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Although only required in very small amounts, iodine is an essential nutrient. This page provides information on the status of Iodine in New Zealand.
Questions and answers
On this page:
- Why is iodine important and what happens if there is not enough in the diet?
- Is iodine deficiency an issue in New Zealand?
- Why is there a re-emergence in iodine deficiency?
- Could we get enough iodine from the food we eat?
- Should supplements containing iodine be recommended to consumers?
- How is it proposed to increase the amount of iodine in the food supply?
- Are there any side effects for consumers from increased iodine levels in food?
- How will we know the effect of increasing iodine in foods?
Why is iodine important and what happens if there is not enough in the diet?
Iodine is an essential nutrient for humans. Although only required in very small amounts, it is an important constituent of thyroid hormones. These hormones maintain the body’s metabolic state and support normal growth and development in children. As iodine is essential for normal brain development, it is particularly important that the unborn baby (fetus) and young children have adequate intakes.
The term iodine deficiency disorders is used to describe the wide range of effects low iodine status can have on health. One of the serious health effects of iodine deficiency disorders is goitre (enlargement of the thyroid gland leading to a swelling of the neck). In very severe iodine deficiency, stunted growth and mental retardation can occur in children. A number of studies have reported adverse effects on hearing capacity, motor and cognitive function in children associated with moderate and severe iodine deficiency.
Is iodine deficiency an issue in New Zealand?
As in many other countries around the world, evidence of iodine deficiency has been observed in New Zealand and in the late 1800s and early 1900s goitre was very common. In order to decrease the incidence, table salt was iodised at a low level from 1924. However, this had little effect and the level was increased to 40-80mg of iodine per kilogram of salt in 1938. When iodised table salt was introduced there was a major public education campaign to ensure people understood the benefits of using iodised salt in the home. However, non-iodised salt has always been available in New Zealand as well as iodised table salt.
Recent evidence from a number of studies has indicated that the iodine status of New Zealanders is now declining to the point where intervention is again required to ensure that iodine deficiency disorders do not once again widely affect the New Zealand population. These studies have provided the evidence for the decision to add iodised salt to commercially prepared bread from September 2009. For example, a study by Skeaff (2005) of breast-fed infants showed that iodine levels were less than half of that of formula-fed infants, reflecting the low iodine concentration of breast milk due to the poor iodine status of breast feeding mothers. The 2002 National Children's Nutrition Survey found that New Zealand children (aged 5-14 years) had mild iodine deficiency and that 28 percent of the children studied had low iodine status. During 2005 a nationwide survey of the iodine status of 170 pregnant women was undertaken by the University of Otago (Pettigrew Porter et al 2006). The results showed moderate iodine deficiency and goitre was found in 7 percent of the women. No differences were found across the regions or between the stages of pregnancy.
At the end of this page there is a further reading that shows the observations and research that demonstrate the extent of the current deficiency.
Why is there a re-emergence in iodine deficiency?
The re-emergence in iodine deficiency appears to be due to:
- the increased consumption of commercially-prepared foods (manufactured mostly with non-iodised salt)
- the declining use of iodine-containing sanitizers by the dairy industry
- less salt being used in home prepared foods as a response to the health messages to reduce salt intake
Could we get enough iodine from the food we eat?
Dietary sources of iodine include seafood (fish, shellfish and seaweed), commercially prepared bread, iodised salt, seameal custard, milk and eggs. The Ministry of Health’s Food and Nutrition Guidelines for Healthy Adults: A background paper (Ministry of Health, 2003) recommend choosing iodised salt when using salt, but do not recommend increasing overall salt intake.
The iodine content of, vegetables, fruits and grains generally reflect the iodine level of the soil in which they were grown. The iodine content of New Zealand soils is low and as a consequence locally produced foods are also low in iodine.
Iodine levels of meat, chicken, eggs and dairy products are reflective of the iodine content of the animal feed used. Currently it is difficult for most consumers to obtain adequate iodine from their normal diet, which is why commercially prepared bread must now have iodine added to it. Requirements for iodine increase during pregnancy and breastfeeding and even with a well balanced diet, it is difficult to get enough iodine from food alone. Women of child bearing age are encouraged to choose foods that are important sources of iodine. These foods include seafood, commercially prepared bread, milk, eggs, meat and cereals and seameal custard. Ensure you follow food safety advice when eating these foods.
For more information, refer to the Health Education publication Eating for Healthy Pregnant Women (HE1805)
Should supplements containing iodine be recommended to consumers?
Due to an increased need for iodine in pregnancy and breastfeeding, the Ministry recommends that pregnant and breastfeeding women take a registered 150 micrograms (mcg) daily iodine only tablet daily available at pharmacies, as well as eating foods which are important sources of iodine. More information can be found in the Health Education publication Folic Acid and Iodine.
While consumption of other iodine containing supplements and kelp tablets will increase iodine intakes, care needs to be taken when considering this type of supplementation as this may lead to an intake of iodine beyond the safe upper level of intake. The margin between too much and too little is very narrow and the iodine content in seaweed products and kelp tablets is extremely variable.
How is it proposed to increase the amount of iodine in the food supply?
Since September 2009, iodised salt is added to bread in New Zealand to address the re-emergence of iodine deficiency. The salt iodisation level is in the range of 25-65 mg of iodine per kg of salt. This range is the same as the current iodised (table) salt range. For details about the decision see the Food Standards Australia New Zealand website and search for 'fact sheets (for iodine)'.
Are there any side effects for consumers from increased iodine levels in food?
The effects of high iodine intakes on thyroid function are variable and depend on the health of the thyroid gland. Very high intakes (in excess of the established upper level of intake) of iodine may inhibit thyroid hormone production. A sudden increase in iodine intake in those used to very low intakes for prolonged periods of time can produce iodine-induced hyperthyroidism or thyrotoxicosis. However, as it is unlikely that this would be an issue for the New Zealand consumer as the decline in iodine levels is relatively recent. More information on iodine sensitivities and mandatory fortification is available on the Food Standards Australia New Zealand website.
How will we know the effect of increasing iodine in foods?
It will be important that the effectiveness and any unexpected consequences of mandatory fortification of bread with iodine are monitored. Monitoring is done by regularly checking the iodine content of foods and by measuring the iodine status of the population, in particular the most vulnerable groups within the population, such as children and pregnant women.
For example, the New Zealand Total Diet Study (TDS) conducted every five years, was most recently undertaken in 2009 by MAF’s New Zealand Food Safety Authority (NZFSA). The TDS includes iodine, and comparison of 2009 results to previous TDSs will provide valuable trend data. For more information about the TDS go to the New Zealand Food Safety Authority website.
National Nutrition Surveys are also a tool which can be used to monitor the iodine status of the New Zealand population. These surveys are the responsibility of the Ministry of Health. In 2002 the Children’s Nutrition Survey collected urine samples for measurement of urinary iodide, which indicates iodine status. The assessment of urinary iodide excretion (to measure iodine status at a population level) has occurred in the 2008/09 New Zealand Adult Nutrition Survey (ANS). Access the ANS results, including iodine, in the 2008/09 New Zealand Adult Nutrition Survey.
This range of information will assist the Government to monitor the effectiveness of mandatory fortification of bread with iodine to improve the iodine status of New Zealanders. Adjustments to the levels of iodine in bread can then be made as required.
Are breast-fed infants and toddlers in New Zealand as risk of iodine deficiency? Skeaff S, Ferguson E, McKenzie J, Valeix P, Gibson R, Thomson S. (2005). Nutrition, 21, 325-331.
Food and Nutrition Guidelines for Healthy Adults: A background paper. Ministry of Health, (2003)
(For other life stages, view the Food and Nutrition Guidelines)
Iodine supplementation improves cognition in mildly iodine-deficient children. Gordon R.C, Rose M.C, Skeaff S.A, Gray A.R, Morgan K, Ruffman T. (2009) American Journal of Clinical Nutrition.
Mild iodine deficiency in a sample of New Zealand school children. Skeaff S, Thomson C, Gibson R. European Journal of Clinical Nutrition, (2002) 56, 1169-1175
The re-emergence of iodine deficiency in New Zealand? Mann J, Aitken E. (2003), New Zealand Medical Journal 351, 1161-1170
Review article – Selenium and iodine intakes and status in New Zealand and Australia, Thomson C. British Journal of Nutrition, (2004), 91, 611-672.
The Thyromobile and iodine in pregnancy (TRIP) survey: Assessing the iodine status of New Zealand pregnant women. Pettigrew Porter A, Skeaff S,Thomson C et al. Paper presented at the New Zealand Dietetic Association 2006, 11-13 September at Te Papa in Wellington.
Victories and Challenges in Optimizing Iodine Intake, Laurberg P. (2004). Thyroid, 14, 589.
If you have further queries about the status of iodine in New Zealand