregnant and thyroid dysfunction and hypothyroidism after
giving birth. The authors of a Cochrane review of hypothyroidism
interventions during pregnancy concluded, based on a trial that
administered supplements containing 200 mcg selenium as selenomethionine
daily to 151 pregnant women with thyroid peroxidase antibodies, that
selenomethionine supplementation in this population is a promising
strategy, especially for reducing postpartum thyroiditis. However, the
authors called for large randomized clinical trials to provide high-quality
evidence of this effect.
Additional research is needed to determine whether selenium supplements can
help prevent or treat thyroid disease.
LifeSource Vitamins - Selenium Journal of the American Medical Association
in 1996 found that people taking 200 mcg of selenium a day for over four
years had a decrease of 46% lung cancer, 63% prostate cancer, and 58%
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