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Selenium and Cancer
Prevention
Introduction
Selenium is a trace
element widely and irregularly distributed in
the environment. Its availability and
consumption in food may vary significantly
depending on the region. The daily requirement
for selenium is between 200 and 250 micrograms
(or a minimum of 1 microgram per kilo body
weight and per day). Selenium occurs in animal
and vegetal foods.
The essentiality of
selenium in human nutrition was derived from its
anti oxidative action, being a part of the
glutathione-peroxidase system (GPx). A deficit
in selenium will result in a decrease of the GPx
activity, and therefore, in an increase of cell
damage which cannot be counter-balanced by other
anti oxidative systems. Selenium is the main
defense against oxidants.
What is
mostly ignored, is that a selenium deficiency
may also result in a thyroid hormone utilization
defect.
Selenium deficiency symptoms
appear in whole population groups when the
selenium content in the diet is
inadequate.
Selenium toxicity states
are rare.
Studies
Numerous
studies have been published documenting the
preventive effect of supplementation with
selenium on the occurrence of
cancer.
In the Linxian study
selenium supplementation resulted in a 13
percent reduction in cancer mortality.
<>Between 1986 and 1991, 29,584 persons
took part in a randomized nutritional
intervention trial in Linxian, China, an area
whose residents had chronically low intakes of
several nutrients and high rates of esophageal
and gastric cardia cancer as well as stroke.
Using a one-half replicate of a 2(4) factorial
design, we randomized individuals to one of
eight groups which received combinations of four
supplements: retinol and zinc (factor A),
riboflavin and niacin (factor B), vitamin C and
molybdenum (factor C), and beta-carotene,
alpha-tocopherol (vitamin E), and selenium
(factor D). Deaths that occurred during 5 years
of supplementation were ascertained and
classified according to cause. At the end of the
supplementation period, we measured blood
pressure readings and determined the prevalence
of hypertension. Participants who received
factor D had reductions in total mortality (9%)
and total cancer mortality
(13%).
(International Epidemiology
Institute, Rockville, MD. Vitamin/mineral
supplementation and cancer risk: international
chemoprevention trials. Proceedings of the
Society for Experimental Biology an Medicine,
1997; Nov, 216(2):291-6)
In another
study, selenium supplementation resulted in 35.1
percent less precancerous lesions An
intervention trial was undertaken among the
general population of 130,471. Individuals in
five townships were involved for observation of
the preventive effect of Selenium. The 8-years
follow- up data showed reduced liver
precancerous lesion incidence by 35.1% in
selenized table salt supplemented vs. the non
supplemented population. On withdrawal of Se
from the treated group, PLC incidence rate began
to increase.
(Yu SY; Zhu YJ; Li WG.
Cancer Institute, Chinese Academy of Medical
Sciences, Peking Union Medical College, Beijing,
China. Protective role of selenium against
hepatitis B virus and primary liver cancer in
Qidong. Biological Trace Element Research, 1997
Jan, 56(1):117-24.)
In a recent
double-blind trial, selenium supplementation
reduced prostate cancer incidence by 63 percent
A total of 974 men with a history of either a
basal cell or squamous cell carcinoma were
randomized to either a daily supplement of 200
microgram of selenium or a placebo. Patients
were treated for a mean of 4.5 years and
followed for a mean of 6.5 years. The selenium
treatment was associated with a significant
(63%) reduction in the secondary endpoint of
prostate cancer incidence
(Clark
LC; Dalkin B; Krongrad A; Combs GF Jr; Turnbull
BW; Slate EH; Witherington R; Herlong JH;
Janosko E; Carpenter D; et al. Decreased
incidence of prostate cancer with selenium
supplementation: results of a double- blind
cancer prevention trial. British Journal of
Urology, 1998 May, 81(5):730-4.)
There
was no effect of selenium on the existing skin
cancers in this trial. Detractors use this fact
to pretend that Nutraceuticals in general and
selenium in particular are useless for cancer
prevention.
National Cancer
Institute
The National Cancer Institute
recognizes the power of selenium to prevent
lung, colorectal, and prostate
cancer.
Selenium
Monitoring
The selenium status of a person
can be assessed by monitoring the selenium
plasma and urine level and the selenium content
of an hair sample. Human plasma should not
contain less than 100-150 and more than 2,800
nanograms of selenium per milliliter. Urine may
contain from 50 to 100 nanogram per milliliter.
Hair selenium should 27
micrograms/g.
Erythrocyte and plasma
selenium and glutathione peroxidase specific
activities, hair and fecal selenium, and urinary
selenium excretion were increased by and were
linearly related to L- selenomethionine dose.
Hair selenium was most sensitive to
L-selenomethionine dose, with an 84- fold
increase in the 300 micrograms selenium/(kg-d)
group relative to controls (r = 0.917). Daily
urinary selenium excretion (80-fold, r = 0.958),
plasma selenium (22-fold, r = 0.885),
erythrocyte selenium (24-fold, r = 0.920), and
fecal selenium (18-fold, r = 0.911) also
responded strongly to L-selenomethionine.
Erythrocyte and plasma glutathione peroxidase
specific activities increased 154% and 69% over
controls, respectively. Toxicity was associated
with erythrocyte selenium > 2.3
micrograms/mL, plasma selenium > 2.8
micrograms/mL, and hair selenium > 27
micrograms/g. Plasma, erythrocyte, and hair
selenium concentrations may be useful for
monitoring and preventing the toxicity of
L-selenomethionine administered to humans in
cancer chemoprevention
trials.
(Hawkes WC; Willhite CC;
Craig KA; Omaye ST; Cox DN; Choy WN; Hendrickx
AG. Effects of excess selenomethionine on
selenium status indicators in pregnant
long-tailed macaques (Macaca fascicularis).
Biological Trace Element Research, 1992 Dec,
35(3):281-97.)
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