[R.B.]

The above paper begs the question how well does iodine intake and urinary iodine correlate. This paper goes some way to confirming in general terms a strong correlation.

http://www.ymj.kr/Synapse/Data/PDFDa...ymj-39-355.pdf

Clearly the difficulties in assessing iodine intake because of the large variations due to processing and food preparation are significant.

But based on this paper it can be reasonably assumed that high urinary outputs do correlate with high intakes, so based on urinary iodine output cited in the papers above we can be certain that some Japanese do indeed consume significant amounts of iodine; further generally there is no indication that this intake has negative health consequences.

Of course nothing is ever simple, so higher urinary outputs for a given intake in some could be due to other factors including differences in transporter function, historic intake and so levels of tissue saturation including fat and muscle, absence of intake of iodine uptake blockers in so far as they can affect net uptake through the gut iodine transporters, other dietary differences such as polyunsaturated fat intake etc. It appears much is still unknown.

However overall if you have not at some point ingested the iodine it is not possible to excrete it, so it is inescapable that on a population basis higher excretion must point to higher intake.

I would find it surprising if nobody has looked at uptake and excretion in population groups known to have a high long term intake of iodine, but have not come across any such papers so far.

In the table in this paper the UK and USA have the lowest intakes, indeed the UK ranks towards the bottom of a global ranking of iodine intakes; no 7 in a list of "The top 10 iodine-deficient countries (based on national median UIC <100 μg/L) with the greatest numbers of SAC with insufficient iodine intake in 2011. SAC, school-age children; UIC, urinary iodine concentration." Fig 3 http://nutrition.highwire.org/content/142/4/744.full