Abstract
Background: New Zealand has a long history of moderate to mild iodine deficiency due to low levels of iodine in the soil. Iodised salt has been used as a vehicle to improve iodine intakes since the 1930’s. More recently, in response to a reemergence of iodine deficiency, the fortification of bread with iodised salt has been used to increase iodine intakes. However, in order to determine whether the use of discretionary iodised salt continues to make a significant contribution to total iodine intakes, it important to determine the quantity of discretionary salt consumed. Lithium-tagged salt has been established to directly measure discretionary salt intake.
Objective: To pilot the use of lithium-tagged salt to determine the contribution of iodised salt, used at the table and in cooking, to the total iodine and sodium intakes of New Zealand adults. This study also compared urinary and dietary methods to assess discretionary iodised salt intakes.
Design: This was a pilot intervention study conducted in Dunedin, August to November 2016. Fifteen healthy adults who were regular users of salt at the table and in cooking were recruited through posters and emails. Participants provided a total of six 24-hour urine collections and six weighed diet records over a nine-day study period, which contained two baseline days and four intervention days. Lithium-tagged salt was provided to participants after the two baseline days. The weight of salt in grams was also determined by weighing the salt shaker before and after intervention.
Results: During the intervention period when lithium-tagged salt was consumed the mean (SD) total sodium intake was 2900 (1365) mg/day and discretionary sodium (lithium excretion converted to a sodium equivalent) was 416 (488) mg/day, contributing 14% of total intakes. When converting to intakes, this equates to 8.2g of total salt per day, with 1.2g coming from discretionary sources, contributing 38% to total iodine intake. From the weighed diet records, the mean sodium intake was 2878 (1435) mg/day with 706 (831) mg/day coming from discretionary sources. This equates to 7.3g of salt per day, with 1.8g or 25% coming from discretionary salt. The mean weight of the lithium-tagged salt used from the salt shaker was 3.2g/day, this equates to a mean (SD) of sodium 1251 (1001) mg/day.
Conclusion: Lithium tagged salt was a feasible method to determine discretionary salt intakes. This method appeared to provide more objective estimates of discretionary salt intake than dietary methods. In this group of adults, discretionary salt intake was 1.2g and contributed 14% of total sodium intake. If discretionary salt was iodised, it would contribute over one third to total iodine intake.