Individuals who choose to drink organic milk as a healthier alternative could be compromising their intake of iodine, of which milk is the main source, according to research published and presented at the Nutrition Society Summer meeting at the University of Reading.
Research from the University of Surrey shows that organic milk has a 42.1% lower concentration of iodine than conventional milk.
This matters because iodine is an essential component of thyroid hormones which are crucial for brain development, especially in the womb and in early childhood. Mild-to-moderate iodine deficiency in pregnancy has been associated with poorer brain and neurological development of the baby in the womb resulting in poorer outcomes for the child that include lower IQ and poorer school performance.
The lower iodine concentration of organic milk has clear public-health implications given the recent evidence of the re-emergence of iodine deficiency in the UK population, particularly in young and pregnant women, and the increasing popularity of organic milk.
Professor Margaret Rayman, Professor of Nutritional Medicine at the University of Surrey said: "As milk is the principal source of iodine in the UK diet, pregnant women and nursing mothers who choose organic milk should be aware that their iodine intake, and hence the brain development of their children, may be compromised. They should therefore ensure that they have an adequate intake of iodine from other dietary sources, such as fish. That said, they should avoid kelp which can provide excessive amounts."
The study aimed to compare the iodine concentration of retail organic and conventional milk. Ninety-two samples of organic milk and 80 samples of conventional milk, purchased from retail outlets in sixteen areas of the UK, were analysed for their iodine concentration.
Professor Rayman suggests that the lower iodine concentration in organic milk is "probably due to the limitations placed on the routine use of mineral preparations in organic farming".
The research paper will be published in the British Journal of Nutrition on the 5th July 2011.