Re-emerging iodine deficiency in high income countries

Iodine nutrition and metabolism

Iodine is an essential mineral regulating overall metabolism. It plays an important role in foetal brain development and the function of several other organs, particularly the thyroid gland. Because of its nutritional relevance in gestation and child development, even moderate iodine deficiencies in pregnancy increase the risk of preventable intellectual impairment in the foetus. This may go on to affect the educational and economic potential in adulthood. Iodine intake and status of the global population is therefore of key importance to public health stakeholders.

Over a century ago, widespread iodine deficiency led to the implementation of salt iodisation as a food-based strategy to combat insufficient iodine intake. This began in High Income Countries (HICs) like the USA and in Europe. Since then, this strategy has become the most successful fortification effort against micronutrient deficiency in developing countries. However, about 30% of the world’s population are still at risk of iodine deficiency with the highest prevalence in Southeast Asia and East Mediterranean regions.

Iodine sources and iodine status in HICs

Iodine deficiency has also begun to re-emerge in HICs like the USA, UK, and Australia, posing a major risk to the health of women and children. Recent studies have confirmed this re-emergence: Data surveys showed inadequate iodine status of <150 μg/L in about 56% of pregnant women in the United States and only 8 out of 21 countries in the European population showed an adequate iodine status. Also, 17% of women aged 16-49 were iodine deficient in the UK, UK National Diet and Nutrition Survey. This could be a result of changing dietary patterns, agricultural and food production practices, Lee et al 1999.

Iodine deficiency is one of the most common nutrient deficiencies globally and it is estimated to affect 2.2 billion people in the world at varying degrees of severity. The prevalence could be more, as mild deficiency causes sub-clinical effects such as functional and intellectual impairments. Severe iodine deficiency is associated with the development of goitre, neurodevelopmental disabilities and mental impairment in babies born to mothers who are iodine deficient. It also causes delayed physical development in children, decreased fertility, stillbirth, and congenital anomalies.  

Iodine occurs naturally in soil and sea water, with meat, dairy, and eggs known to be good dietary sources of iodine. Inland and mountainous regions of the world have iodine deplete soils hence food crops from these areas are not generally rich in iodine compared to coastal regions. This explains why iodine deficiency occurs most commonly in inland regions of the world like continental Africa, Asia, Central and Eastern Europe and parts of the USA.  Other agricultural and industry practices such as use of inorganic fertilisers, iodate dough conditioners for making bread and use of iodine-supplemented feed for livestock are ways to supplement iodine intakes through the diet in developed countries.

Changing iodine levels in the diets of HICs

In recent years, some variability has been observed in the iodine content of dietary food sources known to be naturally high in iodine such as seafood, eggs, and milk and in bread from the use of iodised salt in industrialised countries. Given this variation in iodine content, an individual may need to consume larger portion sizes of these foods to meet daily iodine requirements and thus the potential overconsumption of other nutrients from that food, i.e. carbohydrates and calories in the case of bread.

Most industrialised countries do not mandate salt iodisation and not all who have salt iodisation standards mandate the use of iodised salt in restaurant and processed foods.  In the USA for instance, only half of table salts sold in retail stores are iodised. Hence a majority of out of home meals and commercially processed food, which are major contributors to the diet in these countries, may be lacking in iodine. Also, ongoing public health messaging on salt reduction recommendations for preventing hypertension and other cardiovascular diseases has led to a general reduction in salt use especially for people who are already hypertensive. Similarly, a reduction in the consumption of refined flour-based products such as breads, as a strategy to prevent obesity, limits the intake of iodine in countries where bread is fortified through iodised salt. This is a typical example of one public health drive working against another.  Several other changes to agricultural, food manufacture and individual consumption practices in HICs are contributing to lower supply of iodine in diets and lower iodine status in these contexts.

Factors associated with the changing iodine levels in food and diets of HICs.

Hatch-McChesney & Lieberman in their recent review named several factors as likely causes of low iodine intakes in HICs. Flachowsky et al. suggest that variation in iodine levels in dairy milk and meat may be due to the reduced use of iodine supplemented feed for livestock and seasonal differences in feed iodine content. The increasing preference for organically farmed products with the use of forage feed, found to have 10x less iodine and yet higher goitrogen content compared to non-organic supplemental feed also contribute to lower iodine content of meat, milk, and eggs from livestock. Organic milk is also 25-40% lower in iodine when compared to conventional milk. The processing of milk in manufacturing plants cleaned without the use of iodophors as sanitising agents as was previously done also contributes to lower iodine levels in milk and diary.

Changing dietary practices

At the consumer level, a shift to veganism and some forms of vegetarianism which have become more common in industrialised countries have been found to contribute to the declines in dietary intakes of iodine. The increased avoidance of dairy intake and shifts to the intakes of plant-based food alternatives such as oat and almond milk-alternatives has doubled in recent years in the UK and USA, but they are not typically fortified with iodine. They are hence not comparable in iodine content to dairy milks which are naturally good iodine sources. Other plant-based food alternatives like soy-based milk alternatives are known to interfere with iodine absorption.

Individual’s food choices and perceptions, which may influence their decision to use sea salt and other salt alternatives rather than regular iodised salt also contribute to the reduced iodine consumption in the diet. An increasing focus on high fibre diets without adequate attention to dietary diversification and balance may also contribute to iodine deficiency disease. Consumption of cruciferous vegetables like broccoli, kale and cabbage which are sources of natural goitrogens may interrupt thyroid function and iodine uptake in the body. Cassava, sorghum, maize, and sweet potatoes are also sources of goitrogens that have been associated with endemic goitre in developing countries in Africa and parts of Asia.

Changing iodine levels of dairy products – New Zealand

In the case New Zealand, despite being coastal, soils in are deficient in iodine and hence the mandatory fortification of bread with iodised salt since 2009 following a long period of salt iodisation. The country is also a major producer and exporter of meat and dairy to many other countries in the world, exporting 95% of the milk it produces as dairy products to over 100 countries. This supplies enough milk to meet the two and half servings of dairy requirement for nearly 100 million people daily. Considering current changes in agricultural practices such as reduced use of iodophors as sanitizers in New Zealand dairy industries and how pasture systems alter supply of iodine in feeds of livestock, it is likely that consumers of New Zealand milk may not be getting the expected level of iodine from this otherwise high dietary source of iodine.

Peter Cressey in his study reported significant variation in iodine levels in milk products sold in New Zealand. As the world’s largest dairy exporter, it may be important to explore how this variation affects iodine intakes of consumers of New Zealand milk in other countries where salt iodisation is not mandated and among those with low iodine intakes from other dietary and fortified food sources.

Promoting iodine intakes in HICs

To address the re-emerging situation of iodine deficiency in industrialised countries, the promotion of diary consumption, up to 3-4 servings per day is a feasible option to meet iodine needs of all age and physiologic groups. Getting iodine from milk and dairy offers a nutrient dense alternative to obtaining it from fortified bread which comes with extra calories or from iodised salt for hypertensive individuals on salt reduction regimes.

 Regarding the variability in iodine levels of milk, they could be fortified during processing to increase their iodine content.  In contexts where availability or access to dairy is limited, mandating the use of iodised salt in commercially processed food such as use in bread and other baked products is a useful alternative. Following the mandatory use of iodised salt in most bakeries from 2009, iodine status of New Zealand adult males and females from different ethnic groups was reported to be adequate in 2015.

Exploring the feasibility of iodising food supplies or enriching dairy and milk with iodine while reintroducing practices such as use of iodine fertilisers are beneficial strategies for increasing iodine intakes of populations. Promoting dietary diversification and a focus on nutrient density is another long-term and sustainable approach to addressing all forms of micronutrient deficiencies.

This Thought for Food was written by Justine Coomson, PhD Student, with the support of the SNi team.

Image from Photka on Getty Images.