Contribution of Cow's Milk to Iodine and Selenium Status in Women of Child-bearing Age

Iodine is an essential trace element necessary for the synthesis of thyroid hormones which are critical for human growth and development throughout the lifecycle. Iodine is particularly important during pregnancy and infancy for brain and neurological development. Severe iodine deficiency during pregnancy can cause severe mental impairment and stillbirth while mild-to-moderate deficiency has been associated with impaired infant development. As such, pregnant women and women of child-bearing age are particularly vulnerable to the effects of iodine deficiency.

Globally, 241 million (30%) school aged children are iodine deficient. Historically, the UK and Ireland were believed to be iodine sufficient but concern has been expressed in the last decade regarding the iodine status of the UK and Irish populations. A study of Irish women in 2004 reported 55% of pregnant women in their sample population to be moderately iodine deficient and 53% of non-pregnant women to be moderately iodine deficient. A study of UK schoolgirls found that 51% were mildly iodine deficient and 16% were moderately deficient. Importantly, the prevalence of iodine deficiency was highest in Northern Ireland where 85% of those sampled were iodine deficient.

Iodine is known to interact with selenium in the conversion of the thyroid hormone thyroxine (T4) to the metabolically active triiodothyonine (T3) hormone. Selenium is also important for immune, cardiovascular and cognitive function. Selenium status has been reported to be moderately low using pooled English data on blood selenium concentrations from 1984 to 1992. In addition, the latest National Diet and Nutrition Survey reports that selenium intakes are below recommendations for several population groups including women of childbearing age where mean selenium intakes are 76% of the recommended 60µg/day. This national survey also reported that 49% of women aged 19-64 years failed to meet the lower reference nutrient intake of 40µg/day.

Milk and dairy products are the major source of iodine for the UK population and are also a source of selenium. The contribution of milk and dairy products to iodine and selenium intakes is greatest in adult females (35% and 6% respectively). Observational evidence has repeatedly reported milk consumption to be positively correlated with iodine status and iodine intakes. Research in Iceland and New Zealand has reported that milk consumption is not associated with selenium status; however it is unknown if this is true for the UK population.

To date, no randomised controlled trial has examined the effect of milk consumption on either iodine or selenium status. The possibility of increasing iodine and selenium intake by increasing milk consumption, a widely available and consumed foodstuff, in a population group vulnerable to micronutrient deficiency should be investigated. This would add valuable data to the knowledge base for iodine, where there is ongoing debate over the most appropriate means of increasing iodine intake amongst women of childbearing age in the UK where salt iodisation, the key strategy in preventing deficiency, is not implemented.

The aim of this study is to investigate the impact of cow's milk consumption on the biological status of iodine and selenium among women of child-bearing age.

The study will be a 12 week randomised-controlled human intervention study. Participants will be randomised to consume additional semi-skimmed milk in the Following amounts, control group (0mls/day), intervention group (430mls/day or 3l per week).