The potential use of lithium as a marker for the assessment of the sources of dietary salt: cooking studies and physiological experiments in men.

Lithium was investigated for its possible use as a marker for identifying the various sources of NaCl in the diet. Micromolar concentrations of lithium can be detected in various vegetables, tap water and also in urine specimens of adult volunteers. The lithium content of vegetables varied from 6.1 to 24.5 mumol of lithium/kg dry weight, with the exception of spinach and aubergines which had much higher concentrations. The excretion of the element in 24 h urine specimens ranged from 2 to 4 mumol of lithium/day. Experiments were performed to assess whether both lithium and sodium would penetrate foods at the same rate during cooking. The rates of penetration into food for both elements were proportional to their concentration in the cooking water despite a sodium/lithium ratio of 50:1. Physiological experiments were conducted to investigate the handling of small doses of lithium by the body. A dose of 250 mumol of lithium was chosen as optimal and given orally to healthy volunteers in either single or continuous aqueous doses of lithium carbonate. The recoveries of oral lithium in urine were 92 +/- 5% (SD) and 97 +/- 4 (SD) (n = 5) for single and continuous doses respectively. The daily addition of 100 mmol of oral NaCl to the diet of volunteers receiving a standard dose of lithium did not affect urinary lithium excretion rates nor the final recovery of the administered lithium. These studies suggest that lithium carbonate may be a useful marker for the uptake of NaCl into cooked food; after eating lithium-enriched food the monitoring of urinary lithium output may then be used to quantify the amount of sodium derived from the specific foods.