Abstract
Abstract
A high dietary sodium intake is associated with elevated blood pressure, a major risk factor for cardiovascular disease. Average sodium intake in New Zealand adults exceeds the suggested dietary target of 2000 mg/day and sodium reduction strategies are needed to counter this. A food reformulation programme to lower sodium content in processed foods called the Heart Foundation HeartSAFE programme is the main salt reduction strategy available in New Zealand. Simultaneously, iodine intake in New Zealand is traditionally low and the use of iodised salt in cooking, at the table or added to bread improves the population’s iodine intake. Discretionary salt (added in cooking or at the table), if iodised, contributes to both sodium and iodine intakes. Therefore, the quantification of discretionary salt intake can inform public health policies on both sodium and iodine. However the amount of discretionary salt used in New Zealand has not been quantified. In addition, there are concerns that sodium reduction strategies could adversely impact on iodine intake. Studies examining the reduction in sodium intake while maintaining iodine adequacy in the population are needed. Therefore, this thesis had the following objectives:
1. To simulate the potential impact of the food reformulation programme, HeartSAFE, on sodium intakes in the New Zealand adult population (Chapter 3).
2. To determine the sodium intake from discretionary salt as a proportion of total sodium intake for 18 to 40 year-olds in New Zealand (Chapter 4).
3. To ascertain the level of salt iodisation required for females aged 18 to 40 years in New Zealand to achieve adequate iodine intake at the current amount of discretionary salt intake and if discretionary salt intake were reduced (Chapter 5).
Chapter 3 used 24-hour dietary recall data from the 2008/09 New Zealand Adult Nutrition Survey (n=4721) to estimate the sodium intakes of participants from foods included in the HeartSAFE programme. The sodium content of 840 foods in 35 sub-categories was altered to meet the sodium targets set in the HeartSAFE programme. At baseline, sodium intake from foods included in the HeartSAFE programme was 1307 mg/day (95% CI 1279, 1336). The simulated sodium intake was 1048 mg/day (95% CI 1024, 1072) if all 840 foods met the HeartSAFE’s sodium target. In total, for the foods that were included in the HeartSAFE programme, sodium reduction was 260 mg/day (95% CI 252, 268).
Chapter 4 determined discretionary salt intake using the lithium-tagged salt method. The study recruited 116 adult male and female participants who were healthy, not pregnant or breastfeeding, regularly consume home-cooked meals and use salt during cooking or at the table, aged 18-40 years from Dunedin, New Zealand. Participants took part in the study over a nine-day period. They collected a baseline 24-hour urine sample, replaced their usual salt with the lithium-tagged salt for seven days, with two 24-hour urine samples obtained between day 6 and day 8 and two 24-hour dietary recalls conducted a day after the 24-hour urine collections. The median sodium intake from discretionary salt was estimated to be 13% (25th, 75th percentile: 7, 22) of the total sodium intake or 366 mg/24hr (25th, 75th percentile: 186, 705). The median urinary sodium was 3222 mg/day (25th, 75th percentile: 2516, 3969).
Chapter 5 applied the estimated average requirement (EAR) cut-point method to determine the appropriate level of salt iodisation for 98% of females of reproductive age (18-40 years old) to consume more than the EAR for iodine (100 𝜇g/day). Total iodine intake was calculated by adding the iodine intake from 24-hour dietary recall data collected from 940 women aged 18-40 years in the 2008/09 New Zealand Adult Nutrition Survey and iodine intake from discretionary salt as determined in Chapter 4. At the current discretionary salt intake, salt will have to be iodised at 50 mg/kg salt for 98% of the females of reproductive age to consume above the EAR for iodine. In a reduced discretionary salt intake scenario (304 mg sodium/day), the amount of iodine in salt will have to increase to 60 mg/kg salt for this group to have an adequate iodine intake.
Results from the studies in this thesis show that discretionary salt is not a major dietary source of sodium in the New Zealand diet. However, the HeartSAFE sodium reduction strategy that targets food reformulation of non-discretionary sources of sodium will not lower sodium intake to meet the WHO suggested dietary target of 2000 mg/day. Additional sodium reduction strategies are needed for the New Zealand population to achieve the 2000 mg/day target. This thesis also provided evidence that sodium reduction strategies will not interfere with efforts to ensure adequate iodine intake if appropriate adjustments of the iodine content are made to iodised salt.