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.

S O’Kane,Ruth Weir,Emer Mackle,Alison Yeates,Christian Deitrich,Diana Kmiotek,Mike Johnston,Edel Fitzgerald,L Pourshahidi,Maria Mulhern,Jennifer O’Reilly,J.J Strain,Sarah Hill,Carole Lowis

doi:10.3390/nu10030287

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.

Highlights

Iodine and selenium are essential trace elements required for the maintenance of thyroid hormone homeostasis [1]
When milk samples were categorised as winter/indoor (December–March) or summer/outdoor (June–October), milk iodine concentration was significantly higher in winter/indoor period than summer/outdoor milk (Table 1)
There were no significant differences in milk selenium concentration between winter/indoor or summer/outdoor milk

Summary

Introduction

Iodine and selenium are essential trace elements required for the maintenance of thyroid hormone homeostasis [1]. Iodine is required for the production of thyroid hormones, triiodothyronine (T3) and thyroxine (T4) that are essential during pregnancy and infancy for brain and neurological development [5,6]. Salt iodisation has been the primary approach worldwide to control and prevent iodine deficiency and monitoring of salt iodisation programmes has demonstrated this to be an effective strategy [13,14,15]. In some European countries including the UK, policy makers have been reluctant to implement a salt iodisation programme as this may conflict with public health efforts to reduce salt intake for the prevention of cardiovascular disease [14]. With the growing problem of iodine deficiency and the lack of prophylaxis with iodised salt, alternative strategies are required to increase iodine status

Objectives

Methods

Results

Discussion

Conclusion