Plant milks are low in iodine and as a result, consumers who replace cow’s milk with plant milks are at risk of becoming iodine deficient– so says a report by British investigators published recently in the British Journal of Nutrition.1 Soymilk was one of the plant milks evaluated.
There is a long history of research into the effect of soy on thyroid function. The first study examining this relationship was published in 1933.2 It is well established that soy doesn’t affect thyroid function in individuals with a normal-functioning thyroid3,4 but there has been concern that in those whose iodine intake is marginal, soyfood consumption, because of the exposure to isoflavones, could adversely affect thyroid function. However, in 2011, clinical work by Sosvorova et al.5 strongly suggests this concern is without foundation.
The essential mineral Iodine is required for the production of thyroid hormone. Consequences of iodine deficiency include goiter, cretinism, intellectual impairments, growth retardation, neonatal hypothyroidism, and increased pregnancy loss and infant mortality.6 Furthermore, a UK study recently found significantly decreased IQ scores in children whose mothers were mildly iodine deficient during pregnancy.7
Estimates are that globally 2 billion people have an insufficient iodine intake and that about 50% of Europe remains mildly iodine deficient.6 By comparison, in the United States, iodine status looks pretty good according to recent NHANES data,8 although pregnant women are an at-risk group.9
Good natural dietary sources of iodine are relatively rare as the native content of iodine in vegetables and grains is relatively low and highly variable depending upon the soil content.10 Much of the reason for the contrasting situations between the United States and Europe is that in the US, table salt is fortified with iodine; it is a major source of this mineral. By the way, what is often not appreciated is that most of the salt in the US diet comes from processed foods rather than added table salt11 and that most of the salt used during processing is not iodized.12
Throughout much of Europe, cow’s milk is a major source of iodine. Iodine is found in dairy products due to iodine supplementation of cattle feed and the use of iodophor sanitizing agents for udder washes, teat dips, and cleaning milking equipment.13
With this background in mind, it is not surprising that Bath et al.1 found that UK plant milks are extremely low in iodine in comparison to cow’s milk. In fact, this observation was already made for plant milks in the US14 and Australia15 although the number of milks tested in these studies was not as large as the number evaluated by Bath and colleagues.1
Relatively little is known about the iodine nutriture of consumers of plant milks. In the UK, iodine status (as assessed by 24 h iodine excretion) was lower in consumers of soymilk in women of childbearing age, reflecting the negative correlation between intake of soymilk and cows’ milk. But this study included only 5 women who drank soymilk.16 A US study found that vegetarians are iodine sufficient but also that vegans may be at risk for low iodine intake.17 However, in contrast to the British study,16 higher urinary iodine concentrations (indicating higher iodine intake) were associated with recent ingestion of soymilk.17 The authors of this US study recommended that vegan women of child-bearing age supplement their diet with 150 ug of iodine daily,17 which is the same recommendation that exists for non-vegan women.18
Should manufacturers of plant milks consider fortifying their products with iodine? Given that most of these products are already heavily fortified with nutrients — such as calcium, vitamin B12, and vitamin D — iodine fortification, at least in countries that don’t iodize table salt, is certainly a worthwhile consideration. On the other hand, it doesn’t make sense to consume cow’s milk simply because it is a source of one particular nutrient not routinely found in plant milks. It makes more sense to stick with plant milks and to take a supplement, or to make sure to consume iodine-rich foods. As we’ve noted before, soymilk stands out from the other plant milks because of its high protein content.
- Bath SC, Hill S, Goenaga Infante H, Elghul S, Nezianya CJ, Rayman MP. Iodine concentration of milk-alternative drinks available in the UK in comparison with cows’ milk. Br J Nutr. 2017:1-8.
- McCarrison R. The goitrogenic action of soya-bean and ground-nut. Ind J Med Res. 1933;XXI:179-81.
- Messina M, Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid. 2006;16:249-58.
- EFSA. EFSA ANS Panel (EFSA Panel on Food Additives and Nutrient Sources added to Food), 2015. Scientific opinion on the risk assessment for peri- and post-menopausal women taking food supplements containing isolated isoflavones. EFSA J. 13,4246 (342 pp). 2015.
- Sosvorova L, Miksatkova P, Bicikova M, Kanova N, Lapcik O. The presence of monoiodinated derivates of daidzein and genistein in human urine and its effect on thyroid gland function. Food Chem Toxicol. 2012;50:2774-9.
- Zimmermann MB. Iodine deficiency. Endocr Rev. 2009;30:376-408.
- Bath SC, Steer CD, Golding J, Emmett P, Rayman MP. 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;382:331-7.
- Caldwell KL, Miller GA, Wang RY, Jain RB, Jones RL. Iodine status of the U.S. population, National Health and Nutrition Examination Survey 2003-2004. Thyroid. 2008;18:1207-14.
- Caldwell KL, Pan Y, Mortensen ME, Makhmudov A, Merrill L, Moye J. Iodine status in pregnant women in the National Children’s Study and in U.S. Women (15-44 Years), National Health and Nutrition Examination Survey 2005-2010. Thyroid. 2013.
- Haldimann M, Alt A, Blanc A, Blondeau K. Iodine content of food groups. J Food Compost Anal. 2005;18:461-71.
- Mattes RD, Donnelly D. Relative contributions of dietary sodium sources. J Am Coll Nutr. 1991;10:383-93.
- Dasgupta PK, Liu Y, Dyke JV. Iodine nutrition: iodine content of iodized salt in the United States. Environ Sci Technol. 2008;42:1315-23.
- Hemken RW. Factors that influence the iodine content of milk and meat: a review. J Anim Sci. 1979;48:981-5.
- Ma W, He X, Braverman L. Iodine Content in Milk Alternatives. Thyroid. 2016;26:1308-10.
- Crawford BA, Cowell CT, Emder PJ, et al. Iodine toxicity from soy milk and seaweed ingestion is associated with serious thyroid dysfunction. Med J Aust. 2010;193:413-5.
- Bath SC, Sleeth ML, McKenna M, Walter A, Taylor A, Rayman MP. Iodine intake and status of UK women of childbearing age recruited at the University of Surrey in the winter. Br J Nutr. 2014;112:1715-23.
- Leung AM, Lamar A, He X, Braverman LE, Pearce EN. Iodine status and thyroid function of Boston-area vegetarians and vegans. J Clin Endocrinol Metab. 2011;96:E1303-7.
- Leung AM, Pearce EN, Braverman LE, Stagnaro-Green A. AAP recommendations on iodine nutrition during pregnancy and lactation. Pediatrics. 2014;134:e1282.