Value of overnight urines in the follow-up of people with borderline hypertensive trying to reduce their salt intake.

The purpose of this study was to examine the value of overnight urine samples compared with 24-hour urine collections as a measure of salt intake in a long-term (two years) salt restriction trial which was part of the Finnish North Karelia Salt Project. Persons with slightly elevated blood pressure (without drug therapy) were advised to reduce their salt intake as much as possible. Each urine collection was divided into two time periods: overnight and rest of the 24-hour period. The mean 24-hour urinary sodium excretion among men decreased during a two-year period from 243 mmol to 177 mmol (P less than 0.01) and among women from 194 mmol to 153 mmol (P less than 0.01), respectively. The correlation coefficient between overnight urine sodium excretion rate and 24-hour urine sodium excretion rate was among men 0.49 at baseline and 0.80 at two years and among women 0.82 at baseline and 0.49 at two years. The results suggest that the strength of the correlation between overnight and 24-hour urine sodium excretion rates is modified by the total sodium excretion level. The overnight urine specimen might be valuable in estimating salt intake in groups of people. However, because restriction of salt intake is mainly reflected in the rest of the 24-hour period but not in the overnight collection, the overnight specimen cannot be used to estimate an individual's salt intake or salt intake reduction.