Iodine—Time to Consider Changing U.S.
Recommendations for Daily Intake?
by Lara Pizzorno, MDiv, MA,
LMT
http://www.lmreview.com/articles/view/iodine-the-next-vitamin-d-part-II/
Although the U.S. Institute of Medicine limit for the
tolerable upper intake level for iodine in adults is 1,100 mcg/day, dietary
iodine intake in Asia is much higher. High iodine-containing seaweeds are
frequently consumed and well tolerated by millions of people in Japan, Korea,
and coastal China. In Japan, where seaweed intake averages ~ 4–7 grams/day
(with some estimates as high as 10 gram/day), average dietary iodine intake was
recently estimated to be 1.2 mg/day. However, a study of 4,138 apparently healthy,
euthyroid Japanese men and women found a mean urinary iodine excretion of 5,100
mcg/day, which translates to a daily intake of 5,500 mcg (~5.5 mg) I/day, and
yet other research has estimated daily Japanese iodine consumption ranges as
high as 13,800 mcg/day.
Regardless of which estimate of daily iodine intake among
the Japanese we accept, their consumption of iodine is magnitudes higher than
that in the U.S., where average daily consumption was estimated to be 167
mcg/day11. Not only does the substantially higher intake of iodine among the
Japanese appear to have no harmful effects in this population, but, on the
contrary, incidence rates of autoimmune thyroiditis, hypothyroidism, benign and
malignant breast disease, and prostate cancer are all dramatically lower among
Japanese consuming an iodine-rich diet.
A study of the impact of seaweed consumption on thyroid
function in American women found that iodine intake of 495 mcg/day, an amount
significantly higher than current U.S. DRIs, causes no harm. In this
randomized, placebo-controlled crossover trial, 25 healthy postmenopausal women
(average age 58 years), 10 of whom had a history of early (Stage I or II)
breast cancer but were disease-free at the time of the study, and 15 who had
never been diagnosed with breast cancer, were randomized to receive either 6
weeks of supplemental iodine (in the form of 10 seaweed powder capsules
providing a total of 475 mcg of iodine/day) or placebo (maltodextrose in 10
identical gelatin capsules).
Since soy, a known goitrogen, is also commonly consumed in
Japan, for 1 additional week, the women were also given high-isoflavone powder
providing 141.3 mg of isoflavones and 67.5 g of protein/day in addition to
seaweed or placebo capsules during the last week of each treatment arm.
Neither 7 weeks of seaweed nor 1 week of soy and seaweed
supplementation affected thyroid end points. Seaweed supplementation was
associated with a small increase in TSH, but values remained well within normal
ranges. The women in this study, excreted an average of 587 mcg of I/day while
ingesting supplements providing 495 mcg of I/day.
In the treatment of fibrocystic breast disease, Ghent,
Eskin et al (1993), demonstrated the safety of therapeutic doses of molecular
iodine (I2) of 3 to 6 mg/day over a period of 2-5 years,42 and a more recent
(2004) trial evaluating the effects of varying dosages of I2 on fibrocystic
breast disease (1.5, 3 or 6 mg/day) found that the 6 mg dose produced the best
results: a 50% reduction in pain in 51.7% of women taking this dose with no
adverse effects. No decreases in pain were seen in the groups receiving 1.5 mg
or placebo.