The role of iodine in our diet

Interview with Dr Sarah Bath

Why is iodine important and what are main dietary sources of iodine? The FrieslandCampina Institute spoke with Dr Sarah Bath about her research at the University of Surrey, in the United Kingdom, on the role of iodine in our diet.

The role of iodine in our diet 1

‘Iodine is most important for brain development of the foetus during pregnancy and early life and for the production of thyroid hormones. Iodine has a key role in cognitive development of the child. However it is one of those nutrients that doesn’t necessarily spring to people’s minds as being important before and during pregnancy. Over the past 10 years in the UK there has been more research conducted which has increased the focus on iodine, its role and on population groups which are deficient.’ (1-4)

Is the UK population getting enough Iodine?

‘Whilst there is no widespread deficiency of iodine in the general population in the UK, the picture is more complicated. Bath et al. (2016) reported that iodine intakes are sufficient in school aged children (4) however there is evidence of iodine deficiency in at risk groups, in particular young teenage girls (1). More specifically there is evidence of mild-to-moderate deficiency in pregnant women with 7 studies conducted across the UK documenting deficiencies. This is due to a low intake of iodine-rich foods in young women, in particular milk and fish, which makes the additional requirement in pregnancy harder to meet.’

“Important message for women of child bearing age to have adequate stores of iodine early on before pregnancy begins”

Are there differences between Iodine intakes in the UK and Europe?

The role of iodine in our diet‘There is a mixed picture across Europe with some countries with adequate iodine status, such as the Netherlands and others, such as Belgium which have an adequate status in the general population, although lower intakes than recommended occur in pregnant women. However even in countries with sufficiency it is important to keep monitoring iodine status; indeed the latest Global Scorecard shows that the Netherlands has no recent data in the general population.’

‘I’m currently chair of an ILSI Europe Expert Group which is looking more specifically at actual iodine intakes as opposed to iodine status across Europe. Furthermore I am part of the EUthyroid Project which is a 3 year funded project through the European Union’s Horizon 2020 research and innovation programme (grant agreement No 634453) gathering data on iodine status across Europe and looking at relationship between iodine status in pregnancy and cognitive outcomes in the offspring. In May a meeting was held to highlight The Krakow Declaration on Iodine which aims to raise awareness of iodine deficiency in Europe and beyond and outline tasks for prevention programs.’

What are the requirements for Iodine?

Current recommendations for iodine intakes are shown in Table 1. All apart from the UK recommendations advise higher amounts during pregnancy.

Table 1 Recommended Iodine Intake (µg/day)

Why do UK recommendations differ from EFSA/WHO recommendations?

The UK recommendations were set in 1991 and reviewed again more recently by the UK Scientific Advisory Committee on Nutrition (SACN) in 2014. The SACN statement concluded that the evidence was not strong enough to increase recommendations and therefore UK DRVs have remained the same. However EFSA revised their recommendations and set increased recommendations during pregnancy and lactation. SACN (2014) assumes that adequate iodine intakes prior to and early in pregnancy can maximise iodine stores which can then be utilised during pregnancy and breastfeeding. Therefore it is important for women to have adequate iodine status prior to pregnancy. However NDNS data shows that although there is adequate status (measured through urinary iodine concentration), a good proportion of the UK population, particularly young women, have low iodine intake. Furthermore, in the UK we have no blanket fortification policy, as is the case in Europe where countries such as the Netherlands and Belgium have policies where iodised salt is added to bread. Therefore individual iodine intakes in the UK depend on food choices and dietary patterns, and if fish, seafood, milk and eggs are not consumed then these individuals are more likely to have lower intakes.

What are the best dietary sources of Iodine?

I cannot answer for Europe as a whole as there is no simple answer to that question – there is considerable variation in the iodine content of foods between countries. However in the UK the main dietary source of iodine is milk and dairy products. In addition to milk and dairy products, another good source in the UK is fish – in particular white fish has a higher iodine concentration than oily fish,  although even within white or oily fish categories there is a huge variation in iodine content between fish. The richest fish sources are cod and haddock and there are lower amounts in salmon. Eggs are also a useful source of iodine.

Seaweed and kelp are a very concentrated source of iodine and are a cause for concern as it’s important to guard against excessive intakes. It is therefore important to advise people to be very careful about buying and using kelp supplements as this can lead to excess iodine intake. I have written a fact sheet for the BDA which includes more specific advice on kelp and iodine supplements.

Are you able to share news and latest developments on your work as chair of the ILSI expert working group and the EUthroid project?

It is still early days for the ILSI Expert group which is looking at Iodine intake and sources of Iodine across Europe. There is presently a lot of data on iodine status measured via urinary iodine concentration but a lack of data on dietary sources and intakes across Europe which is more difficult to measure (for example through food diaries or food frequency questionnaires). Iodised salt is a useful mechanism for increasing iodine intakes however salt reduction policies are also a factor that needs to be taken into account and so it is important to look at other dietary sources.

The EUthyroid project has a number of work packages and I am involved in looking at the relationship between iodine status in pregnancy and outcomes in the child. In our work package we have been looking at iodine status and thyroid hormones in pregnant women in three cohorts based in the UK, Netherlands, and Spain. We have just published a paper looking at the effect of thyroid function during pregnancy and cognition in the offspring (6). I am also a member of the UK Iodine Group, who are involved with awareness raising in the UK and I run the group website

Looking ahead over the next 5 years what questions would you like to address in your research and expert advisory roles?

There is still a lot of work to be done in terms of raising awareness in the UK of the important role of iodine in brain development and food sources in vulnerable groups.

  • In particular looking at the relationship between iodine status in early pregnancy and brain development and cognitive scores. We still need to know a lot more about the effect of mild-to-moderate iodine deficiency as opposed to severe deficiency which results in goitre and cretinism and improving management in pregnant women.
  • Another role for iodine is in pre-pregnancy nutrition advice which I think will form part of the research area over the next 5 years.
  • A key focus of future research is the role of iodine supplementation during pregnancy and the effect on cognition in the offspring as there is a lack of good quality evidence (RCTs) from regions with moderate iodine deficiency.
  • Dietary management of low iodine intake and the role of diet in specific groups of the population for example vegetarians, who do not consume fish, and vegans, who consume no dairy products, will also be a focus.

Pregnant women in the UK are generally not told about iodine and studies show poor awareness of iodine and how to get it from the diet. So it’s important to focus on developing greater knowledge and awareness. This has improved in the UK over the past 10 years within the nutrition and dietetic professions however it has not spilt over to the wider information available to other healthcare professions and the general public.

References

  1. Vanderpump, MP., Lazarus, JH., Smyth, PP., Laurberg, P., Holder, RL., Boelaert, K., Franklin, JA; British Thyroid Association UK Iodine Survey Group. (2011) Iodine status of UK schoolgirls: a cross-sectional survey. Lancet, Jun 11;377(9782):2007-12. doi: 10.1016/S0140-6736(11)60693-4
  2. Bath, SC., Steer, CD., Golding, J., Emmett, P., Rayman, MP. (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, Jul 27;382(9889):331-7. doi: 10.1016/S0140-6736(13)60436-5
  3. Bath, SC., & Rayman, MP. (2015). A review of the iodine status of UK pregnant women and its implications for the offspring. Environ Geochem Health, Aug;37(4):619-29. doi: 10.1007/s10653-015-9682-3.
  4. Bath, SC., Combet, E., Scully, P., Zimmerman, MB., Hampshire-Jones, KH, Rayman, MP. (2016). A multi-centre pilot study of iodine status in UK schoolchildren, aged 8-10 years. Eur J Nutr. Sep;55(6):2001-9. doi: 10.1007/s00394-015-1014-y
  5. Bath, S., Hill, S., Infante, H., Elghul, S., Nezianya, C., & Rayman, M. (2017). Iodine concentration of milk-alternative drinks available in the UK in comparison with cows’ milk. British Journal of Nutrition, 118(7), 525-532. doi:10.1017/S0007114517002136
  6. Levie, D., Korevaar TIM., Bath, S., Dalmau-Bueno A., Murcia, M., Espada, M., Dineva, M., Ibarluzea, JM., Sunyer, J., Tiemeier, H., Rebagliato, M., Rayman, MP., Peeters, RP., Guxens, M. (2018). Thyroid Function in Early Pregnancy, Child IQ, and Autistic Traits: a Meta-analysis of Individual-participant Data.J Clin Endocrinol Metab. May 10. doi: 10.1210/jc.2018-00224