[This is one of the finalists in the SMTM Mysteries Contest, by a reader writing under the pseudonym Lee S. Pubb. We’ll be posting about one of these a week until we have gotten through all the finalists. At the end, we’ll ask you to vote for a favorite, so remember which ones you liked.]

Background

Element 53, iodine, is a mineral essential to human health largely because it is utilized in the production of thyroid hormones.[1] For much of recorded human history, societies the world over suffered from the iodine deficiency disorder known as goiter–swelling in the neck resulting from an enlarged thyroid gland. It was common in regions where topsoil was regularly eroded (e.g., by flooding), as topsoil is rich in iodine, normally leading to its uptake in food crops. It’s also abundant in foods derived from the ocean, as ocean water contains iodine.

While Chinese doctors are said to have prescribed the consumption of animals’ thyroid glands to treat goiter as early as the 7th century, it was only in the early 20th century that large-scale research was done on iodine supplementation. It was found that sufficient levels of iodine consumption eliminated goiter in the vast majority of cases, and we began iodizing salt soon after. (Which is why I had to give this lengthy background on goiter, instead of just saying “that giant lump in your friend’s neck”.) The FDA recommends that Americans consume 150 mcg of iodine a day, and expect that the vast majority of Americans will achieve this through salt consumption.

That said, while the level of iodine supplementation present in modern-day table salt is clearly sufficient to greatly reduce the incidence of goiter, that doesn’t necessarily mean that it’s the ideal level for overall thyroid health.[2] (The FDA’s upper limit is set nearly 8 times higher than the RDA, at 1,100 mcg, suggesting that considerably more than the recommended amount isn’t expected to be deleterious to human health.) The thyroid largely regulates metabolism, and you may be familiar with some mysteries raised about human metabolism over the past century.[3] There are also a number of diseases directly associated with thyroid function, and these are pretty common in the present day (especially among women), with around 20 million patients in the United States estimated to have some kind of thyroid disorder. Thyroid functioning is screened for by doctors via blood tests measuring thyroid hormone concentration. Common disorders include hypothyroidism, in which an abnormally low level of thyroid hormone is produced, and hyperthyroidism, in which too much is produced.

INTERESTING!:

The Blue Zones are regions around the world renowned for their high proportion of centenarians, people living to the age of 100 or later. These are Okinawa, Japan; Sardinia, Italy; Nicoya, Costa Rica; Ikaria, Greece, and Loma Linda, California. These zones are now getting the shit studied out of them by anthropologists, doctors, etc., so we can try and figure out why they live so long and hopefully use it to improve human healthspan elsewhere.

You will not be surprised, given the background I just provided, to find out that iodine appears to be related to the mystery of the blue zones.

In the study Association of endemic goitre and exceptional longevity in Sardinia: evidence from an ecological study, the authors note “The spatial analysis revealed that the goitre rate (p < 0.0001), the proportion of inhabitants involved in pastoralism (p = 0.016), the terrain inclination (p = 0.008), and the distance from the workplace as a proxy for physical activity (p = 0.023) were consistently associated with population longevity at an aggregated level in the 377 municipalities.” Which is to say, a higher goiter rate in a municipality was the measure they studied that was most clearly associated with greater longevity. This association had an extremely–to my mind, an almost outrageously–low p-value.

 The study goes on to say that, “from a worldwide perspective, the finding of an epidemiological association between goitre prevalence and longevity does not seem to be limited to Sardinia, but partially shared also by other populations where long-lived subjects are numerous. Most of the Longevity Blue Zones in the past were niches of endemic goitre as well. In Costa Rica, where the Nicoya LBZ was identified in 2007 (Rosero-Bixby 2008), high prevalence of endemic goitre has been reported since the 1950s (Perez et al. 1956) possibly aggravated by a gross excess of calcium ingested with drinking water (Boyle et al. 1966). In another LBZ, Ikaria island, the iodine level in spring water is remarkably low (Pes and Poulain 2014).”

So great, restrict iodine and you’ll live forever, right? AGING SOLVED! WE DID IT, YOU’RE WELCOME, aging is just iodine damage.

…Except.

Another Blue Zone is Okinawa, where most of the population routinely consumes nutritionally large quantities of seaweed–one of the richest natural sources of iodine. How much iodine are they consuming? I don’t have great numbers on Okinawa itself, but mean consumption of iodine in Japan is estimated at around 1-3 milligrams a day–that’s 1,000-3,000 micrograms of iodine. Remember that the FDA puts the RDA of iodine at just 150 micrograms, and the UL at 1,100 micrograms! Apparently, many Japanese people are consuming considerably more than the “tolerable upper intake level” for iodine. Centenarianhood is apparently their reward. Japan has 86,000 centenarians (.06% of their population, the highest percentage in the world), and a life expectancy of 84.62 years, second only to Hong Kong (the population of which, as of 2011, had a high risk of iodine deficiency).

These studies, taken together, suggest that having a moderate iodine intake leads to early death. As the Buddha said, “seek extremes in all things and maybe you can live to be a hundred”.

It is difficult not to propose any explanations for this, but that is the task that has been assigned to me, so, there you have it. Low iodine is correlated with longevity. High iodine is correlated with longevity. The thyroid is truly the most mysterious organ.

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[1] If that first sentence surprised you, you’re in good company; iodine is “the heaviest element commonly needed by living organisms.”

[2] Plus, there’s bound to be population variance–I, for one, use kosher salt in my cooking, because Alton Brown told me to, and it’s not iodized. I’m surely exposed to iodized salt via processed foods and restaurant meals, but I do consume those somewhat rarely, at least compared to the number of meals I prepare at home.

[3] Iodine was once used to treat flour, oxidizing it to allow gluten to more easily form bonds and create the texture all us non-celiacs love in bread and pizza. But around the same time we began to study iodine supplementation, we invented potassium bromate, and started brominating flour instead to achieve the same effect–potassium bromate produced a very white flour that rapidly oxidized. Iodine is a necessary mineral, though, and bromine isn’t. So there was likely a point in recent American history where iodine consumption in bread decreased. You’re also really not supposed to consume the potassium bromate, but the thought is that baking temperatures sublimate the bromine–ah fuck I guess you really *shouldn’t* eat raw cookie dough? fuck.