Surveillance
The following sections discuss surveillance for both sodium intake and sources of sodium in the food supply.
Measuring Dietary Sodium Intake
Spot urine vs. 24-hour urine for population values
Spot urine samples are much easier to collect and require far fewer resources than the gold standard, 24-hour urine method. This section looks at evidence examining how spot urine samples perform compared to 24-hour urine in determining population sodium intake.
Based on a review of multiple systematic reviews and analyses, the International Consortium for Quality Research on Dietary Sodium/Salt (TRUE) concludes that the recommended method for assessing population dietary sodium intake is to collect “single complete 24-hour urine samples, collected over a series of days from a representative population sample” and for individual level estimations, “at least 3 non-consecutive complete 24-hour urine collections collected over a series of days that reflect the usual short-term variations in dietary pattern.” Generally, spot urine samples systematically underestimate changes in dietary sodium intake at the population level. To understand the role of single spot or short duration timed urine collections in assessing population average sodium intake will require more research. Single spot or short duration timed urine collections are not recommended for individual sodium intake estimation.
Spot urine concentrations and 24-hour urinary sodium excretion collected by the Intersalt study (1885-1987 in 32 North American and European countries) were compared to determine the utility of estimating 24-hour urinary sodium excretion using spot urine specimens. The correlation (Pearson r) between observed and estimated 24-hour sodium excretion was 0.50 for individual men and 0.51 for individual women, and .79 for men at the population-level and 0.71 for women at the population-level. Bias was minimal (-1.8 mmol men; 2.2 mmol women) at the population level. Using spot urine estimations, the percent of people with sodium intake above the recommended levels was slightly higher than with observed 24-hour sodium excretion. Spot urine specimens may be able to provide useful estimates of population but not individual sodium intake.
This study (a review of 29 studies estimating daily salt intake based on both spot and 24-hour urine) provides evidence that spot urine samples can be used to provide estimates of mean population salt intake; however, because spot urine systematically overestimated lower levels of salt intake and underestimated higher levels compared to 24-hour urine, spot urine samples should not be used to detect changes in average population salt consumption over time. For estimating mean population salt intake as above or below a 5g/day threshold, spot urine had a sensitivity of 97% and specificity of 100% (assuming 24-hour urine to be gold standard).
The gold standard method to measure population salt intake is complete 24-hour urine measurements, and previous studies have demonstrated challenges with using spot urine samples to estimate overall sodium intake. This study, however, found that measurement of change in mean population sodium intake over time can be conducted using the sodium concentration of spot urine samples. This was true in both random and representative population samples taken to monitor population salt reduction programs. Data was pulled from intermittent 24-hour urine samples (n=2020) and spot urine samples (n=21711) collected in 4 separate cross-sectional surveys between the years 2005 and 2014 conducted among a representative and randomly selected sample of the UK population. The study found that the relative changes collected in spot urine and 24-hour urine samples were similar across the various time intervals between the surveys, providing evidence towards a more cost-effective monitoring method for sodium reduction interventions. However, spot urine samples should not be used to measure individual sodium intake. Further, the study found that the Kawasaki, Tanaka, and INTERSALT formulas are not suitable to estimate 24-hour urine or monitor changes in population salt intake, as they systematically underestimate higher salt intakes and overestimate lower salt intake.
Population Sodium Intake
Using 24-hour urine-based and diet-based surveys from 66 countries (61% high-income regions, 40% low- or mid-income regions), authors estimated a mean level of global sodium consumption of 3.95g per day (or 10.06g/day of salt) as of 2010 (regional mean levels ranged from 2.18 to 5.51g sodium per day), with 99.2% of adults worldwide exceeding recommended level of 2.0g/day and 88.3% exceeding the recommended level by more than 1.0g/day. Men had slightly higher intake than women (4.14g/day vs 3.7g/day). Variation by age was small. Highest intakes were found in Asia (East Asia: 4.8g/day, Asia Pacific (high income): 5.0g/day, and Central Asia 5.51g/day. Next highest were Eastern Europe (4.18g/day), Central Europe (3.92g/day), and the Middle East and North Africa (3.92g/day). Lowest intakes were found in Sub-Saharan Africa, Latin America and the Caribbean and Oceania, although data was limited in these areas. A statistically non-significant increase in sodium intake globally was found using 24-hour urine data between 1990 and 2010.
Dietary Sources of Sodium
Knowing the main sources of dietary salt in a country is essential for the development of effective salt reduction interventions. To determine baseline data on the major contributors to salt intake, authors recommend determining the level of discretionary salt in the household by a combination of direct and indirect methods (e.g., qualitative surveys or interviews, household food disappearance studies, “subtraction” methods), determining food consumption patterns through established survey instruments (if they exist) or by 24-hour recalls or food frequency questionnaires, and identifying the proportional importance of discretionary sources versus commercially prepared foods. They emphasize the importance of directing resources toward maintaining national or international food composition databases, either from primary or secondary sources.
This review identifies the sources of dietary salt in all countries with available data. The average daily salt intake of adults in these studies ranged from 5.2 in Guatemala to 15.5 g/day in China. Brazil, China, Costa Rica, Guatemala, India, Japan, Mozambique, and Romania appear to obtain more than half of their daily salt during cooking or at the table (discretionary salt). Populations in Jordan, Portugal, South Korea, Taiwan, and Turkey appear to receive between 25 and 50% of their daily salt from discretionary sources. On the other hand, discretionary salt intake accounts for less than 25% of daily salt intake in Australia, Austria, Canada, Denmark, Finland, New Zealand, the United Kingdom, and the United States of America. While there were no obvious regional patterns in the food sources of dietary salt, bread and bakery products, cereal and grain products, meat products, and dairy products appeared to be the main global contributors to daily salt intake. The study additionally found a significant inverse correlation between a country’s GDP per capita and the proportion of daily salt intake from discretionary sources.
