Iodine is not a nutrient you hear much about. In fact, until recently overt iodine deficiency seemed to be confined to the developing world.
But could all of that be changing? Recent research suggests that sub-clinical iodine deficiency may be growing in the developed world, particularly in groups with high iodine needs like pregnant women. Why is the once-forgotten iodine back in the limelight and what could the DASH diet have to do with it?
It’s been a long time since iodine has been on the public health radar in the developed world. In an effort to eradicate iodine deficiency in North America table salt was first iodized in the 1920s, which resulted in a dramatic drop on deficiency. In the rest of the world, however, iodine deficiency continues to be the leading cause of preventable mental impairment in infants and children (1,2).
What is Iodine?
Iodine is a non-metallic, trace mineral that is required in the diet in small amounts. Iodine’s key role is in the production of thyroid hormones and proper functioning of your thyroid gland, essentially the metabolic engine of the body; iodine is also important in immune response and breast health (1). While iodine is important throughout the lifespan, particular groups are at greatest risk for deficiency – namely during fetal development and infancy – when iodine needs are highest to support rapid normal brain development (2).
Without adequate iodine, severe and irreversible cognitive impairments, poor growth and congenital hypothyroidism (cretinism) may occur. Later in life, the hallmark of iodine deficiency is an enlarged thyroid gland or goiter, but poor iodine status without goiter can lead to poorer learning in children, infertility in adults and increased risk of developing thyroid cancer and possibly prostate, breast, endometrial, and ovarian cancer too (1,2).
Iodine in Food
The iodization of salt started for a good reason – dietary iodine is relatively scarce as it is not widespread in the diet. Iodized salt remains the number one contributor of iodine to the diet of most people (3,4).
Other sources of natural iodine include seaweed, fish and shellfish (particularly saltwater fish/shellfish), and dairy products. While there is *some* iodine in fruits and vegetables, it varies depending on soil concentrations; unfortunately, soil erosion leads to loss of iodine. For more information on food sources of iodine, Dietitians of Canada has a good list.
What about goitrogens? These are substance that can interfere with iodine absorption and utilization in the body. They are present in some plant foods including cassava, soy, peanuts, leafy greens and cruciferous vegetables such as broccoli, cauliflower and Brussels sprouts.
They DO NOT cause goiters or hypothyroidism and do not make you gain weight (though some websites would have you believe there that they do!) but are chiefly of concern if a). your risk of inadequacy is very high and b). your goitrogen intake is excessive. Moderate intakes of these otherwise healthy foods (who would ever say kale is bad for you???) are not a concern for most people.
Are Iodine Intakes Falling to Dangerous Levels?
The groups at highest risk of iodine deficiency include those with poor intakes, like vegans, and those with higher needs, such as during pregnancy and infancy. Historically, iodine intakes have been relatively stable since salt iodization, with most people in North American considered “iodine-sufficient” since monitoring began in the 1970s… but that has changed in recent years.
There have been sharp declines in iodine intake and status since the 1980s, at first coinciding with dropping dairy intakes, and later in the 1990s with falling sodium consumption (3-5). As people were encouraged to reduce their sodium intake, iodine went with it! The growing trend in non-iodized salt use could also be a contributor – kosher salt, grey salt, pink Himalayan salt and the like DO NOT contain iodine (note: they do contain the same amount of sodium though!).
Current estimates suggest that iodine status in the general population has stabilized (4), but there is growing concern that poor iodine status is occurring more frequently during pregnancy (6-10). Based on recent NHANES data, the median iodine level among pregnant women was inadequate, particularly early on in the first and second trimester (6).
In a small cohort of pregnant women in Toronto, 25% had iodine levels less than adequate and over 5% were considered moderately deficient (7). Gahche et al. (8) had similar findings regarding suboptimal iodine status with fewer than 25% of women taking a prenatal supplement containing iodine.
What impact could suboptimal iodine status have? Bath et al. (9) found that even mild iodine deficiency during pregnancy had significant negative impacts on cognitive development in children; Hykes et al. (10) had similar findings in Australia. Could the increasing prevalence of neurodevelopmental disorders such as autism, ADHD and learning disorders be a result of mild iodine deficiency? Some experts think this is a possibility and are calling for renewed education regarding the importance of adequate iodine intake during pregnancy and lactation (11,12).
No Baloney’s advice? We are by no means suggesting people return to the iodized salt shaker in droves nor start popping iodine supplements. Too much iodine causes the same problems as too little iodine, namely goiter and hypothyroidism, so it is still advisable to limit your intake to less than the UL of 1,100 mcg for adults. Iodine has a relatively narrow range of safety – you want not too little, not too much, but just right.
Look to natural sources of iodine in the diet to fill the void left by iodized salt, such as heart healthy fish and seaweed (we love Sea Snax roasted seaweed as a potato chip-craving alternative!). And don’t forget about low-fat dairy! Children and adults with adequate dairy intakes are more likely to be iodine-sufficient! (6,12)
If you are pregnant or lactating, it seems prudent to look for a prenatal multivitamin-mineral that contains iodine. Take a close look at the label because not all supplements contain iodine.
- Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington, DC: National Academy Press, 2001.
- Pearce EN, et al. Global iodine nutrition: where do we stand in 2013? Thyroid 2013; 23:523-8.
- Hollowell JG, et al. Iodine nutrition in the United States. Trends and public health implications: iodine excretion data from National Health and Nutrition Examination Surveys I and III (1971-1974 and 1988-1994). J Clin Endocrinol Metab 1998; 83:3401-8.
- Caldwell KL, et al. Iodine status of the U.S. population, National Health and Nutrition Examination Survey, 2005–2006 and 2007–2008. Thyroid 2011; 21:419-27.
- Johner SA, et al. Iodine status in preschool children and evaluation of major dietary iodine sources: a German experience. Eur J Nutr 2012; [epub ahead of print].
- Caldwell K, et al. Iodine Status in Pregnant Women in the United States: National Children’s Study and National Health and Nutrition Examination Survey. Thyroid 2013 [epub ahead of print].
- Katz PM, et al. Iodine nutrition during pregnancy in Toronto, Ontario. Endor Pract 2013; 19:206-11.
- Gahche JJ, et al. The prevalence of using iodine-containing supplements is low among reproductive-age women, NHANES 1999-2006. J Nutr 2013; 143:872-7.
- Bath SC, et al. 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 2013 [epub ahead of print].
- Hynes KL, et al. Mild Iodine Deficiency During Pregnancy Is Associated With Reduced Educational Outcomes in the Offspring: 9-Year Follow-up of the Gestational Iodine Cohort. J Clin Endocrinol Metab 2013; 98:1954-62.
- Bath SC, Rayman MP. Iodine deficiency in the U.K.: an overlooked cause of impaired neurodevelopment? Proc Nutr Soc 2013; 72:226-35.
- Stagnaro-Green A, et al. Iodine supplementation during pregnancy and lactation. JAMA 2012; 308:2463-4.
- Perrine CG, et al. Intakes of dairy products and dietary supplements are positively associated with iodine status among U.S. children. J Nutr 2013; [epub ahead of print].