jikeda at garnet.berkeley.edu
jikeda at garnet.berkeley.edu
Mon Mar 3 13:10:05 PST 1997
Selenium as an Anticarcinogenic Agent
An Up-date for Nutrition Professionals
Chris Hawkes,PhD and Joanne Ikeda,MA,RD
The findings of a recent study designed to evaluate the effectiveness of
selenium supplementation on reducing skin cancer startled the scientists who
conducted this research. Giving subjects selenium supplements did not
protect against the development of basal or squamous cell carcinomas of the
skin. However, results from secondary end-point analysis indicated that
supplemental selenium significantly reduced the incidence of, and mortality
from, lung, colorectal and prostate cancers. In their December, 1996, JAMA
article, "Effects of Selenium Supplementation for Cancer Prevention in
Patients with Carcinoma of the Skin," Dr. Larry Clark and his colleagues
state, "These effects of selenium require confirmation in an independent
trial study of appropriate design before new public health recommendations
regarding selenium supplementation can be made."
This study was done in the Southeast where many people have low
selenium intakes because crops are grown in low selenium soil. "In parts of
the country where people get more selenium from their food, the effect may
be less dramatic," says Clark. Clark initiated this study after doing a
geographical study that found cancer mortality rates were significantly
lower for total cancer and cancers of the lung, colon and rectum, bladder,
esophagus, pancreas, breast, ovary, and cervix, in U.S. counties with
intermediate or high soil selenium levels compared with counties with low
soil selenium levels.
A number of scientists are investigating selenium's antitumorigenic
properties, and there are various hypotheses as to how this compound may
inhibit tumorigenesis. Selenium may act by protecting against oxidative
damage as an essential component of the antioxidant enzyme glutathione
peroxides; altering carcinogen metabolism; effecting the endocrine and
immune systems; producing cytotoxic selenium metabolites; inhibiting
protein synthesis; inhibiting specific enzymes, and/or stimulating apoptosis.
Dr. Chris Hawkes, a Research Scientist at the USDA Western Human
Nutrition Research Center in San Francisco, expects that a number of
research articles linking selenium supplementation to reduced cancer risk
will be published this spring and summer in scientific and medical journals.
"Quite a few scientists are in the process of summarizing findings from
their studies on selenium supplementation and the incidence of various types
of cancers. I expect they will be publishing their results soon." The
media is quite apt to publicize the results. "Consumers will be asking
questions about selenium supplementation," predicts Hawkes. "We need to
warn people about the possible adverse consequences of indiscriminately
taking selenium," states a concerned Hawkes, who has conducted research on
selenium toxicity in non-human primates and is well aware of the potential
dangers associated with overdosing.
Unlike many other nutrients, the gap between enough selenium and
too much is fairly narrow. The RDA for selenium is 55 mcg. per day for
adult women, and 70 mcg. for adult men. In Clark's study, subjects were
given supplements of 200 mcg. per day, but these subjects were not getting
very much selenium from food.
Symptoms of overdosing on selenium supplements become evident at
levels of about 1,000 mcg. daily. Symptoms include the development of
white streaks in fingernails - the nails become very brittle; also, hair
starts to fall out. Acute selenium toxicity, which in the U.S. are usually
due to industrial accidents, results in lassitude, depression and fatigue; a
very noticeable garlic odor breath; a metallic taste in the mouth; nausea,
vomiting, diarrhea; skin lesion that become red and swollen, then
blistered and eruptive with intense itching; pain in extremities;
convulsions; partial paralysis and finally, death.
Dr. Hawkes notes that most of what we know about selenium toxicity
in humans is the result of an unfortunate environmental situation in
selected areas of China where the soil has traditionally been fertilized
with ash from the burning of sulpher-containing coal, which is very rich in
selenium. People living in these areas consume food grown in this selenium
contaminated soil, and are thus susceptible to chronic selenium toxicity.
Unfortunately, there is no effective treatment for this condition except
decreasing exposure. "In China, they relocate people suffering from
selenium toxicity to a region where the soil has not been contaminated with
this mineral. It may take up to a year for a person to completely recover,"
reports Dr. Hawkes, who has learned about this situation during his visits
"People need to be very careful when supplementing their diets with
selenium," says Hawkes, "I recommend that people limit supplementation to
a maximum of 200 mcg of selenium a day if they feel they want to take
supplements. The selenium content of the soil in California varies, and the
food we eat is grown in both high and low selenium soils. It is quite
likely that we are getting enough selenium from the food we eat." Dr.
Hawkes is planning to do a study on selenium supplement users in the San
Francisco Bay Area. "Then I'll be in a much better position to comment on
this practice; we will know allot more about both the benefits and the risks
of selenium supplementation and can decide if the benefits outweigh the
possible adverse effects of overdosing."
In the meantime, Dr. Hawkes recommends that Dietitians and
Nutritionists take a conservative approach with respect to supplementation.
"There are very few nutrients that are potentially lethal. Selenium happens
to be one of those nutrients that has the capacity to cause death. With
other potentially toxic nutrients like vitamin A and iron, there is a large
margin of safety between the recommended intake and the amount that is
toxic. With selenium, the gap is much narrower, so we need to be very
regards to promoting supplementation. We certainly don't want to end up
doing more harm than good."
Clark, Larry C. et al. Effects of selenium supplementation for cancer
prevention in patients with carcinoma of the skin. JAMA, December 25,
1996, Vol. 276, No.24, pp.1957-1963.
Clark, Larry C. et al. Selenium in forage crops and cancer mortality in U.S.
counties. Archives of Environmental Health, Vol. 46, 1991, pp.37-42.
Liebman, Bonnie. The selenium surprise. Nutrition Action Healthletter,
January/February, 1997, pp.8-9.
Hawkes, WC and NL Keim, The effect of selenium on triiodothyronine and
weight changes in healthy men. FASEB. Vol 9, 1995, p.A160.
Subcommittee on the Tenth Edition of the RDA's, Food and Nutrition Board,
National Research Council, Recommended dietary allowances, 10th edition.
Washington, D.C.: National Academy Press; 1989.
Written by Joanne P. Ikeda,MA,RD, in collaboration with Dr. Chris Hawkes,
Research Chemist, USDA Western Human Nutrition Research Center (WHNRC), the
Presidio, San Francisco, California. Ms. Ikeda is the Cooperative Extension
Nutrition Education Specialist and a Lecturer in the Department of
Nutritional Sciences at the University of California, Berkeley, CA 94720-
3104. She is currently on sabbatical leave at WHNRC.
Joanne P. Ikeda,MA,RD
Cooperative Extension Nutrition Education Specialist
Department of Nutritional Sciences
University of California, Berkeley
E-mail jikeda at garnet.berkeley.edu
E-Mail: jikeda at garnet.berkeley.edu
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