Evidence suggests
soy foods can contribute to problems with the thyroid gland, a widespread
condition many Americans might not even realize they have, yet health experts
contend soy's benefits outweigh any risks of thyroid malfunction.
One of the most
prevalent thyroid problems with a link to soy consumption is called hypothyroidism.
It is a condition in which the gland, a butterfly-shaped organ in the neck,
slows its activity and produces less thyroid hormone. The slowing down
is gradual and can take months or even years before an individual notices
any change.
Hypothyroidism's
symptoms are rather subtle. They include fatigue, slight weight gain, constipation
and often feeling cold. The condition also contributes to menstrual irregularities
and even interferes with fertility.
The first link
between soy consumption and hypothyroidism occurred when researchers began
suspecting soy-based baby formulas were responsible for reported increases
in the condition among infants.
The most recent
such study was published in the February 2000 issue of the New Zealand
Medical Journal. Its leading author, Mike Fitzpatrick, an environmental
researcher in New Zealand and an expert on the subject, found the isoflavones
in soy adversely affected the thyroid gland. Isoflavones have estrogen-like
activity, which means they could behave like a hormone.
"As well as
the effects on thyroid function, there is also strong evidence that soy
may affect the uptake of essential minerals, immune system function, pancreatic
function and sex-steroid hormones," Fitzpatrick told United Press International.
"On a very dark
note, there is even evidence that certain cancers may be accelerated by
the isoflavones that are found in soy products. This is particularly worrying
given the widespread promotion of soy as a foodstuff that will help both
prevent and fight cancer."
Although there
has been some question whether soy products are dangerous for breast cancer
patients, there is no concrete evidence linking soy consumption with the
spread of various cancers.
A 1997 study
published by the National Center for Toxicological Research in Jefferson,
Ark., a division of the U.S. Food and Drug Administration, also found isoflavones
in soybeans suppressed thyroid function and interfered with thyroid hormone
balances. A researcher there, who spoke on condition of anonymity, told
UPI ongoing animal studies investigating this perceived connection "make
some clear conclusions" suggesting soy products influence proper thyroid
function.
"The risks and
benefits are different for different populations," the researcher said.
"The idea is to educate."
For example,
"we see the postmenopausal women population most at risk for thyroid problems
and they're a group being heavily targeted for an intense marketing campaign
for soy."
Thyroid conditions,
particularly hypothyroidism, are more common among women than men, regardless
of their dietary habits, according to the American Thyroid Association.
Dr. Hossein
Gharib, president of the American Association of Clinical Endocrinologists
and a professor of medicine at the Mayo Clinic in Rochester, Minn., told
UPI the link between soy and thyroid problems is "an unproven belief. There's
no proof that this is so."
Although there
is some evidence suggesting soy-based infant formulas are linked to thyroid
dysfunction, no such evidence exists for adults consuming moderate amounts
of soy, Gharib said.
"There's a general
belief there's some interference, but there's no documentation," he said.
Gharib said
it is estimated about 10 percent of the U.S. population -- about 25 to
30 million people -- have hypothyroidism and the causes could be many,
including genetic.
Keith Ayoob,
a spokesman for the American Dietetic Association, said soy is widely consumed
and therefore this is evident not everyone who eats soy foods has a thyroid
problem.
"Billions of
people eat this stuff, so if there was anything glaring in there, we'd
probably know it on some level," Ayoob told UPI. "I'm not aware of any
studies of any vegetarians who have increased problems with hypothyroidism."
All health experts
agree more research is needed to determine what link, if any, exists between
soy and thyroid problems.
Ayoob noted
soy's health benefits outweigh any downsides. Soy protein has been conclusively
linked to lower risks of heart disease, he explained, and soy is also a
good source of dietary fiber and protein that is low in fat and cholesterol.
Many grocery store products are supplemented with soy, such as soy milk,
as soy's popularity grows in the United States.
Foods high in
soy also include tofu, tempeh and soybeans, as well as many of the substitute
meat products on grocery store shelves, such as soy burgers, soy hotdogs
and soy sausages.
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Copyright 2003 by United
Press International.
All rights reserved.
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