The Effect of Processing and Seasonallity on the Iodine and Selenium Concentration of Cow's Milk Produced in Northern Ireland (NI): Implications for Population Dietary Intake

Abstract: Cow’s milk is the most important dietary source of iodine in the UK and Ireland, and alsocontributes to dietary selenium intakes. The aim of this study was to investigate the effect ofseason, milk fat class (whole; semi-skimmed; skimmed) and pasteurisation on iodine and seleniumconcentrations in Northern Ireland (NI) milk, and to estimate the contribution of this milk to consumeriodine and selenium intakes. Milk samples (unpasteurised, whole, semi-skimmed and skimmed) werecollected weekly from two large NI creameries between May 2013 and April 2014 and were analysedby inductively coupled plasma-mass spectrometry (ICP-MS). Using milk consumption data from theNational Diet and Nutrition Survey (NDNS) Rolling Programme, the contribution of milk (at iodineand selenium concentrations measured in the present study) to UK dietary intakes was estimated.The mean ± standard deviation (SD) iodine concentration of milk was 475.9 ± 63.5 µg/kg and themean selenium concentration of milk was 17.8 ± 2.7 µg/kg. Season had an important determiningeffect on the iodine, but not the selenium, content of cow’s milk, where iodine concentrations werehighest in milk produced in spring compared to autumn months (534.3 ± 53.7 vs. 433.6 ± 57.8 µg/kg,respectively; p = 0.001). The measured iodine and selenium concentrations of NI milk were higherthan those listed in current UK Food Composition Databases (Food Standards Agency (FSA) (2002);FSA (2015)). The dietary modelling analysis confirmed that milk makes an important contributionto iodine and selenium intakes. This contribution may be higher than previously estimated ifiodine and selenium (+25.0 and +1.1 µg/day respectively) concentrations measured in the presentstudy were replicable across the UK at the current level of milk consumption. Iodine intakes weretheoretically shown to vary by season concurrent with the seasonal variation in NI milk iodineconcentrations. Routine monitoring of milk iodine concentrations is required and efforts should bemade to understand reasons for fluctuations in milk iodine concentrations, in order to realise thenutritional impact to consumers.