Dear Friends,

Last week, I posted a series of articles
pertaining to soy consumption. I've been
away, and returned to find thousands
of EMAILS which included a few hundred
comments about those soy articles.

The most frequently occurring concern of
readers of the NOTMILK column dealt with
soy consumption and human thyroid functioning.

I've found a recent article addressing
that concern:

SOY AND THYROID FUNCTION:
STUDIES SHOW LITTLE EFFECT
By Mark Messina, Ph.D.

There has been much discussion of late over
the possible adverse effects of soy consumption
on thyroid function in both infants and adults.

In fact, researchers from the National Center
for Toxicology (NCTR), which is part of the U.S.
Food and Drug Administration, have even
expressed some concerns. But concerns about
effects of soy on thyroid function are not new.

The first report that soybean produced goiter in
iodine-deficient rodents was published in 1933.
Subsequent animal studies published in the 1930s
and 1940s produced similar findings. However,
iodine fortification of the diet was shown to
largely eliminate this problem, although soy-fed
animals required approximately twice as much
iodine to prevent enlarged thyroids as animals
fed soy-free diets, and in some cases, slight
histological abnormalities of the thyroid gland
still persisted.

In the late 1950s, 10-15 cases of goiter were
identified in infants fed non-iodized soy
flour-based infant formula. However, this type
of formula has not been used since the 1960s.
Today, soy formula is based on soy protein
isolate and is fortified with iodine. No cases
of goiter in infants, due to the consumption of
soy protein isolate-based iodized formula as is
used today, have been reported in the scientific
literature.

Unfortunately, no studies solely designed to
study the effect of soy formula on thyroid function
in infants has been conducted. Still, given the
millions of infants fed soy formula over the
past three to four decades, it is reasonable
to assume that if a problem existed, more
than likely it would have been reported upon
by pediatricians in the medical literature.

There are, however, at least limited data
suggesting that infants with congenital
hypothyrodism who consume soy formula
require about 25 percent more synthetic
hormone than infants with congenital
hypothryoidism on non-soy formulas. But this
may not be a systemic effect, since fiber
supplements also necessitate that patients
increase their thyroid hormone medication.

This suggests soy, like fiber, may interfere with
either the absorption of thyroid hormone (in the
case of medication), or may interfere with
reabsorption by interrupting the enterohepatic
circulation of thyroid hormone.

Furthermore, a recently conducted human trial
showed no effects of isoflavone supplements
on thyroid function. And several other human
studies have also found little or no effect of
soyfoods.

One study conducted in Japan did find that soy
Consumption was associated with adverse effects
(increase TSH levels) on thyroid function in older
women, including an increase incidence of goiter.

Women in this three-month study consumed
30 g/ day of pickled soybeans stored in rice
vinegar. However, this study suffers from many
design flaws, and although these results should
not be ignored, they directly conflict with the
results from several, better-designed studies.

For example, recent work from the University of
Minnesota indicates that the consumption of
isoflavone-rich soy over a three-month period
had little effect on thyroid hormone levels in
either pre- or post-menopausal women. And a
recent double-blind study involving 38
postmenopausal women over the age of 64 who
were not on hormone therapy, found no differences
in thyroid function, based on measures of thyroid
stimulating hormone (TSH), total thyronine (T4),
and triiodothyronine (T3), between women given
daily either a placebo or a supplement that
provided 90 mg of isoflavones (expressed as
aglycone units).

Overall, there appears to be little reason to think
that in healthy adults, either soy isoflavone
supplements, or soyfoods, will exert adverse
effects on thyroid function. Even the NCTR
researchers acknowledge that soy is not likely
to be a problem in iodine-replete individuals.

In contrast, arguably, in people who are
predisposed to goiter or who are consuming
marginally iodine sufficient diets, soy could
conceivably a risk factor for goiter. So, it is
important to note that as many as 10 percent of
post-menopausal women may have subclinical
hypothyroidism. This group may be sensitive
to the adverse effects of weak goitrogens.

The iodine status of the U.S. population is
considered adequate, although there is a
downward trend in iodine intake and subsets
of the population may have marginal intakes.

In conclusion, there is no reason to restrict
soy consumption over concerns about the
impact on thyroid function. When consuming
large amounts of soy, it is important to make
sure iodine intake is adequate. But of course,
all people, regardless of their dietary pattern,
need to consume sufficient amounts of iodine.

Any concerns about the effect of soy on thyroid
levels can be definitively addressed by having
thyroid hormone levels measured. Even this
step is not unordinary, since the American
Thyroid Association recommends that all people
have their thyroid hormone levels checked every
five years beginning at the age of 35.
_______________________________________________

Robert Cohen
http://www.notmilk.com