HERITABILITY OF SALT INTAKE USING HALF-DAY URINE SAMPLES: THE HEALTHY TWIN STUDY

Abstract

Salt is essential for both life and diet, but excess consumption of salt is an established risk factor of hypertension. Historically, salt intake has increased along with civilization, and the traditional Korean diet, although generally considered to be healthier than westernized one, has been reported to contain high level of salt on average about 13.5g/day which is 2.7-fold higher than the recommended values of WHO. Whether there is a genetic predisposition toward sodium intake level is a basic but interesting question to ponder in the Korean population.A half-day urine (HU) samples were collected for all participants of the Healthy Twin Study. HU collection starts around 7 pm of the day before visit, after completely voiding when time record starts. All the urine after then was collected in a bag until the next day visit for health examination. On site, in the morning, remaining urine was further voided and the time was recorded as final. We selected samples collected more than 8 hours. Urine samples with less than 8 hours collection were not included as these samples require a special formula to estimate 24 hour levels using sodium, potassium and creatinine concentrations. Among 3079 participants of the Healthy Twin Study, urine samples from 1312 individuals (143 pairs of MZ twins, 31 pairs of DZ twins and 961 singletons) sufficed both accurate information on volume and more than 8 hours collection, which were included in the analyses after projecting 24 hour sodium excretion level from simple volume-time calculation.Heritability of 24 hour sodium intake was estimated using a variance components model (SOLAR). The crude heritability of 24 hour sodium intake was 0.34±0.05. After further adjustment for household income which is one of the main factors of socio-economic status (SES) the heritability was not materially changed (0.28±0.1 p=0.007). Various types of shared environments, such as overall household effects, sibling effects, and generation effects, were included in ACE model, but did not account significant contribution to the variation of salt intake. We concluded that although salt intake is mediated through diet and meals are shared among families, genetic predisposition will play an important role in controlling salt intake. Analysis on the preference for salty food and on total amount of food intake is ongoing which will further dissect the genetics and shared environments related to salt intake.