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

Abstract

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.

Iodine nutrition among the adult population of the Faroe Islands: a population-based study

“…Thyroid hormones are essential for growth, neuronal development, reproduction and regulation of energy metabolism (3,4) . Mild and moderate iodine deficiency presents primarily as non-toxic goitre, an enlarged thyroid gland with normal production of thyroid hormones, and prolonged iodine deficiency may lead to hyperthyroidism and shorter lifespan (5) . Furthermore, severe iodine deficiency may lead to hypothyroidism in adults and to cretinism in infants and children (1,6) with iodine deficiency being the single most important, preventable cause of developmental brain damage worldwide (7,8) .…”

Raised mortality in old adults with a history of hyperthyroidism following iodine fortification

“…The overall long‐term benefit of IF in iodine‐deficient populations cannot be questioned 1,7,10 . The increased demand on the thyroid gland during pregnancy and the parallel increased renal iodine clearance raise the need for iodine to support the developing foetus and prevent brain damage 26,27 .…”

“…The increased demand on the thyroid gland during pregnancy and the parallel increased renal iodine clearance raise the need for iodine to support the developing foetus and prevent brain damage 26,27 . Furthermore, the structural and functional effects on the thyroid caused by long‐term iodine deficiency can be prevented for the benefit of older adults in whom sufficient iodine intake is associated with lower long‐term incidence of hyperthyroidism and even may increase longevity 6,7 . However, the risk of short‐term adverse effects triggered by IF must be addressed.…”

“…Thyroid peroxidase (TPO) antibodies were measured by an enzyme‐linked immunosorbent assay (detection limit 4 U/L, reference standard code 66/387 NIBSC) with human TPO as antigen. Urine samples were analysed for iodine and creatinine content using the robust standard method of our laboratory 7,10,12,13 …”

“…The proportion of older adults in the global population is growing. In older adults, optimal iodine nutrition is associated with longevity, 7 but comorbidities such as atrial fibrillation and heart failure make older adults more vulnerable to the iodine‐induced hyperthyroidism that may follow an increase in iodine intake 8,9 . Hence, the higher prevalence of multinodular toxic goitre in iodine‐deficient older adults is a concern and the benefit of iodine supplementation should be balanced against the increased incidence of hyperthyroidism shortly after IF and the potential adverse effects hereof 10 .…”