1. An epidemiological study was conducted in the market town of March, Cambridgeshire, to assess the quantitative importance of cooking and table salt to total dietary salt intake by the use of a fused mixture of lithium carbonate and sodium chloride. 2. Men and women aged 20–60 participated in a 12 day study with sequential 24 h urine collections to assess salt sources over a 7 day period. 3. Total salt consumption estimated from urinary chloride excretion amounted to 10.6 ± 0.55 ( sem ) g in 33 men and 7.4 ± 0.29 ( sem ) g in 50 women. The cooking salt eaten was only 0.45 ± 0.09 ( sem ) g in men and women, with men eating more table salt (0.77 g/day) than women (0.46 g/day). 4. Discretionary sources, i.e. cooking and table salt use, contributed only 15% to the total intake. Salt from manufacturing foods and catering in purchased food therefore provided on average 85% of total salt intake. These results are consistent even when an allowance is made for the slightly poorer pouring quality of the lithium-tagged salt. 5. The importance of food as a source of salt was reflected in the significant relationship between the weight of the individual and the amount of salt eaten (for males P < 0.05 and for females P < 0.001). 6. Cooking salt consumption did not relate to the amount of salt derived from purchased food nor did table salt use relate to the amount of salt in cooked foods. 7. Husbands and wives showed a high correlation in their salt use but the husbands had higher intakes of salt from purchased food and from cooking salt. They also used more table salt than their wives.
1. 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 μmol 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 μmol of lithium/day. 2. 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. 3. Physiological experiments were conducted to investigate the handling of small doses of lithium by the body. A dose of 250 μmol 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. 4. 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 administerd lithium. 5. 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.
1. 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°C followed by grinding and sieving the fused salts to a defined grain size; magnesium carbonate was added as a hygroscopic agent. 2. 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. 3. 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. 4. This study suggests that the lithium method is suitable for epidemiological use to measure the sources of dietary salt.