A paper published in the journal Food Chemistry has analysed the levels of iodine in supermarket milk, reporting lower levels in milk labelled as organic compared with conventional milk. The authors suggest that this may increase the risk of iodine deficiencies in at-risk groups such as pregnant women.
Prof. Jean Golding, Emeritus Professor of Paediatric and Perinatal Epidemiology, University of Bristol, said:
“Extremely low levels of iodine in pregnant mothers was shown in the past to be associated with the birth of children with major reductions in cognitive function [cretin was the technical name]. Women are no longer so deficient but I was a co-author on a Lancet paper published in 2013 that showed a positive association between maternal prenatal iodine level and both the child’s IQ and his/her reading ability at ages 8-9 years. The original Lancet study indicated that many women in the UK have a lower level of iodine than is optimum for their child’s development. Therefore information on ways in which to increase the population’s intake of iodine is to be welcomed.
“Milk is a major source of iodine, but it is important to know which sort of milk is most appropriate. This new study has compared different types of milk and shown that, reassuringly, there is no difference in iodine content between full fat, semi-skimmed and skimmed milk. The findings mirror those already shown by Dr Sarah Bath at the University of Surrey, which adds to their reliability. Pregnant women should be aware, however, that organic milk appears to have lower levels of iodine in the UK. Perhaps organic farmers could increase the iodine content of their herds with iodine supplementation – meanwhile pregnant women should be aware of the findings of this study.”
Prof. Margaret Rayman, Professor of Nutritional Medicine, University of Surrey, said:
“This study complements our own comparison of organic and conventional milk in the summer (Bath et al. 2012 Iodine concentration of organic and conventional milk: implications for iodine intake. Br J Nutr 107, 935-940) as the authors have measured iodine concentration in winter milk samples. The study results are in agreement with our finding of a lower iodine concentration in organic milk compared to conventional milk (up to 35% in the current study and 42% in our own study) and echo similar results from other studies in Europe.
“The interpretation of the data is limited by the fact that, unfortunately, the authors have not provided details on the accuracy of the laboratory method for iodine measurement. This is important because measurement of iodine in milk is challenging and without measures of accuracy (from certified reference materials), the results may not be reliable. The study is also limited by the relatively small number of organic and conventional milk samples and the fact that the geographical origin of the milk was not taken into account.”
Dr Sarah Bath, MRC Population Health Scientist Fellow, University of Surrey, said:
“While there are limitations in this study, as stated by Margaret Rayman, the lower iodine concentration in organic milk may have implications for risk of iodine deficiency, as milk and dairy products are the principal source of iodine in the UK diet. However, the study has no measures of iodine status in an individual, so we cannot conclude that those who consume organic milk have a lower iodine status, as that depends on the quantity of organic milk consumed and intake of other iodine-rich foods (such as fish).
“An adequate intake of iodine during pregnancy is vital and we, and others, have previously shown an association between low maternal iodine status in pregnant women and lower IQ, reading and spelling scores in the child [Bath et al. 2013 Effect of inadequate iodine status in UK pregnant women on cognitive outcomes in their children: results from the Avon Longitudinal Study of Parents and Children (ALSPAC). Lancet 382, 331-337; Hynes et al. 2013 Mild iodine deficiency during pregnancy is associated with reduced educational outcomes in the offspring: 9-year follow-up of the gestational iodine cohort. J Clin Endocrinol Metab 98, 1954-1962].”
Catherine Collins, Spokesperson for The British Dietetic Association, said:
“This is an interesting piece of research that confirms the lower iodine content of pasteurised organic and UHT-treated milks compared to pasteurised conventional milks produced during winter months. These results add to existing data on summer milks, which has also shown a lower level of iodine in organic milks compared to conventional milks.
“Iodine is important as it’s an absolutely essential component of thyroxine, the ‘master’ hormone that controls all metabolic processes and in particular our metabolic rate. Thyroxine cannot be made without sufficient iodine. Low levels of thyroxine are associated with a lower metabolic rate and weight gain, so it’s essential for us to have sufficient dietary iodine to maintain metabolic rate at its optimum level.
“Iodine is found in small amounts in plant foods, although the majority of UK dietary iodine comes from milk, seafood and seaweed. ‘One-a-day’ multivitamin and mineral supplements usually provide 100% of daily iodine requirements per tablet.
“Milk contributes 40% of our dietary iodine intake, so variations in the levels found in milk can have significant impact on dietary iodine intake. The research shows that organic milk provides a third less iodine than conventionally farmed milks, which may be due to differences in the iodine content of winter feeds or different practices in cleaning cow teats prior to milking.
“All milk, whether organic or not, provides high quality protein and a valuable source of calcium, but this research suggests that if you choose organic milk you would need to consume 30% more to reach the same iodine content of non-organic milk.”
‘Effect of milk type and processing on iodine concentration of organic and conventional winter milk at retail: Implications for nutrition’ by Payling et al. published in Food Chemistry on Tuesday 28th April.
Jean Golding: “I was a co-author on the Lancet paper that showed a negative association between maternal prenatal iodine level and the child’s IQ.”
Margaret Rayman: “Member of UK Iodine Group. Received a grant from Wassen International to support a PhD studentship.”
Sarah Bath: “Member of UK Iodine Group. Has received payment from the Dairy Council Northern Ireland for a lecture. PhD studentship partly funded by Wassen International.”
Catherine Collins: “No conflict of interest declared.”