Abstract
Repeated 24-hour urine collection is considered to be the gold standard for assessing salt intake. This is often impractical in large-population studies, especially in low–middle-income countries. Equations to estimate 24-hour urinary salt excretion from a spot urine sample have been developed, but have not been widely validated in African populations. This study aimed to systematically assess the validity of four existing equations to predict 24-hour urinary sodium excretion (24UNa) from spot urine samples in a nationally representative sample of South Africans. Spot and 24-hour urine samples were collected in a subsample (n = 438) of participants from the World Health Organisation Study on global AGEing and adult health (SAGE) Wave 2 in South Africa in 2015. Measured 24UNa values were compared with predicted 24UNa values from the Kawasaki, Tanaka, INTERSALT and Mage equations using Bland–Altman plots. In this subsample (mean age 52.8 ± 16.4 years; body mass index 30.2 ± 8.2 kg/m2; 76% female; 73% black African; 42% hypertensive), all four equations produced a significantly different population estimate compared with the measured median value of 6.7 g salt/day (IQR 4.4–10.5). Although INTERSALT underestimated salt intake (−3.77 g/d; −1.64 to −7.09), the other equations overestimated by 1.28 g/d (−3.52; 1.97), 6.24 g/d (2.22; 9.45), and 17.18 g/d (8.42; 31.96) for Tanaka, Kawasaki, and Mage, respectively. Bland–Altman curves indicated unacceptably wide levels of agreement. Use of these equations to estimate population level salt intake from spot urine samples in South Africans is not recommended.
Highlights
It is widely accepted that excess dietary sodium or salt consumption is a major determinant of population blood pressure (BP) levels [1], contributing to the epidemic of hypertension and cardiovascular disease (CVD) [2]
The aim of this study was to investigate the validity of four such equations (INTERSALT [30], Tanaka [31], Kawasaki [32], Mage [33]) to predict 24-hour urinary Na excretion (24Una) from spot urine samples in South African adults
Reasons for missing data related to uploading issues of sociodemographic survey data and anthropometric and blood pressure measurements by the computer assisted personal interview system
Summary
It is widely accepted that excess dietary sodium or salt consumption is a major determinant of population blood pressure (BP) levels [1], contributing to the epidemic of hypertension and cardiovascular disease (CVD) [2]. Of the 56.9 million deaths worldwide in 2016, CVD including ischaemic heart disease and stroke were the world’s biggest killers, accounting for over a quarter or 15.2 million deaths in that year alone [3]. Hypertension is estimated to be responsible for around half of these heart disease and stroke deaths, translating to 9.4 million deaths worldwide every year [4], and highlighting the need for prevention and treatment of raised BP to prevent these vascular events [5].
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