The Effect of Processing and Seasonality 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 also contributes to dietary selenium intakes. The aim of this study was to investigate the effect of season, milk fat class (whole; semi-skimmed; skimmed) and pasteurisation on iodine and selenium concentrations in Northern Ireland (NI) milk, and to estimate the contribution of this milk to consumer iodine and selenium intakes. Milk samples (unpasteurised, whole, semi-skimmed and skimmed) were collected weekly from two large NI creameries between May 2013 and April 2014 and were analysed by inductively coupled plasma-mass spectrometry (ICP-MS). Using milk consumption data from the National Diet and Nutrition Survey (NDNS) Rolling Programme, the contribution of milk (at iodine and 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 the mean selenium concentration of milk was 17.8 ± 2.7 µg/kg. Season had an important determining effect on the iodine, but not the selenium, content of cow’s milk, where iodine concentrations were highest 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 higher than 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 contribution to iodine and selenium intakes. This contribution may be higher than previously estimated if iodine and selenium (+25.0 and +1.1 µg/day respectively) concentrations measured in the present study were replicable across the UK at the current level of milk consumption. Iodine intakes were theoretically shown to vary by season concurrent with the seasonal variation in NI milk iodine concentrations. Routine monitoring of milk iodine concentrations is required and efforts should be made to understand reasons for fluctuations in milk iodine concentrations, in order to realise the nutritional impact to consumers.

Large Variability of Iodine Content in Retail Cow’s Milk in the U.S.

“…Just as we found no significant difference in iodine content among retail milk with differing fat content, similar results were obtained in other studies [50][51][52]. However, one study found significantly higher iodine levels in skim than in whole or in semi skimmed [53], although the difference was inconsequential in terms of nutritional impact.…”

Influence of Iodine Feeding on Microbiological and Physico-Chemical Characteristics and Biogenic Amines Content in a Raw Ewes’ Milk Cheese

“…The iodine concentration in milk is directly proportional to the iodine levels in feedstuffs. Moreover, the season of milk production and fat content of milk can significantly affect its rate [ 35 ]. Manca et al [ 36 ] found that iodine supplementation did not influence the goat milk fatty acid profile, except for some short-chain fatty acids.…”

What is the availability of iodised salt in supermarkets on the Island of Ireland?

“…This is possibly due to concerns that iodisation may undercut population-level efforts to reduce salt intakes and, consequently, hypertension (2). Neither the UK nor Ireland has ever had a salt iodisation policy, with dietary iodine relying heavily on adventitious intakes, particularly from dairy produce where presence of iodine in cleaning solutions or in animal feed has historically led to iodine entering milk and dairy produce (2,3). Without any iodised salt policy, a number of studies have investigated iodine intake and status in UK and Irish populations, suggesting some concern (4)(5)(6)(7)(8).…”