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Despite concerns, soy probably safe for thyroid
Reported April 14, 2010
NEW YORK (Reuters Health) - Despite some concerns to the contrary,
the soy-based dietary supplement genistein may not harm postmenopausal
women's thyroid function, a new study finds.
Genistein is a type of soy isoflavone, a plant chemical that is structurally
similar to estrogen and may have certain estrogen-like effects in the human
body. In a 2007 clinical trial, Italian researchers found that genistein
supplements, along with calcium and vitamin D, appeared to help boost bone
mass in postmenopausal women with thinning bones.
In this latest study, the researchers evaluated data from the same clinical
trial -- this time looking at whether the genistein supplements had any
effects on the women's thyroid function.
The question stems from lab research showing that genistein and other
isoflavones may decrease thyroid-hormone production. Thyroid hormones help
govern metabolism, and an underactive thyroid gland, called hypothyroidism,
can lead to problems like fatigue, weight gain and intolerance to cold.
The earlier research suggested that isoflavones can affect thyroid hormones
by interfering with iodine, which is needed for thyroid- hormone production,
explained Dr. Francesco Squadrito of the University of Messina, the senior
researcher on the study.
However, he told Reuters Health by email, those studies used genistein doses
that were 10 to 250 times higher than the doses used in his team's clinical
trial -- 54 milligrams (mg) per day.
Squadrito and his colleagues found that among 77 study participants they
followed for three years, those who used the genistein supplement during
that time showed no overall differences in thyroid function compared with
women who were given a placebo.
The findings are published in the Journal of Clinical Endocrinology and
Metabolism.
According to Squadrito, it is not surprising that studies would find thyroid
effects of very high doses of genistein. However, he said, women are
unlikely to consume such levels from soy-protein products, or from soy foods
like tofu.
As far as thyroid function is concerned, Squadrito said, "it is possible to
conclude that genistein therapy is safe in postmenopausal women -- at least
at the dose of 54 mg a day."
However, soy contains several types of isoflavones, and more studies are
needed to establish the safety of those compounds, according to Squadrito
and his colleagues.
SOURCE: here Journal of Clinical Endocrinology and Metabolism, online
March 31, 2010. |
