Iodine in the traditional Japanese diet

In an effort to reap the perceived benefits of the traditional Japanese diet, some alternative medicine practitioners advocate high-dose iodine supplements. Alan Gaby reviews the evidence for this kind of treatment in the Townsend Letter:

While iodine therapy shows promise, I am concerned that two concepts being put forth could lead to overzealous prescribing of this potentially toxic mineral. First is the notion that the optimal dietary iodine intake for humans is around 13.8 mg per day, which is about 90 times the RDA and more than 13 times the "safe upper limit" of 1 mg per day established by the World Health Organization. Second is the claim that a newly developed iodine-load test can be used as a reliable tool to identify iodine deficiency.
Is the optimal human requirement 13.8 mg per day?
The argument, developed by one investigator, (5) that the optimal human iodine intake is around 90 times the RDA is based mainly on two points. The first point is that the average iodine intake of adults living in Japan is 13.8 mg per day, and the Japanese are among the healthiest people in the world, with low rates of cancer. The second point is in regard to the amount of oral iodine that it takes to saturate the thyroid tissues.
The idea that Japanese people consume 13.8 mg of iodine per day appears to have arisen from a misinterpretation of a 1967 paper. (6) In that paper, the average intake of seaweed in Japan was listed as 4.6 g (4,600 mg) per day, and seaweed was said to contain 0.3% iodine. The figure of 13.8 mg comes from multiplying 4,600 mg by 0.003. However, the 4.6 g of seaweed consumed per day was expressed as wet weight, whereas the 0.3%-iodine figure was based on dry weight. Since many vegetables contain at least 90% water, 13.8 mg per day is a significant overestimate of iodine intake. In studies that have specifically looked at iodine intake among Japanese people, the mean dietary intake (estimated from urinary iodine excretion) was in the range of 330 to 500 mcg per day, (7,8) which is at least 25-fold lower than 13.8 mg per day

Gaby has debunked a pervasive myth. Concerning the second point, Gaby examines the rationale for an innovative new testing procedure for iodine deficiency ('iodine loading") and finds it wanting.

Gaby concludes:

The possibility that high-dose iodine/iodide can relieve certain common conditions is intriguing. Considering the positive anecdotal reports, an empirical trial of iodine/iodide therapy, based on the clinical picture, seems reasonable. The case has not been made, however, that the average person should markedly increase his or her iodine intake in an attempt to saturate the tissues with iodine. Nor has the case been made that the iodine-load test can provide reliable guidance regarding the need for iodine therapy. Thyroid function should be monitored in patients receiving more than 1 mg of iodine per day

Reasonable suggestions, I think, based on the evidence.

The original article by Gaby sparked a debate (which I have yet to read). Make of it what you will, here are those posts:

• A Rebuttal of Dr. Gaby's Editorial on Iodine by Guy E. Abraham, MD and David Brownstein, MD
  
• Alan R. Gaby, MD's Response to: A Rebuttal of Dr. Gaby's Editorial on Iodine
  
• Iodine Debate Continues by Guy E. Abraham, MD and David Brownstein, MD
  
• Gaby Responds to Rebuttal #2 by Alan R. Gaby, MD

Update:
Here is a comprehensive study of idodine toxicity.