Vitamins and minerals:
dietary sources, supplements and deficiencies
What is it?
Iodine is an essential nutrient that plays an important role in thyroid hormone production. Thyroid hormones maintain metabolic state and support normal growth and development.10
Who needs it?
Everybody needs a small amount of iodine in their diet to maintain normal growth and metabolism, however iodine is especially important for pregnant and breastfeeding mothers. Pregnancy alters thyroid function due to an increase in hormone requirements, beginning in the first trimester.11
Most table salt in New Zealand is fortified with 25–65 mg/kg iodine. However, recent evidence shows that many New Zealanders have low iodine levels. A study in 2005 found that babies who had been breastfed had lower levels of iodine (due to the low concentration of iodine in their mother’s milk) than babies who had been bottle fed.12 The 2002 Children’s Nutrition Survey found that most New Zealand children aged 5–14 years had mild iodine deficiency (assessed by urinary iodine excretion).13
The re-emergence of iodine deficiency is thought be to due to:10
- Increased consumption of commercially prepared foods which are manufactured with non-iodised salt
- Declining use of iodine sanitisers (iodophores) in the dairy industry (opportunistic source of iodine)
- Less salt being used due to healthy eating messages
- Increased use of non-iodised sea salt or rock salt
Symptoms of iodine deficiency
Iodine deficiency causes thyroid dysfunction, resulting in stunted growth, developmental brain damage and adverse effects on hearing, motor and cognitive function.10
Iodine deficiency during pregnancy can cause impaired mental development of the foetus and in severe cases may result in miscarriage, still birth or congenital abnormalities.14
Adverse effects of excess iodine
The primary adverse effect of excess iodine is inhibition of thyroid hormone production. Adverse effects have been observed at iodine levels of 1700 mcg/day but people with thyroid disorders or a long history of iodine deficiency may experience adverse effects at lower levels.14
Tests for iodine status
Laboratory testing of iodine status on an individual basis is not usually indicated when iodine deficiency is suspected.
A normal TSH level may be used as a “rule out” test for hypothyroidism caused by iodine deficiency.
Dietary sources and supplements
Dairy products are the main source of iodine in the typical New Zealand diet. Iodised salt should be used in preference to non-iodised salt, however overall consumption of salt should not be increased. It is difficult to obtain adequate iodine from a normal diet. Good dietary sources of iodine include seafood (fish, shellfish, seaweed), seameal custard, milk and eggs.
The iodine content in New Zealand soil is low, therefore plants grown locally are not a good source of iodine. Brassica vegetables (cabbage, broccoli, brussel sprouts), kumara, cassava and lima beans can impair iodine absorption. The iodine content of foods can also be depleted by cooking.10, 14
The World Health Authority and other researchers recommend that women who are considering a pregnancy, are pregnant or are lactating, consume a supplement of potassium iodide, 150 mcg/day.15, 16 Prenatal vitamin supplements usually contain potassium iodide.
Routine use of iodine supplements is not indicated for other groups of people. Iodine supplements such as kelp tablets are not recommended as the iodine content is variable and not subject to control.
In March 2008 a proposal to recommend the use of iodised salt in all bread making in New Zealand was accepted (excluding organic breads). The bread making industry has been given 18 months to prepare for iodine fortification. The standard will become enforceable in September 2009. This also coincides with the implementation date for mandatory fortification of bread with folic acid.
See www.nzfsa.govt.nz for more information.
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