Dr Carol Wham

For many years we have been advised to eat less salt. This is largely recommended to help decrease the average blood pressure levels in the general population. A further benefit of reducing salt intake is related to the bone thinning disease, osteoporosis. A high amount of salt in the diet increases the loss of bone building calcium in the urine. This increases the body’s overall need for calcium, a nutrient already in short supply.

New Zealand is an extraordinarily low iodine environment. Our vegetables, fruit and other plant foods have low iodine content, as they are grown in relatively iodine deficient soils. Typically iodine has been leached from the soil by high rainfall. Meats and poultry similarly reflect this low iodine content. In the early part of last century there was a high prevalence of goitre in New Zealand caused by iodine deficiency. The fortification of table salt in 1924 was a vital public health measure. Presumably consumption of relatively large amounts of seafood protected Maori from any problem before the arrival of the Europeans.

In recent years our iodine intake has been compromised with lifestyle changes including the trend to eat more pre-prepared foods and to dine out. The salt used in manufactured food is non-iodised. Until recently milk was ‘contaminated’ with iodine by the use of iodophor disinfectants to clean milking equipment and milk storage vats but this can no longer be relied upon.

The outcome is that New Zealanders now have perilously low intakes of iodine. A survey of adult blood donors carried out in Dunedin and the Waikato suggested some had critically low iodine intakes, which placed them at risk of goitre. Of more concern a study of over 300 school children at 30 Wellington and Dunedin schools showed more than half were iodine-deficient. Of interest, about a third of the children’s caregivers did not use iodised salt in cooking and about half of the children did not use iodised salt at the table.

This cannot be taken lightly. A lack of iodine may impair mental growth and physical development in children. Indeed it is the world’s greatest single cause of preventable brain damage and mental retardation. Goitre (enlarged thyroid gland) that occurs at lesser degrees of iodine deficiency threatens to re-emerge.

It is an anomaly that we are reliant on salt as a key source of iodine but discouraged to sprinkle it on our food. Our public health authorities are working on alternative vehicles for fortification such as bread. Meanwhile, the public health view is that where salt is used in food preparation or added to food it should always be iodised. We need 150 mcg of iodine a day. Good food sources include eggs, fish, shellfish as well as seaweed (found in sushi). Enjoy.