Question:
My GP recommended about 30 years ago that I cut out salt and salty foods from my diet, which I did. Now I don’t like the taste of salty foods. Am I at risk of not having enough iodine in my diet?
Answer:
In decades past, iodised table salt was a staple on the Kiwi dinner table, providing the much-needed iodine lacking in our local produce and meats because of New Zealand’s low soil-iodine levels. However, subsequent public health campaigns to reduce our copious salt intake had the unfortunate side effect of lowering our iodine intake. This led to iodine deficiency re-emerging in the 1990s. Fortunately, in the past decade matters have been improved since bread has been fortified with iodised salt.
Iodine is an essential mineral that the body cannot make so we need a regular supply in our diet. It is an integral part of thyroid hormones that maintain our body’s metabolic rate and support normal growth and development in children and infants. Low soil and groundwater levels of iodine are common throughout the world and result in diets that are low in iodine.
An iodine deficiency can cause several problems. The most visible of these is a goitre, an enlarged thyroid gland at the base of the neck. It can also cause significant harm in unborn babies through to children, given iodine’s critical role in brain development. The World Health Organisation notes that, globally, iodine deficiency is the leading cause of brain damage in childhood.
Iodine deficiency was largely eradicated in New Zealand by adding the chemical to table salt in the 1920s and increasing the level in the 1930s. However, it re-emerged in the 21st century as our discretionary salt intake declined and the dairy industry moved away from using iodophors (iodine-containing preparations) as sanitisers for milk equipment.
In 2009, New Zealand opted to fortify bread with iodised salt to halt the re-emergence of widespread iodine deficiency. Bread now contributes nearly half of an adult’s iodine intake in their diet, followed by milk and milk products. Dairy products are a naturally rich source of iodine – the mammary gland bio-concentrates iodine into the milk, so any dairy products produced from dairy milk contain iodine.
If you regularly consume commercially prepared bread, your iodine status is probably adequate. A 2016 study found Kiwi kids had an adequate iodine status thanks to the fortification of bread. About the same time, another study found adults were likely to have adequate iodine status too, thanks to introducing iodised salt in bread.
In contrast, about 29% of elderly New Zealand rest-home residents were mildly iodine deficient in the same year. In 2020, researchers found New Zealand toddlers typically had a moderate or mild iodine deficiency. Likewise, a study published in 2021 showed breastfeeding mothers were also iodine deficient.
Given the crucial role of iodine in the development of unborn babies and infants, the Ministry of Health recommends pregnant and breastfeeding women take a 150 microgram (mcg) iodine supplement daily.
Breastfeeding mothers who take an iodine supplement have markedly better iodine status, as do toddlers who drink an iodine-fortified milk drink to boost their iodine status; however, the fortified drink would not be enough for toddlers with mild-to-moderate iodine deficiency.
It’s important to note that New Zealanders who do not eat commercial bread or eat only organic bread will not benefit from the mandatory iodine fortification of bread. However, you can get a general idea of your iodine status by considering how much commercially prepared bread and other iodine-rich foods such as seafood, milk, eggs, meat and cereals you typically consume (see table below).
