Errors in estimating usual sodium intake by the Kawasaki formula alter its relationship with mortality: implications for public health

• Published in: International journal of epidemiology 2018-12, Vol.47 (6), p.1784-1795
• Main Authors: He, Feng J, Campbell, Norm R C, Ma, Yuan, MacGregor, Graham A, Cogswell, Mary E, Cook, Nancy R
• Format: Article
• Language: English
• Subjects: Adult; Female; Humans; Hypertension - prevention & control; Male; Middle Aged; Mortality; Proportional Hazards Models; Public Health; Sodium - urine; Sodium, Dietary - administration & dosage; The Perils of Sodium
• ISSN: 0300-5771; 1464-3685
• DOI: 10.1093/ije/dyy114

Several cohort studies with inaccurate estimates of sodium reported a J-shaped relationship with mortality. We compared various estimated sodium intakes with that measured by the gold-standard method of multiple non-consecutive 24-h urine collections and assessed their relationship with mortality. We analysed the Trials of Hypertension Prevention follow-up data. Sodium intake was assessed in four ways: (i) average measured (gold standard): mean of three to seven 24-h urinary sodium measurements during the trial periods; (ii) average estimated: mean of three to seven estimated 24-h urinary sodium excretions from sodium concentration of 24-h urine using the Kawasaki formula; (iii) first measured: 24-h urinary sodium measured at the beginning of each trial; (iv) first estimated: 24-h urinary sodium estimated from sodium concentration of the first 24-h urine using the Kawasaki formula. We included 2974 individuals aged 30-54 years with pre-hypertension, not assigned to sodium intervention. During a median follow-up of 24 years, 272 deaths occurred. The average sodium intake measured by the gold-standard method was 3769 ± 1282 mg/d. The average estimated sodium over-estimated the intake by 1297 mg/d (95% confidence interval: 1267-1326). The average estimated value was systematically biased with over-estimation at lower levels and under-estimation at higher levels. The average measured sodium showed a linear relationship with mortality. The average estimated sodium appeared to show a J-shaped relationship with mortality. The first measured and the first estimated sodium both flattened the relationship. Accurately measured sodium intake showed a linear relationship with mortality. Inaccurately estimated sodium changed the relationship and could explain much of the paradoxical J-shaped findings reported in some cohort studies.