Iodine Supplementation: Should it be considered for people consuming vegetable-based diets?

In 2009, I wrote an article on evidence supporting iodine supplementation during pregnancy (1). Iodine intake has decreased over the last 30 years (primarily due to decreased consumption of iodized salt and dairy products), and studies show that maternal iodine supplementation has beneficial effects on neurocognitive development of infants. I am happy to report that the Council for Responsible Nutrition recently released a guideline recommending for dietary supplement manufacturers and marketers to include a daily serving of at least 150 micrograms iodine in all multivitamin/mineral supplements intended for pregnant and lactating women in the United States (2). If dietary supplement manufacturers follow this guideline, pregnant or lactating mothers who take prenatal supplements should receive an adequate intake of iodine.

Iodine is an essential component of triiodothyronine (T3) and thyroxine (T4), which regulate cell activity in virtually all tissues, and are responsible for maintenance of metabolic processes. Adequate iodine is required for optimal thyroid function throughout life, not just during development. Symptoms of iodine insufficiency in adults include slower response times, impaired mental function, fatigue, weight gain, cold intolerance, and constipation (3). Many of these symptoms are associated with aging, so iodine deficiency may be overlooked in older people.  As consumption of iodine is waning, it is time to take a fresh look at consumption of iodine in segments of the population who may be at risk of iodine deficiency: vegans, vegetarians and omnivores consuming very little meat or dairy products. With a few exceptions (e.g. seaweed, prunes) (4), vegetables and fruits are notoriously poor in iodine. Vegans with low iodine intake are particularly vulnerable to hypothyroidism because goitrogenic substances found in cruciferous vegetables and soy have been shown to inhibit thyroid hormone synthesis (3).

To date, studies that have examined iodine status in vegans and vegetarians (5-12) indicate that, in general, vegans are at risk of iodine deficiency unless they consume seaweed (see Table 1).  However, the range of iodine values in vegans is large, and includes some values in the excessive range. Using the urinary iodine and intake values from the American Thyroid Association (12), one can conclude that urinary iodine values should be approximately 2/3 of iodine intake values. As shown in Table 1, this relationship does not hold for any of the studies that have measured iodine intake and urinary iodine. Urinary values are thought to be more accurate indicators of iodine status than intake values because almost all ingested iodine is excreted in the urine, and not all ingested iodine may be accounted for in intake surveys (particularly from salt). Because the iodine concentration in urine depends on the intake of both iodine and fluid, urinary values may be artificially low or high, dependent on fluid intake.  Iodine values obtained from twenty-four house urine samples are more reliable than spot urine samples because there is considerable variability in daily iodine intake (13).

Clearly, there is a need for a better method to assess iodine intake.  Current information about the iodine content of foods sold in the United States is lacking; a commonly used reference (Pennington et al., 1995) cited for the iodine content of foods in the United States (14) is twenty years old and does

Table 1. Iodine status in vegetarians, vegans and omnivores

not contain information about several of the types of foods that are consumed today.  Further, the colorimetric method used for the Pennington et al., 1995 study was not optimal (14), and improved methods of detection have been developed (e.g. ICP-MS) (15). The time has come for the FDA (or some other authoritative body) to take a new look at iodine content of foods sold in the United States and develop a strategy to combat iodine deficiency (particularly in vegans and vegetarians).

There is a fairly large margin between the recommended daily allowance of iodine (150 µg/day) and the tolerable upper limit for iodine in the adult population (1,100 µg/day), so daily use of a dietary supplement containing 150 µg iodine/serving should not have any adverse effects on healthy vegans or vegetarians, provided they do not consume seaweed. An alternative to use of iodine supplements is increased consumption of iodine-containing foods. This is somewhat of a challenge, particularly in vegans or vegetarians who make their own bread. Of the types of foods that vegans/vegetarians may consume, store-bought bread made with iodized salt and/or iodate dough conditioners is the most important source of iodine.  Achieving adequate iodine status without consuming dairy, fish, meat or iodized salt is challenging.  Recognizing that salt may not be the ideal vehicle for iodine fortification, alternative vehicles to salt are being investigated as iodine carriers, including water, rice, edible vegetable oils and fats, wheat and maize flour, and condiments and seasonings (16). The World Health Organization has commissioned The Cochrane Collaboration to perform an assessment of the effects of fortifying foods other than salt with iodine on iodine status and health-related outcomes in the general population (17). Iodine fortification of ingredients other than salt is an area ripe for development.

References:

1. Dolan, LC and Burdock, GA (2009). Evidence that iodine supplementation should be considered during pregnancy. Supplement to AgroFood Industry Hi-tech, volume 20, number 6.

2. Council for Responsible Nutrition (2015). Recommended Guidelines: Iodine Quantity in Multivitamin/Mineral Supplements for Pregnancy and Lactation. http://www.crnusa.org/pdfs/CRNRecommendedGuidelinesIodine_Final.pdf, released January 27, 2015.

3. Drake,VJ (2010). Iodine. http://lpi.oregonstate.edu/infocenter/minerals/iodine/, site visited January 28, 2015.

4. National Institutes of Health (2011). Iodine. Fact sheet for professionals. http://ods.od.nih.gov/factsheets/Iodine-HealthProfessional/, site visited February 4, 2015.

5. 5  Abdulla, M, Andersson, I, Asp, N-G, et al.  (1981). Nutrient intake and health status of vegans. Chemical analyses of diets using the duplicate portion sampling technique. Am J Clin Nutr 34: 2464-2477.

6. Draper, A, Lewis, J, Malhotra, N, Wheeler, E (1993). The energy and nutrient intakes of different types of vegetarian: a case for supplements? Br J Nutr 69, 3-19.

7. Krajcovicová-Kudlácková, M, Bucková, K, Klimes, I, Seboková, E (2003). Iodine deficiency in vegetarians and vegans. Ann Nutr Metab 47(5):183-5.

8. Leung, AM (2011). Iodine status and thyroid function of Boston-area vegetarians and vegans. J Clin Endocrinol Metab 96(8): E1303-E1307.

9. Lightowler, HJ and Davies, GJ (1998). Iodine intake and iodine deficiency in vegans as assessed by the duplicate-portion technique and urinary iodine excretion. Br J Nutr 80: 529-535.

10. Rauma, AL, Tormala, ML, Neonen, M, Hanninen, O (1994). Iodine status in vegans consuming a living food diet. Nutr. Res 14: 17898-1795.

11. Remer, T, Neubert, A, Manz, F (1999). Increased risk of iodine deficiency with vegetarian nutrition. Br J Nutr 81:45-49.

12. American Thyroid Association (2012). Iodine deficiency. http://www.thyroid.org/iodine-deficiency/, site visited January 28, 2015.

13. Vejbjerg, P, Knudsen, N, Perrild, H, Laurberg, P, Andersen, S, Rasmussen, LB, Ovesen, L, Jørgensen, T (2009). Estimation of iodine intake from various urinary iodine measurements in population studies. Thyroid 19(11):1281-6.

14. Pennington, JAT, Schoen, SA, Salmon, GD, et al. (1995). Composition of core foods of the U.S. food supply. J Food Consump Analysis 8:171-217.

15. Haldimann, M, Alt, A, Blanc, A and Blondeau, K (2005). Iodine content of food groups. J Food Comp Analysis 18: 461-471.

16. World Health Organization (2013). Salt reduction and iodine fortification strategies in public health. Available at http://apps.who.int/iris/bitstream/10665/101509/1/9789241506694_eng.pdf?ua=1, site visited January 29, 2014.

17. Land MA, Christoforou A, Downs S, Webster J, Billot L, Li M, Peña-Rosas JP, Neal B (2013). Iodine fortification of foods and condiments, other than salt, for preventing iodine deficiency disorders (Protocol). Available at http://onlinelibrary.wiley.com/doi/10.1002/14651858.CD010734/pdf, site visited January 29, 2014.