Evaluating population salt reduction programmes worldwide: the risk of cutting corners!

The question of how to assess salt intake in individuals and populations has been framed since the 1970s. Salt intake is extremely variable between individuals as well as from day to day in the same person. Therefore, even a single measurement of the daily amount of Na excreted in the urine (often regarded as the ‘gold standard’) is inadequate for assessing the salt consumption of an individual. In a well-conducted physiological study, single 24 h urine collections at intakes ranging from 6 to 12 g salt/d were not suitable to detect a 3 g difference in individual daily salt intake. Repeated measurements of 24 h urinary Na improve precision, suggesting multiple 24 h urine collections over time are necessary to assess a person’s salt intake. However, the interest in using easier alternative methods has led to a renewed curiosity into the validation of methods based primarily on estimating 24 h urinary Na excretion (hence salt intake) from spot urine samples. In an original systematic review including 1 380 130 participants from twenty studies, spot, timed and overnight urine samples showed greater intraindividual and inter-individual variability than 24 h urine collections. There was a wide range of correlation coefficients between 24 h urine Na and other measures of Na excretion. Subsequently, numerous validation studies have been published, comparing 24 h urine collections v. estimates of daily Na excretion from spot urines extrapolated with the application of different formulas. From a variety of population analyses spot urines (irrespective of the formulas used to estimate daily consumption) lead to biased estimates of 24 h urinary Na excretion with overestimates at lower levels and underestimates at higher levels, making these measures not suitable to assess an individual’s Na excretion (salt intake) in cross-sectional and prospective studies. A different question has been put more recently as to whether we can estimate the average Na excretion for the population using spot urine samples, avoiding the burden of 24 h urinary collections in epidemiological settings. This question has become repeatedly common given the need to assess average population salt consumption and to monitor and evaluate intervention programmes of population salt reduction over time within the UN resolution and WHO action plan to reduce global CVD by 2025. The analysis of different South African population samples from various ethnic backgrounds (white, black, Indian), reported by Swanepoel et al. in this journal, is an important step forward. It validates three formulas (Kawasaki, Tanaka and INTERSALT) commonly used to estimate 24 h urinary Na excretion from spot urine samples against the direct measurements of Na excretion using a 24 h urine sample. The authors pooled data from three cross-sectional population studies in South Africa: the African-PREDICT study, including 470 black and white men and women aged 20–30 years; the Thusa-Bothle study, including 104 black women aged 35–65 years; and the KwaZulu-Natal study, including 107 Indian women aged 18–50 years. All three studies used common methodologies for collecting and analysing urine samples. Several measurements were considered: comparison of mean population Na excretions (estimated and measured); Bland–Altman plots to calculate the bias in the spot estimate v. the ‘gold standard’ (24 h urinary excretion); proportional bias by linear regression of the difference in estimates v. the mean; interclass correlation coefficients between estimated and measured Na; and sensitivity and specificity of estimated Na excretion to correctly classify the mean population Na excretion below the WHO’s recommended target of 2000mg Na/d (5 g salt/d). The search for validity in the framework of the global action plan is not so much to simply estimate the average population mean, or the proportion of the population with a salt intake ‘above’ a specific threshold, since the majority of populations globally are above that threshold. The validations of two alternative measures should be able to: (i) estimate the absolute difference in salt consumption between two time points (measuring the effectiveness of a population programme of salt reduction); and (ii) estimate the proportion of the population ‘below’ a threshold of 5 g salt/d. Crude direct comparisons between population means by Swanepoel et al. indicated that the Kawasaki and Tanaka formulas overestimate urinary Na excretion compared with 24h urinary Na, whereas the INTERSALT formula – in a couple of cases – underestimates it. However, the degree of bias obtained from the Bland–Altman plots and the proportional bias measured indicate that all three formulas fall short of an ideal scenario. Both the Kawasaki and the Tanaka Public Health Nutrition: 21(12), 2161–2163 doi:10.1017/S1368980017003408