Beware the hidden health damage of Veganuary: How plant milks could be secretly harming our IQ

You’ve probably never been concerned about iodine deficiency. And why would you? Up until 15 years ago, it was a thing of the past in this country.

But worrying data shows it now is. The iodine status of women of reproductive age in the UK is now among the lowest in the world, below countries such as Ghana and Mozambique. No other major western country is as low as us.

And the health of unborn babies could be affected as a result – leading to miscarriage, stillbirth, irreversible brain damage and lower IQs in younger generations.

Iodine deficiency also increases the risk of women and men developing thyroid enlargement and nodules, lumps in the thyroid gland and thyroid disease.

Iodine is an essential mineral we need in small amounts to produce thyroid hormones. Too few of these, as with an under-active thyroid, causes your metabolism to slow down and symptoms including weight gain and tiredness. But during pregnancy, the effects of low iodine can be particularly devastating – a severe deficiency can not only lead to possible complications such as lower birth size, but also death of the baby; or, if it survives, brain damage.

Iodine is essential in producing thyroid hormones for mother and foetus. These regulate the development of the foetal brain and nervous system – which is why iodine requirements increase substantially for a mother during pregnancy.

Even a mild deficiency can affect the child’s cognitive abilities such as speech and reading – lowering IQ scores by eight to ten points, the World Health Organisation (WHO) says.

But there is an incredibly simple solution: add iodine to salt, something already done for up to 90 per cent of the world’s population.

I first became aware of a possible iodine deficiency problem in the UK back in 2011, after being asked to help provide an up-to-date measurement of our iodine status, as there had been no national studies previously.

My clinical research interests include the management of thyroid conditions and endocrine disorders in pregnancy.

Given the importance of the mineral during pregnancy, my colleagues from nine centres in the UK and I, along with the British Thyroid Association and the UK Iodine Survey Group, chose to focus on girls aged 14 to 15 – a group likely to become pregnant in the coming few years.

Results from urine tests (the only way to measure iodine levels) on 750 girls showed more than 50 per cent were either mildly or moderately iodine deficient. It was a shock.

Deficiency worldwide used to be very common, including in the UK. One of the most obvious symptoms is goitres – a lump at the front of the neck caused by a swollen thyroid.

In 1924, a survey in England and Wales reported visible goitres in up to 30 per cent of 12-year-old children.

Another study – in Oxfordshire and Dorset during the Second World War – noted visible goitres in 50 per cent of adult women and 26 to 43 per cent of girls.

When caused by iodine deficiency, as this was, it was entirely reversible.

Recognising a similar problem across the world, many countries started to introduce salt iodisation – adding potassium iodate solution to wet salt as a simple way to raise iodine levels through diet.

In 1993, the WHO established a global salt iodisation programme and 140 countries including many in Europe, the US and Australia signed up – but not the UK, as we thought we had eradicated our iodine deficiency during the early 20th century.

It’s not always easy to get enough iodine from diet.

It originates in the soil and is often washed by rainwater into the sea. Fish and shellfish are good sources but, in the UK, we eat less than one portion a week.

In the 1930s, we started to add iodine to animal feed to boost fertility and lactation; iodine-based sterilisers were also used in milking.

Suddenly, cow’s milk – not naturally rich in iodine – became an abundant source. Thanks to a post-war push to drink milk, involving giving it to children, it became our main source of iodine – around 40 per cent of our intake – and led to a three-fold increase, eradicating any deficiency.

Or so we thought until those 2011 study results.

At the time the Government was, understandably, worried about introducing salt iodisation because they feared it would promote people to eat more salt, not least as this coincided with a salt-reduction campaign.

However, the WHO stressed that iodised-salt programmes can run concurrently with salt reduction campaigns. This is so long as individual countries adjust the concentration of iodine in salt to ensure adequate intake comes from lower salt consumption.

British adults are advised to eat no more than 6g of salt per day, about one level teaspoon. But you only need a quarter of a teaspoon of iodised salt to get around 78mcg of iodine, instantly providing 52 per cent of the daily recommended 150mcg for those aged over 12. This is well within the guideline amounts.

It is also just over 30 per cent of the 250mcg recommended for pregnant women and those who are breastfeeding.

Ultimately, the Department of Health did not deem it necessary. And now the situation has got worse. Much worse.

The number of people drinking cow’s milk – our main source – has declined.

The average consumption has dropped by nearly 50 per cent in the UK since 1974, according to the Defra Family Food Survey.

Many women in particular avoid it, as well as younger age groups, who are increasingly opting for plant-based alternatives such as almond and coconut. [There is ten times more iodine in cow’s milk than plant-based milks, according to a 2023 study in the Journal of Trace Elements in Medicine and Biology.] The surge in veganism contributes to this decline. 

In the National Diet and Nutrition Survey 2019-2023, the average urinary iodine concentration of women of reproductive age (16 to 49 years old) was 82 mcg per litre (ug/L) – with 30 per cent recording levels below 50ug/L.

Both are far below the 100-199 threshold noted by the WHO – and well below what is required for reproductive and foetal neurodevelopmental health.

Women considering becoming pregnant are careful, quite rightly, to get enough folic acid – essential for the development of the baby’s brain and spinal cord – but iodine is equally important in protecting their baby’s developing brain.

If we don’t do something about this, I fear we will see more complications at birth, more children with reduced comprehension and slower processing speeds – and our IQs will drop.

There may also be an increased risk of thyroid enlargement and thyroid disease, too.

I always seek out iodised salt for my family. It is not available everywhere but you can find it at some supermarkets and online. I also ensure my family drink cow’s milk – any type is beneficial; there is no evidence to suggest that iodine content is different when comparing full-fat, semi-skimmed or skimmed milk.

But clearly we can’t hope that individuals will ensure they get enough – something needs to be done on a population-wide basis.

Along with implementing salt iodisation, there needs to be regular surveillance of urinary iodine – particularly in women of reproductive age – and we need to raise awareness among health professions and the public, too.

Iodine deficiency is the world’s single most preventable cause of brain damage and intellectual disability.

Taking action right now would prevent entirely avoidable harm to future generations.

Professor Boelaert is a professor of endocrinology at the University of Birmingham. She was speaking to Charlotte Dovey