Spouse-spouse, sib-sib, and parent-offspring correlations were calculated for urinary, plasma, and intracellular sodium levels on over 1,900 persons aged 3-86 years in 98 Utah kindreds. For 36 hours prior to their clinic visit, 31% of the sample was salt-loaded with salt tablets, while the rest followed their normal diet. For those on their normal diet, urine creatinine-, age-, and sex-adjusted urinary sodium excretion from a timed 12-hour overnight sample showed similar and significant correlations between spouses (r = .29), sibs less than 20 years old (r = .38), and parent-offspring pairs for offspring less than 20 years old (r = .29). This contrasted with the lower correlations between sibs 20 years of age and older (r = .10) and parent-offspring pairs for offspring 20 years of age and older (r = .13), presumed to live in different households. Adult plasma sodium sib-sib (r = .13) and parent-offspring (r = .15) correlations were similar to the urinary sodium correlations, while the spouse-spouse (r = .48), the sib-sib (r = .64), and the parent-offspring (r = .63) correlations for those presumed to live in the same household nearly doubled. Intracellular sodium correlations for the adult sibs (r = .32) and offspring (r = .36) were over twice as large for urinary or plasma sodium, although the spouse-spouse correlation (r = .37) remained large also. Using the salt-loaded families, only the sib-sib and parent-offspring correlations for those sibs and offspring who were less than 20 years of age for plasma sodium and the spouse-spouse correlation for intracellular sodium changed (decreased). These results indicate that there is a large environmental component not only for urinary sodium excretion but also for plasma sodium concentration, and that plasma sodium may also reflect changes in dietary sodium intake. Intracellular sodium concentration, on the other hand, seems to have a larger genetic component, although because the spouse-spouse correlations were large and decreased upon salt-loading, there also appears to be an environmental component that can modify the genetically determined sodium handling in an individual.
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