‘Iodine concentration of organic and conventional milk: implications for iodine intake’, by Sarah Bath, Suzanne Button and Margaret Rayman was recently published in the British Journal of Nutrition (BJN).

The authors noted that iodine is vital for adequate thyroid hormone production, which is essential for infant brain development, particularly in the first trimester of pregnancy.

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.

Identifying milk as the principle iodine source in UK diets - with dairy sources accounting for 42% of intake - and the growing popularity of organic milk (drunk by around 3% of consumers), the researchers compared iodine concentrations of retail organic and conventional milk and evaluated regional differences.

Samples of organic milk (92) and conventional milk (80), were purchased from retail outlets in 16 areas of the UK: in Southern England, Wales and Northern Ireland, all of which were analysed for iodine content.

Worrying UK deficiencies

Professor Rayman told FoodManufacture.co.uk that this was the first sizable study, and principal finding held important public-health implications, particularly in view of emerging evidence of iodine deficiency in UK population sub-groups, including pregnant women.

Iodine deficiency was only thought to be a mainstream problem in "so-called developing countries", Rayman said, and could lead to cretinism, various adverse effects on the motor system, lower IQ scores, fertility issues and even deafness.

But she said evidence was emerging of worrying UK deficiencies and alluded to an as yet unpublished pilot study with colleagues from the University of Bristol that indicated adverse effects on intelligence, school performance and reading ability in children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.

“Fish is one alternative source, but lots of people don’t like fish or don’t want to eat it,” she said. “Women can take a supplement during pregnancy, but from a public health perspective you don’t want to tell everyone to do this.”

OMSCo response

Rayman added that health food stores sometimes advised consumers to buy kelp as a rich iodine source, but she said that this was “potentially dangerous” as the product could be very high in iodine.

She also challenged the Organic Milk Suppliers Co-Operative (OMSCo) over its response to a recent interview she undertook with the BBC’s Farming Today programme, and said the body should have focused on organic milk positives, such as the lack of pesticides and antibiotics used in production.

“They said the research was alarmist, but the work isn’t rubbishable,” she said, adding that the BJN was a peer-reviewed journal and that tests on milk samples were conducted at the government's principal laboratory.

Huw Bowles, corporate affairs director of OMSCo, said in response: “Although we have not seen the full report, we are concerned that consumers are being advised to switch out of organic milk on the basis of lower levels of one nutrient when there are a whole host of benefits, including higher nutrient levels, of consuming organic milk.

“We are particularly concerned that the report’s advice is directed towards pregnant women when research has shown many benefits to mother and baby of consuming organic foods and dairy in particular.”

Bowles added: “We are also concerned at the very strong messaging regarding human health implications of these lower levels. Despite a growing body of evidence showing the higher levels of beneficial nutrients in organic food, we are not allowed to state positive health outcomes without full human health trials and you would expect that the same were true with regard to these claims.”

Trying to raise awareness

But Rayman said: "We're not being alarmist. I think our findings are worrying. We're trying to raise awareness about the fact that, pregnant mothers, say, perhaps think they're doing what's best by buying organic milk, when in terms of iodine intake, they are increasing their risk of deficiency."

She added that she had spoken to the chair of the Food Standards Agency and Department of Health advisory committee on nutrition (SACN), who she said indicated that the most that might happen in the short-term was monitoring of the UK population's iodine status within the context of the National Diet and Nutrition Survey (NDNS).

Study title: ‘Iodine concentration of organic and conventional milk: Implications for iodine intake’

Authors: Sarah C. Bath, Suzanne Button and Margaret P. Rayman: Nutritional Sciences Division, Faculty of Health and Medical Sciences, University of Surrey, Guildford.

Source: British Journal of Nutrition (March 2011)

Doi: 10.1017/S0007114511003059