Iodine is one of the few nutrients Australia has gone to the trouble of putting into a staple food by law. How it got there is a story about soil, dairy cleaning products and a change in the law.
What iodine does
Iodine runs your thyroid, which in turn runs a good deal of your metabolism. You need very little of it, 150 micrograms a day for an adult, but you need it regularly.
A soil problem
Australian soils are naturally low in iodine, so food grown here carries less of it than food grown in a lot of other places.
For a while that was masked. Dairies used iodine based sanitisers, and enough of it ended up in the milk to cover the gap.
Through the eighties and nineties the industry moved away from those sanitisers. By the early 2000s, testing showed mild iodine deficiency had returned. In 2009 the law changed, and all commercial bread in Australia has been made with iodised salt ever since.
It worked, broadly. Women eating a decent amount of bread are around five times more likely to be getting enough iodine than women who eat very little.
What low iodine affects
Your thyroid. Iodine is the raw material thyroid hormones are made from. Without enough, the gland cannot produce them properly, and over time it enlarges trying to compensate. That is a goitre.
Brain development. Iodine deficiency is the leading preventable cause of intellectual impairment in the world. The effect is on brains that are still forming, so young children, and babies in utero and while breastfeeding. That is why requirements rise substantially in pregnancy and breastfeeding, and why those groups are the focus of Australia's iodine program. Adult cognition is a different question, and the evidence there is far weaker.
Energy and metabolism. Thyroid hormones set your metabolic rate, so when they drop the usual picture is fatigue, feeling cold, and weight that shifts without much explanation.
Too much is also a problem
Iodine is one of the few nutrients where more is clearly not better. Excess causes thyroid dysfunction too, and it can go either way, overactive in some people and underactive in others.
The upper level for adults is 1,100 micrograms a day. That sounds a long way from 150 until you look at seaweed, where a single serve of some products carries several thousand.
Where the gaps are
If the national iodine strategy is bread, then not eating much bread leaves a gap. Low carbohydrate eating, gluten free, or simply not being a bread person.
Organic bread is exempt, as is any bread you make at home from a mix. There is more on checking your loaf and on where else iodine comes from.
The takeaway: Australian soils are low in iodine. Dairy sanitisers covered the gap until the industry stopped using them, deficiency returned, and since 2009 commercial bread has carried iodised salt. Low iodine affects the thyroid, energy, and developing brains in particular. Too much is also harmful, which is why raw seaweed and kelp supplements are best avoided.
The science behind this post:
- Food Standards Australia New Zealand. Iodine fortification. Mandatory iodine fortification implemented in 2009 through Standard 2.1.1, requiring iodised salt in all bread except organic bread and bread mixes for home baking. foodstandards.gov.au
- Charlton K, et al. (2016). Dietary iodine intake of the Australian population after introduction of a mandatory iodine fortification program. Nutrients, 8(11), 701.
- National Health and Medical Research Council. Nutrient Reference Values for Australia and New Zealand: Iodine. RDI 150 micrograms per day for adults; Upper Level 1,100 micrograms per day. nrv.gov.au
- World Health Organization. Iodine deficiency disorders. who.int
- Li M, et al. (2006). Are Australian children iodine deficient? Results of the Australian National Iodine Nutrition Study. Medical Journal of Australia, 184, 165-169.