Tuesday April 28 2015
Milk and dairy products are a common source of iodine
"Pregnant women who switch to 'healthier' organic milk may be putting the brain development of their unborn babies at risk," The Guardian reports after researchers found organic milk had lower levels of iodine than standard milk.
Iodine is needed for the healthy function of the thyroid gland. Thyroid hormones are needed for the development of the brain and spinal cord in an unborn baby. This means a sufficient iodine intake during pregnancy is important, as it is throughout life.
As a result of different farming systems, milk produced from grass-fed organic cows during the summer is known to contain less iodine than standard milk.
This study compared samples of milk taken from supermarkets during the winter. It found that in the winter, organic milk still contains around a third less iodine than standard milk. This is regardless of fat content. But the iodine content in a normal 346ml glass of organic milk was still enough to provide the recommended daily iodine intake.
Despite the headlines, the researchers did not actually look at the effects of milk consumption on any measure of child health, including intelligence. The study also did not consider the iodine content of other dairy products or non-dairy sources, such as eggs, fish and certain grains.
This study therefore does not provide any evidence to suggest drinking organic milk during pregnancy could have a negative impact on a child's IQ.
But it is worth being aware that organic milk is likely to contain less iodine than standard milk, so you may need to balance your intake through other sources.
Where did the story come from?
The study was carried out by researchers from the University of Reading and was published in the peer-reviewed journal, Food Chemistry.
It was funded by the University of Reading, and the authors declare no conflicts of interest.
The media headlines give the impression the study found evidence organic milk can harm babies' development. This is not the case.
While it is true iodine is needed for the development of a healthy brain and nervous system, this study only compared the iodine content of a sample of different milks. It didn't look at any health outcomes for babies whose mothers drank organic or non-organic milk during pregnancy.
What kind of research was this?
This was a cross-sectional study that aimed to compare the iodine content of organic and standard milk produced during the winter; whole, semi-skimmed and skimmed milk; and pasteurised and ultra-high-temperature (UHT) treated milk.
The researchers explain how iodine is a key component of the hormones released by the thyroid gland. These hormones are important for the development of the foetal brain and spinal cord. This makes iodine intake in pregnancy important.
The recommended intake for adults in the UK is 140mcg (0.14mg) a day. The World Health Organisation recommends this is raised to 250mcg during pregnancy and breastfeeding.
Previous studies have observed an increase in iodine deficiency in the UK, particularly among teenage girls, as we reported in 2011. Milk and dairy foods are the main source of iodine intake in this country.
One study found organic milk produced during the summer has lower iodine concentration than standard milk. But there has been little research comparing organic milk produced in the winter with standard milk, or looking at the effect of the fat content in milk or the processing method. This is what this study aimed to investigate.
What did the research involve?
The researchers carried out two studies to investigate this. In the first study, they purchased 22 samples of organic and standard milk (whole, semi-skimmed and skimmed) from two supermarkets in Reading in late January 2014.
In the second study, they purchased 60 samples of milk from four supermarkets in Reading over three consecutive weeks starting from the beginning of February.
They bought five different types of milk product:
- standard semi-skimmed
- organic semi-skimmed
- branded organic semi-skimmed
- UHT semi-skimmed
- Channel Island standard whole milk
Milk samples were then analysed in the laboratory for fat, protein, lactose and iodine concentrations.
What were the basic results?
In the first study, the researchers found standard or organic production systems made no difference to the fat, protein or lactose content of milk (whole, semi-skimmed or skimmed).
However, organic milk had significantly lower iodine concentration than standard milk – about a third lower. The difference here was 595mcg/l of iodine in each litre of standard milk, compared with 404mcg/l in each litre of organic milk.
The second study similarly found organic milk had a significantly lower iodine concentration than standard milk. Again, this was about a third lower, with standard milk having iodine of 474mcg/l versus 306mcg/l in organic. Branded milk tended to have lower iodine content than own-brand organic.
UHT milk also had a significantly lower iodine content than standard milk and was no different from organic milk. The iodine content in the standard Channel Island whole milk was no different from the other standard milks.
How did the researchers interpret the results?
The researchers say their results "indicate that replacement of [standard] milk by organic or UHT milk will increase the risk of suboptimal iodine status, especially for pregnant or [breastfeeding] women".
Previous studies have shown that organic milk produced during the summer has lower iodine content than standard milk. This is said to have been the first study comparing the milks in the winter. It also found iodine concentration is lower in organic milk.
During the winter, cows housed indoors receive more iodine supplement through their feed concentrate than grazing cows in the summer. Winter milk is therefore known to contain more iodine than summer milk.
It may have been expected there would be less of a difference between organic and non-organic cows during the winter. But organic systems are known to rely more heavily on foraged feed than standard systems, which is why the iodine content of the milk is still expected to be lower in organic milk.
However, before leaping to the conclusion that everyone should avoid organic milk – particularly pregnant and breastfeeding women – there are some points to bear in mind.
- The study only compared samples from a small number of supermarkets from two months in the winter of 2014. Though these are likely to give a good indication, the iodine content of milk may vary across the country and in different years.
- Although there was almost 200mcg (0.2g) less iodine per litre in organic milk compared with standard milk, this may not mean a person who drinks this milk has an insufficient iodine intake. The amount of iodine in organic milk was still sufficient to provide the daily recommended intake of iodine in a standard glass of 346ml.
- The study also has not taken into account other dietary sources of iodine beyond milk. It did not compare the iodine content of other organic and standard dairy products, such as cheese and yoghurt. Nor did it look at non-dairy sources, such as eggs, fish and grains. Pregnant women have to be careful about eating some non-milk sources of iodine, such as soft cheeses, undercooked eggs and seafood, and are advised to limit their intake of certain fish, such as tuna.
- Iodine is needed to help the development of the foetal brain and nervous system. But this study did not look at health outcomes for the foetus or infant. The study did not compare the outcomes for a group of pregnant women who drank organic milk throughout pregnancy against outcomes for women who drank non-organic milk. News reports that organic milk may harm an unborn baby or affect IQ are therefore not supported by the results of this study.
- Excess iodine intake could have an impact on the way the thyroid works. It should be possible to get all the iodine you need through a balanced diet without the need for supplements, even during pregnancy. The current advice is you should take no more than 500mcg (0.5mg) of iodine supplements a day.
The possible benefits and drawbacks of organic versus non-organic farming methods have often been debated. There is no firm evidence that organic foods offer any health benefits.
The choice about whether or not to go organic is often prompted by ethical concerns about animals and the environment. Pregnant and breastfeeding women still have this option – there is no evidence that drinking organic milk could harm an unborn baby.
If you drink organic milk, it is likely to contain less iodine than standard milk, so you may need to balance your intake through other sources. Good food sources of iodine include fish and shellfish.
Pregnant women should never eat raw shellfish and should also avoid eating shark, swordfish and marlin because of their high mercury content.
Read more about foods pregnant women should avoid
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.