[Background and aims]: The importance of milk intake to the supply of dietary iodine is not fully known. We therefore undertook a study in Spain of the iodine concentration in cow's milk and the impact of the frequency of milk consumption on urinary iodine concentrations in three study populations. [Methods]: We studied the iodine concentration in 362 samples of milk from 45 commercial brands and compared it with the milk iodine status in studies undertaken 17 years earlier. The epidemiologic studies were performed in three different places in the south of Spain: two in school-age children (N = 757 and N = 1205 children) and one in adults (N = 1051). A milk consumption questionnaire was given and urinary iodine concentrations measured. [Results]: The mean concentration of iodine in the milk rose from 1991 (117 ± 37 μg/L) to 2008 (259 ± 58 μg/L) (P < 0.001). The iodine concentration was greater in skimmed milk (273 ± 52 μg/L) than in semi-skimmed milk (254 ± 57 μg/L) or whole milk (251 ± 61 μg/L) (P < 0.0001). The winter samples had a greater concentration of iodine (270 ± 55 μg/L) than the summer samples (247 ± 58 μg/L) (P < 0.0001), independently of the type of milk. The urinary iodine concentrations in all three epidemiologic studies were significantly associated with the frequency of milk intake. [Conclusions]: The concentration of iodine in cow's milk has risen over recent years, and it is higher in skimmed milk. The results also show that cow's milk is a relevant source of dietary iodine. © 2010 Elsevier Ltd and European Society for Clinical Nutrition and Metabolism.

This work was supported in part by a grant from the Spanish Society of Endocrinology and Nutrition. CIBER Fisiopatología de la Obesidad y Nutrición (CB06/03) and CIBER de Diabetes y Enfermedades Metabólicas Asociadas (CIBERDEM) are ISCIII projects.

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