Vegans, Vegetarians, and Omnivores: How Does Dietary Choice Influence Iodine Intake? A Systematic Review

Abstract

Vegan and vegetarian diets are becoming increasingly popular. Dietary restrictions may increase the risk of iodine deficiency. This systematic review aims to assess iodine intake and status in adults following a vegan or vegetarian diet in industrialised countries. A systematic review and quality assessment were conducted in the period May 2019-April 2020 according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Studies were identified in Ovid MEDLINE, Embase, Web of Science, PubMed, Scopus, and secondary sources. Fifteen articles met inclusion criteria. Participants included 127,094 adults (aged ≥ 18 years). Vegan groups presented the lowest median urinary iodine concentrations, followed by vegetarians, and did not achieve optimal status. The highest iodine intakes were recorded in female vegans (1448.0 ± 3879.0 µg day^-1) and the lowest in vegetarians (15.6 ± 21.0 µg day^-1). Omnivores recorded the greatest intake in 83% of studies. Seaweed contributed largely to diets of vegans with excessive iodine intake. Vegans appear to have increased risk of low iodine status, deficiency and inadequate intake compared with adults following less restrictive diets. Adults following vegan and vegetarian diets living in countries with a high prevalence of deficiency may be more vulnerable. Therefore, further monitoring of iodine status in industrialised countries and research into improving the iodine intake and status of adults following vegan and vegetarian diets is required.

Keywords: iodine deficiency; iodine intake; iodine status; vegan; vegetarian.

Figure 1

Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) flow diagram for included studies.

Figure 2

Visual representation of iodine status by median urinary iodine concentration (UIC) for included studies. The shaded grey bar represents the optimal range for iodine status (100–299 μg L⁻¹). Significance values are not presented within this figure. See Table 3.

Figure 3

Visual representation of estimated average iodine intake (µg day⁻¹) for included studies. The grey dashed line represents the adequate intake recommended by the WHO of 150 µg day⁻¹. (a), mixed gender values. Significance values are not presented within this figure. See Table 5.

Figure 3

Visual representation of estimated average iodine intake (µg day⁻¹) for included studies. The grey dashed line represents the adequate intake recommended by the WHO of 150 µg day⁻¹. (a), mixed gender values. Significance values are not presented within this figure. See Table 5.