Researcher Network member, Elizabeth Eveleigh, presents a systematic review to consider how dietary choice may influence the intake of iodine.
Vegan diets have become increasingly popular over the past decade, particularly in industrialised countries. In the UK, well-planned vegan diets are considered to be suitable throughout the lifespan1 and have been associated with reduced incidence of obesity, cardiovascular disease, hypertension, diabetes and some cancers.2 Despite these health benefits, concerns have been raised regarding the ability of vegan diets to adequately provide essential micronutrients, such as iodine.3,4
Iodine is an essential micronutrient, vital for the synthesis of thyroid hormones - triiodothyronine (T3) and thyroxine (T4) - which regulate metabolism, growth and neurological development.5,6 Iodine deficiency presents as a spectrum of clinical disorders termed ‘iodine deficiency disorders’ (IDDs) that occur when the recommended intake (of 150 µg per day) is not achieved,6 the most recognisable being goitre (the enlargement of the thyroid gland). Iodine excess is also detrimental to health and daily intake of above 1000 µg can induce thyroid dysfunction in susceptible individuals.7
Research has identified that individuals who consume diets relying on plant foods for iodine, exclude iodine-rich food, principally dairy, eggs, fish, seaweed, iodised salt, and/or supplements have increased risk of iodine deficiency.8,10 Moreover, recent research has discovered that regular consumers of unfortified alternative milk products may also be at risk of deficiency.11 Characteristically, vegans do not consume any animal-derived products and tend to consume greater quantities of alternative milks,11 therefore individuals selecting these diets may be more prone to poor iodine nutrition.
In our systematic review, we investigated dietary iodine intake and iodine status in those following a vegan diet in industrialised countries.12 Our review including 127,094 adults (aged 18 years and over) confirms that vegans living in industrialised countries appear to have increased risk of lowered iodine status, iodine deficiency and inadequate iodine intake compared to adults following other diets (vegetarians, pescatarians and omnivores). It’s important to recognise that, in our review, those following omnivorous diets also had low iodine status and mild to moderate deficiency, signifying that iodine deficiency is not solely predicted by dietary choices and highlighting the need to improve iodine nutrition throughout the whole population.
We found the iodine status of vegans to be low. According to WHO criteria for assessing population iodine status by urinary iodine concentration,13 those following vegan diets were most frequently seen to exhibit either mild (50–99 µg L−1) or moderate deficiency (20–49 µg L−1), and in two studies were found to be severely deficient (<20 µg L−1). Although clinical manifestations are rare in industrialised nations experiencing mild-to-moderate iodine deficiency, severe deficiency (as observed in some vegan populations) may potentiate the development of IDDs.
We found that those with the lowest dietary iodine intake tended to identify as vegan. In all studies, males following vegan diets had average dietary iodine intake values lower than almost all comparative dietary groups. Similarly, females following vegan diets presented the lowest iodine intake in 75% of included studies. This finding supports the possibility of a gender difference within vegan groups and that vegan males have more difficulty achieving iodine recommendations. Further, our findings indicate that there are risks of iodine excess associated with frequent seaweed consumption in vegan diets. Five studies recorded seaweed consumption in vegans, with these individuals having iodine intake values close to, or over, the maximum tolerant level. Seaweed is a naturally rich source of iodine, but the relative content is highly variable and can provide excessive quantities. Therefore, seaweed might not be a useful vehicle to improve iodine intake in individuals reliant on plant sources to achieve safe recommended daily intake.
A finding of particular interest is that vegans living in industrialised regions, where national population iodine status is inadequate, or where dietary intake is insufficient are more susceptible to iodine deficiency. This suggests that the degree of vulnerability to iodine deficiency appears to be impacted by not only individual dietary choices, practices and restrictions, but also country-specific dietary determinants and national food fortification strategies.
In conclusion, iodine deficiency remains a public health problem and is of concern following the “re-emergence” of iodine deficiency, especially in industrialised countries. This review agrees with findings from the previous systematic reviews exploring this topic, confirming that vegans living in industrialised countries and not consuming seaweed or iodine-containing supplements appear to have an increased risk of iodine deficiency. The evidence suggests that the degree of vulnerability appears to be relative to the prevalence of deficiency at the national level.
Further monitoring of iodine status in industrialised countries and research into iodine nutrition in vegan diets is needed. Efforts need to be made to devise a means of safe iodine consumption whether by fortification of foods (such as alternative milks), iodised salt provision or iodine-containing supplements, in addition to information delivery intended for tolerable consumption of seaweed varieties. It is vital to make efforts to increase awareness of the importance of iodine in the diet and to effectively communicate how iodine recommendations can be achieved by all individuals, regardless of dietary choice.
For more information about iodine click here. You can also download The Vegan Society's iodine pdf here and compare your diet to our guidelines using the free VNutrition app.
Original source and references available: https://www.mdpi.com/2072-6643/12/6/1606/htm#B3-nutrients-12-01606
References
1. British Dietetic Association. British Dietetic Association confirms well-planned vegan diets can support healthy living in people of all ages https://www.bda.uk.com/resource/british-dietetic-association-confirms-well-planned-vegan-diets-can-support-healthy-living-in-people-of-all-ages.html (accessed 24 February 2020)
2. Dinu M. & Abbate R. et al. Vegetarian, vegan diets and multiple health outcomes: A systematic review with meta-analysis of observational studies. Critical Reviews in Food Science and Nutrition 2017; 57(17): 3640-3649 https://www.tandfonline.com/doi/full/10.1080/10408398.2016.1138447 (accessed 18 March 2019)
3. Craig WJ. & Mangels AR. Position of the American Dietetic Association: vegetarian diets. Journal of the American Dietetic Association 2009; 109(7): 1266-1282 http://www.ncbi.nlm.nih.gov/pubmed/19562864 (accessed 13 September 2019)
4. Public Health England. SACN Statement on Iodine and Health. UK; 2014 https://www.gov.uk/government/publications/sacn-statement-on-iodine-and-health-2014 (accessed 8 April 2021)
5. Ahad F. & Ganie SA. Iodine, Iodine metabolism and Iodine deficiency disorders revisited. Indian Journal of Endocrinology and Metabolism 2010; 14(1): 13-17 http://www.ncbi.nlm.nih.gov/pubmed/21448409 (accessed 24 February 2020)
6. Zimmermann MB. Iodine Deficiency. Endocrine Reviews 2009; 30(4): 376–408
7. Leung AM. & Braverman LE. Consequences of excess iodine. Nature Reviews Endocrinology 2014; 10(3): 136-142
8. Fuge R. Soils and Iodine Deficiency. In Selinus O. (ed) Essentials of Medical Geology: Revised Edition Netherlands: Springer Netherlands; 2013
9. Fields C. & Borak J. Iodine Deficiency in Vegetarian and Vegan Diets: Evidence-Based Review of the World’s Literature on Iodine Content in Vegetarian Diets. In Preedy V. & Burrow G. et al. Comprehensive Handbook of Iodine US: Elsevier Inc.; 2009
10. Fields C. & Dourson M. et al. Iodine-deficient vegetarians: a hypothetical perchlorate-susceptible population? Regulatory Toxicology and Pharmacology 2005; 42(1): 37-46 http://www.ncbi.nlm.nih.gov/pubmed/15896441 (accessed 13 September 2013)
11. Dineva M. & Rayman MP. et al. Iodine status of consumers of milk-alternative drinks versus cows’ milk: data from the UK National Diet and Nutrition Survey. British Journal of Nutrition 2020 https://pubmed.ncbi.nlm.nih.gov/32993817 (accessed 27 January 2021)
12. Eveleigh ER. & Coneyworth LJ. et al. Vegans, Vegetarians, and Omnivores: How Does Dietary Choice Influence Iodine Intake? A Systematic Review Nutrients 2020;12(6) /pmc/articles/PMC7352501/?report=abstract (accessed 20 July 2020)
13. World Health Organization (WHO). Assessment of Iodine Deficiency Disorders and Monitoring their Elimination: A guide for programme managers, Second edition. WHO; 2001 https://apps.who.int/iris/handle/10665/61278 (accessed 8 April 2021)
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