A test of the validity of the lithium-marker technique for monitoring dietary sources of salt in man.

A lithium-tagged salt for assessing the sources of salt in the human diet was prepared by fusing lithium carbonate and sodium chloride at 900 degrees C followed by grinding and sieving the fused salts to a defined grain size; magnesium carbonate was added as a hygroscopic agent. To validate the use of this tagged salt, which replaced alternately table salt and cooking salt in daily use, a 44 day metabolic study was conducted on five volunteers. Measurements of sodium and lithium balance throughout the study showed that 93% of both dietary sodium and dietary lithium were excreted in the urine and 1.7% of dietary lithium and 2% of dietary sodium in the faeces. Collections of sweat for 48 h at intervals throughout the study showed that only 1.7% of dietary lithium and 1.4% of dietary sodium were recovered in sweat. Thus both elements were excreted equivalently by the three main routes. The apparent mean sodium retention over the whole period of dietary control was 7.4 +/- 4.4 mmol/day or 3.4% of the total intake. The excretory patterns of lithium after a period of lithium ingestion were exponential, thus allowing a method to be developed for the prediction of total lithium output from a shorter period of urine collection. This study suggests that the lithium method is suitable for epidemiological use to measure the sources of dietary salt.