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Food Focus February 2008

The importance of iodine in our diet

Foods grown in New Zealand soils are low in iodine which means we don’t get enough iodine in our food. Low iodine levels in our diet may lead to health issues such as poor growth and development in children, thyroid diseases and goitre

Iodine is an essential nutrient for humans. It is only required in very small amounts, but is important for 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 and function, it is particularly important that unborn babies and young children have enough iodine.

Foods that are rich in iodine include seafood (fish, shellfish and seaweed), iodised salt, seameal custard, milk and milk products, and eggs.

Iodine deficiency emerging

New Zealand-grown vegetables, fruits and grains have very low levels of iodine compared with food produced in other parts of the world. Even with a balanced diet, it is difficult for New Zealanders to get enough iodine from the food they eat. NZFSA’s Total Diet Survey (2003/04) estimated the iodine intake of New Zealanders is at best only 57% of the recommended dietary intake (the adequate daily intake is 200 μg per day for both men and women).

Here, as in many other countries, iodine deficiency has led to health problems. In the late 1800s and early 1900s, goitre was very common. In 1924, iodine was added to table salt to increase the iodine in people’s diets. The amount of iodine added to table salt was increased in 1938.

Some New Zealand children are now showing signs of mild iodine deficiency. The re-emergence of iodine deficiency appears to be due to:

• people eating more commercially prepared foods (which tend to be made with non-iodised salt)

• reduced use of iodine-containing sanitisers by the dairy industry (we used to get small amounts of iodine in cow’s milk when the dairy industry used disinfectants containing iodine)

• less salt being used in home-prepared foods because of health messages encouraging consumers to reduce their salt intake.

A serious health effect of iodine deficiency is goitre (enlargement of the thyroid gland) and 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.

Increasing iodine consumption

The Ministry of Health recommends choosing iodised salt if using salt, although their key health message is to reduce our overall salt intake.

Internationally the preferred option for increasing iodine levels in food is to ensure all salt (including salt used in processed foods) is iodised. This is a simple and low-cost way of increasing the iodine content of a range of foods. Food Standards Australia New Zealand is working on a proposal for mandatory fortification of iodine to redress the issue of low iodine intake.

While iodine intakes are an area of concern, advice for adult New Zealanders needs to be mindful of the risks associated with a higher intake of salt. For these reasons, the Ministry of Health offers the following advice.

Salt intake should be limited but when salt is used for cooking and at the table it should be iodised salt. (Rock salt and other salts have negligible levels of iodine and are not recommended.)

Consider including other important sources of iodine such as low-fat milk products, eggs, fish and seafood. Foods containing seaweed, such as sushi, seameal and alginates (food-thickening agents), are also a good source of iodine.

Use of kelp and iodine supplements is not recommended, unless under the supervision of a doctor or a dietitian, as these are usually highly concentrated sources of iodine. High intakes of iodine can be toxic and may have adverse health effects.