A new study by Bath and colleagues from the University of Surrey and published in the British Journal of Nutrition examines the iodine concentration in organic versus conventional cow's milk and concludes that the relative lack of iodine in organic milk could compromise iodine intakes and therefore increase the risk of deficiency, as milk is the primary source of iodine in UK diets.

While only 3% of the British population chooses organic milk, the report continues, they apparently choose it for claimed health benefits and this may have a negative impact on iodine intakes.

"Low levels in organic milk were attributed to factors such as the lack of iodine-containing vitamin and mineral supplements in cattle feed stock, and the use of feed with 'goitrogenic' properties that could lower milk iodine concentration, such as white clover.

"But [Professor Margaret Rayman, co-author of the study] said evidence was emerging of worrying UK deficiencies and alluded to an as yet unpublished pilot study with colleagues from the University of Bristol that indicated adverse effects on intelligence, school performance and reading ability in children of mothers who were iodine deficient by World Health Organisation criteria.

"She said that individuals who choose organic milk should be aware that their iodine intake may be compromised and should ensure adequate iodine intake from alternative sources."