Many thanks for your post and questions. The answers below are based on a collective opinion and existing evidence, and are not exhaustive.
1) Why some countries use KI, but other countries use KIO3?
It’s most likely a combination of technical, economic, and historical reasons.
Importantly, KI is cheaper than KIO3. It’s also more water-soluble which means it is more easily dispersed on salt crystals by atomized sprays.
However, KI in salt is not very stable and can easily be lost by oxidation to iodine if the iodized salt is subjected to any of the following conditions: (1) moisture in the salt, (2) humid or excessively aerated environment; (3) exposure to sunlight; (4) exposure to heat; (5) acid reaction in the salt; or (6) presence of impurities. It can also be lost if the iodized salt packages become damp, resulting in the migration of iodide from the salt to the fabric, and subsequent evaporation if the fabric is pervious. This loss can be lessened when the salt iodized with KI is very pure (+ 99.5%) and dry (moisture less than 0.1%), and by the addition of stabilizers such as sodium thiosulfate and calcium hydroxide, and/or drying agents such as magnesium carbonate or calcium carbonate. However, in most impure salt, KI stability is poor due both to oxidation and to migration and segregation in the presence of moisture.
Historically, KI was used to fortify salt when iodization programs were first initiated in the U.S. and Switzerland in the 1920s. KIO3 is an oxidant which was used in the past to strengthen the structure of wheat dough in bread production. In the 1930's, this application led to an excessive supply of iodine, and KIO3 was replaced by bromate, which was later replaced by ascorbic acid due to bromate toxicity risks. The benefits of KIO3 as a salt fortificant were not discovered until the 1950s, and since then it has been successfully adopted around the world (including in Australia, New Zealand, Germany, Italy, Spain, and Switzerland). A WHO guideline published last year permits the use of both (1).
The salt producers in both the U.S. and Canada are large companies with long experience of using KI. In the U.S., KI remains stable during the short market turn-over between production and purchase. Given the lower cost, convenience, and the absence of official recommendation (either from the government or the scientific community) to switch to KIO3, there is little incentive for the salt producers to do so.
2) What are the side effects of KI and KIO3 added in salt? if any?
The issue of side-effects is extensively studied in many countries, and such studies are still continuing. The physiological responses by the thyroid gland across sub-groups in the population have been described in a recent review (2). It has been shown that side-effects do indeed occur after a rise in population iodine intake (especially if iodization is excessive and iodine deficiency was severe to start with), but it is not possible to accurately predict the individuals that will be affected.
In 1991, the Joint FAO/WHO Expert Committee on Food Additives endorsed iodate and iodide as safe at doses below the tolerable upper limit (1 mg iodine/day from all sources). In practice, even at the highest dosages currently used, iodine intake through iodized salt is unlikely to exceed 20% of this value.
An official statement on the safety of iodate was published by the ICCIDD (3) and more recently the evidence was summarized in a comprehensive review (4). To cite some of the key points from the latter:
- Iodate has been used for decades as an additive to salt and bread without notable toxic effects;
- It is added to salt in low amounts; is probably reduced to iodide in food or at the latest in the intestinal mucosa, thus decreasing systemic exposure; and
- It has shown no toxic effect levels with regard to organ toxicity that are at least 100 times higher than the exposure to be expected from the intake of iodized salt.
3) If true, why US FDA bans using KIO3 in salt?
Both KI and KIO3 have been granted “GRAS” (“generally recognised as safe”) status by the U.S. FDA, which means both can be and are being added to foods. Although KI is the only compound mentioned in relation to iodization, it appears that there is no specific ban on KIO3.
1) WHO. Guideline: Fortification of Food-Grade Salt with Iodine for the Prevention and Control of Iodine Deficiency Disorders. Geneva: World Health Organization, 2014
2) Zimmermann MB and Boelaert K. Iodine deficiency and thyroid disorders. Lancet Diabetes Endocrinol 2015; 3: 286–95
3) Stanbury JB. The safety of iodate as a salt additive. IDD Newsletter 1991, 7: 23.
4) Bürgi H et al. The Toxicology of Iodate: A Review of the Literature. Thyroid 2001, 11: 449-456