Agreement Between 24-hour Salt Ingestion and Sodium Excretion in a Controlled Environment

Hypertension. 2015 Oct;66(4):850-7. doi: 10.1161/HYPERTENSIONAHA.115.05851. Epub 2015 Aug 10.


Accurately collected 24-hour urine collections are presumed to be valid for estimating salt intake in individuals. We performed 2 independent ultralong-term salt balance studies lasting 105 (4 men) and 205 (6 men) days in 10 men simulating a flight to Mars. We controlled dietary intake of all constituents for months at salt intakes of 12, 9, and 6 g/d and collected all urine. The subjects' daily menus consisted of 27 279 individual servings, of which 83.0% were completely consumed, 16.5% completely rejected, and 0.5% incompletely consumed. Urinary recovery of dietary salt was 92% of recorded intake, indicating long-term steady-state sodium balance in both studies. Even at fixed salt intake, 24-hour urine collection for sodium excretion (UNaV) showed infradian rhythmicity. We defined a ±25 mmol deviation from the average difference between recorded sodium intake and UNaV as the prediction interval to accurately classify a 3-g difference in salt intake. Because of the biological variability in UNaV, only every other daily urine sample correctly classified a 3-g difference in salt intake (49%). By increasing the observations to 3 consecutive 24-hour collections and sodium intakes, classification accuracy improved to 75%. Collecting seven 24-hour urines and sodium intake samples improved classification accuracy to 92%. We conclude that single 24-hour urine collections at intakes ranging from 6 to 12 g salt per day were not suitable to detect a 3-g difference in individual salt intake. Repeated measurements of 24-hour UNaV improve precision. This knowledge could be relevant to patient care and the conduct of intervention trials.

Keywords: dietary; hypertension; salt; sodium; urine specimen collection.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Blood Pressure / physiology*
  • Circadian Rhythm / physiology*
  • Environment, Controlled*
  • Follow-Up Studies
  • Humans
  • Hypertension / physiopathology*
  • Hypertension / urine
  • Male
  • Reference Values
  • Sodium / urine*
  • Sodium Chloride, Dietary / administration & dosage*
  • Urine Specimen Collection


  • Sodium Chloride, Dietary
  • Sodium