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.
This section focuses on four
diseases and disorders in which selenium might play a role: cancer,
cardiovascular disease, cognitive decline, and thyroid disease.
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
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
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 activity.
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
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
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% colorectal cancer.*
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