I am troubled because these women are very likely putting the developing brains of their unborn babies at risk of suffering intellectual impairment and other neurological disorders. Not because the plant-based alternative milks are naturally harmful, but unlike dairy milk, these products do not contain the micronutrient iodine that is essential for optimal maternal thyroid gland function that regulates normal foetal brain development.

As Angus Dalton highlighted in his article I’ll have a half-oat, half-soy decaf in The Sun-Herald last week, a quarter of the customers in the cafés he canvassed had ditched dairy milk in their coffees for plant-based alternatives. This causes me a great deal of concern for future generations of Australian children.

The World Health Organisation states iodine deficiency during pregnancy and early infancy – the first 1000 days of life when development of the human brain is so critical – is the commonest global cause of preventable intellectual disability.

Added to this, there is also good evidence implicating iodine deficiency during pregnancy – as a causal or possible contributory factor – in the development of other neurological disorders such as ADHD and autism, which are on the increase in Australia without any plausible explanation. These consequences are not simply theoretical. Recent studies performed in Tasmania, where dietary iodine deficiency has been more prominent than elsewhere in Australia, have confirmed that children born to women with mild iodine deficiency during pregnancy had a 10 per cent reduction in literacy and numeracy performance, compared with children born to mothers who had a sufficient iodine intake.

Unfortunately, follow-up of these affected children has shown impaired school-performance has persisted and is irreversible. When you see these results, it is not unreasonable to ask: is iodine deficiency the reason for the continuing decline in the academic standards of Australian schoolchildren compared with their peers in many other countries? As with the increased rates of ADHD and autism, no plausible explanation has been advanced for the comparatively poor performances in these international examinations.

Iodine in the Australian diet comes mainly from dairy milk, iodised salt, and to a lesser extent seafoods. There is good evidence from studies of pregnant women in Sydney that dairy milk has been the main source of iodine for the great majority not taking an iodine supplement.

Iodine deficiency re-emerged in Australia several decades ago because of a decline in the content of iodine in dairy products, coupled with decreased household use of iodised salt and it not being used by the food manufacturing industry.

This problem continues as it is frequently difficult to find any iodised table salt on display in our grocery stores, where the shelves are dominated by un-iodised crude, pink crystalline salts imported from salt mines in Pakistan and labelled as “Himalayan salt” – somehow conveying some mystic qualities on these products. When I get the opportunity, I furtively look for the iodised salt products and bring them to the front of the shelf and push the un-iodised products to the back.

In response to a national survey of children in 2005-2006 by Westmead Hospital and Sydney University, which established widespread insufficient dietary iodine intake in the Australian population, WHO labelled Australia as an iodine deficient country. State and federal health departments eventually mandated iodine fortification of food in 2009 requiring that all salt used in the making of bread and such products must be iodised salt.

While this initiative has been successful in raising the intake of iodine to satisfactory levels in most of the population, it is insufficient to meet the natural increased requirements of iodine during pregnancy and breastfeeding.

To address this deficit in 2009, our National Health and Medical Research Council (NHMRC) recommended a daily supplement of 150 ug of iodine for women during pregnancy and breastfeeding. Unfortunately, this message has not been well promoted as current research indicates approximately 50 per cent of pregnant women in Australia are still not getting enough iodine.

We know from very good studies conducted in pregnant women before mandatory fortification of bread with iodised salt in 2009, that the principal source of iodine in the diet of pregnant women in Australia was dairy products.

Given the increasing preference for plant-based milks it is likely that this new fad, coupled with the consumer preference for non-iodised salt in the home and by the food industry, will likely contribute to a further decline in iodine intake in vulnerable sections of our population.

It is unquestionable that the outcome will be some degree of impairment of normal brain development in the most critical first 1000 days of life, leading to lower IQs, lifelong disability, and disadvantage in a large section of our population, unless we take action to prevent it. We know what the problem is, how to fix it, and we should do it now.

Creswell Eastman is a Professor of Medicine at Sydney University Medical School, Principal of the Sydney Thyroid Clinic and Consultant Emeritus to the Westmead Hospital.