Salt of the earth

Seaweed is one good source of iodine. Photo: Getty Images
Seaweed is one good source of iodine. Photo: Getty Images
Getting enough iodine doesn’t have to be a pain in the neck, writes Dr Kirsty Fairbairn.

Dr Kirsty Fairbairn
Dr Kirsty Fairbairn

Iodine is a somewhat innocuous, yet important mineral for New Zealanders. How many of you have even thought about your iodine status?

Perhaps you think about your thyroid hormone function, but did you consider that iodine is necessary to make those thyroid hormones? Our body contains only a tiny 15-50mg of this trace element, yet if we don’t consume it regularly, a deficiency of iodine can have major consequences. We can’t make iodine in our body, so must get it from our diet.

Iodine deficiency is visible as goitre (an enlarged thyroid gland; like a swelling in the throat area). Goitre is a sign that the thyroid gland is trying to adapt to low iodine availability. Thyroid hormones (thyroxine and tri-iodo-thyronine) are critical to normal growth and development, the development of our brain and central nervous system, and also play a key role in energy metabolism and oxygen use throughout our body. Due to the reduced production of thyroid hormones, iodine deficiency is particularly concerning during pregnancy, in early infancy and during childhood, as it can compromise growth, as well as brain and cognitive development in this important phase of life.

The iodine status of New Zealanders has a fluctuating and interesting history, and University of Otago staff have played a leading role in our understanding of iodine deficiency. 

Back in the 1920s University of Otago researchers Charles Hercus, Noel Benson and Charles Carter  demonstrated that goitre was common in New Zealand, and that our soils were low in iodine; meaning that our land-grown agricultural products are typically low in iodine. Those of you raised on Stewart Island can clap yourselves on the back for having by far the highest soil iodine content in the entire South Island, and much of New Zealand!

This tradition of iodine insight has continued through the  work of Prof Christine Thomson and Associate Prof Sheila Skeaff, of the university’s department of human nutrition, who have  studied the iodine status of New Zealanders as we grappled to improve iodine intakes.

Introducing iodised salt in 1939 had a successful impact on the iodine status of New Zealanders,  but the declining use of iodised salt in the face of higher rates of high blood pressure and heart disease means that our iodine intake from salt is dropping. It is important to note that the salt contained in processed foods is usually not iodised salt. Research conducted in the 1990s by Prof Thomson indicated that iodine deficiency was once again on the rise in New Zealand, and  Prof Skeaff and her team reported mild iodine deficiency in school children as part of the 2002 National Children’s Nutrition Survey.

The Annual Nutrition Survey of 2008-2009 also found that New Zealanders were showing mild iodine deficiency. Food Standards Australia New Zealand implemented the mandatory use of iodised salt to replace non-iodised salt in bread production in 2009 to address this (organic and unleavened bread,  such as crumpets, are exempt). This move has had a positive impact on the iodine status of school children  and adults since.

The recommended dietary intake of iodine peaks at 150 micrograms  per day for most of us over the age of 14. Due to the importance of iodine during pregnancy and infancy, the recommended dietary intake for pregnant women is raised to 220 micrograms per day and for breastfeeding women to 270 micrograms per day.

Currently the Ministry of Health recommends that pregnant and breastfeeding women take a registered, iodine-only supplement of 150 micrograms of iodine per day. Research released by Dr Andrew Reynolds and Prof Skeaff just this month found that only 38% of pregnant women are following supplement recommendations for folic acid and iodine. Discuss this with your lead maternity carer and/or doctor who may be able to prescribe these for you, and purchase safe supplements from pharmacies.

And of course, we also need to ensure that our children get iodine regularly in their diet by encouraging them to eat good sources of iodine.

So what foods are good sources of iodine? The best sources of iodine are foods from the sea!  Fish, seafood and seaweed are all good sources. The Ministry of Health advises pregnant women to limit their intake of large fish to three 150g servings per week to manage their mercury intake.

However, the consumption of canned tuna and smaller fish such as blue cod and hoki accumulate less mercury so are safe to eat regularly.  Dairy foods such as milk, yoghurt and cheese are also a good source of iodine in the New Zealand diet. Iodised salt is an obvious one, but with rising rates of high blood pressure and heart disease, this is not a food source that we should increase in our diet.

- Dr Kirsty Fairbairn is a health, wellness and sports dietitian at Invigorate Nutrition (www.invigoratenutrition.com), based at Eclipse Health, Wellness and Performance, Hanover St, Dunedin.

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