Long-term Iodine nutrition is associated with longevity in older adults: A 20-year 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-year 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 μg/L in Randers and 139 μg/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 12-12-2017 using Danish registries. Complete follow-up data were available on 428 out of 430 of participants (99.5%). At baseline, the median urinary iodine concentration was 55 μg/L in Randers and 160 μg/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 to Randers was associated with lower hazard ratio (HR) of death in both age- and sex-adjusted analysis (HR 0.60, 95% CI: 0.41-0.87, P = 0.006), but also after adjustment for age, sex, number of drugs, Charlson Comorbidity 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 to residency in an iodine deficient environment.