Long-term iodine nutrition is associated with longevity in older adults: a 20 years' follow-up of the Randers-Skagen study

Iodine intake affects the occurrence of thyroid disorders. However, the association of iodine intake with longevity remains to be described. This led us to perform a 20 years' follow-up on participants from the Randers-Skagen (RaSk) study. Residents in Randers born in 1920 (n 210) and Skagen born in 1918-1923 (n 218) were included in a clinical study in 1997-1998. Mean iodine content in drinking water was 2 mu g/l in Randers and 139 mu g/l in Skagen. We collected baseline data through questionnaires, performed physical examinations and measured iodine concentrations in spot urine samples. Income data were retrieved from Danish registries. We performed follow-up on mortality until 31 December 2017 using Danish registries. Complete follow-up data were available on 428 out of 430 of participants (99 center dot 5 %). At baseline, the median urinary iodine concentration was 55 mu g/l in Randers and 160 mu g/l in Skagen residents. Participants were long-term residents with 72.8 and 92.7 % residing for more than 25 years in Randers and Skagen, respectively. Cox regression showed that living in Skagen compared with Randers was associated with a lower hazard ratio (HR) of death in both age- and sex-adjusted analyses (HR 0.60, 95 % CI 0.41, 0.87, P = 0.006), but also after adjustment for age, sex, number of drugs, Charlson co-morbidity index, smoking, alcohol and income (HR 0.60, 95 % CI 0.41, 0.87, P = 0.008). Residing in iodine-replete Skagen was associated with increased longevity. This indicates that long-term residency in an iodine-replete environment may be associated with increased longevity compared with residency in an iodine-deficient environment.

Automated summary

Long-term residency in an iodine-replete environment (Skagen) is associated with a lower risk of death and increased longevity compared to residency in an iodine-deficient environment (Randers) according to a 20-year follow-up study on participants.