Salt and cardiovascular disease in PURE: A large sample size cannot make up for erroneous estimations

Abstract

The latest Prospective Urban Rural Epidemiology (PURE) study claims that salt reduction should be confined to settings where its intake exceeds 12.7 g/day and that eating less than 11.1 g/day of salt could increase cardiovascular risk. More specifically, Mente et al. suggested that (a) salt intake was positively associated with stroke only when it exceeded 12.7 g/day, (b) salt intake was inversely associated with myocardial infarction and total mortality, and (c) these associations were largely independent of blood pressure. These provocative findings challenge the robust evidence on the role of salt reduction in the prevention of cardiovascular disease and call into question the World Health Organization's global recommendation to reduce salt intake to less than 5 g/day. However, Mente et al.'s re-analysis of the PURE data has several severe methodological problems, including erroneous estimations of salt intake from a single spot urine using the problematic Kawasaki formula. As such, these implausible results cannot be used to refute the strong evidence supporting the benefits of salt reduction for the general population worldwide.

Keywords: Salt; cardiovascular disease; mortality; prevention; salt reduction programmes.

Figure 1.

Spline plots on the association between salt intake and all-cause mortality in 2974 individuals who took part in the Trials of Hypertension Prevention, but were not in the salt reduction intervention group, followed up for over 20 years. (a) Salt intake assessed by the average of 3–7 24-hour urinary sodium excretions (i.e. gold standard method) during the Trials of Hypertension Prevention trial period of 18 months to 4 years. (b) Salt intake calculated as the average of 3–7 estimated 24-hour urinary sodium excretions from sodium concentrations using the Kawasaki formula. (c) Salt intake assessed by the first measured 24-hour urinary sodium excretions at baseline of Trials of Hypertension Prevention trials. (d) Salt intake assessed by the estimated 24-hour urinary sodium excretions from the first 24-hour urinary sodium concentration using the Kawasaki formula. 24h UNa: 24-hour urinary sodium excretions.

Figure 2.

Changes in salt intake as measured by 24-hour urinary sodium excretion, blood pressure, stroke and ischaemic heart disease in England from 2003 to 2011. 24h UNa: 24-hour urinary sodium excretion; IHD: ischaemic heart disease.