WASH & CASH response to Publication in the Lancet by Mente et al on salt and blood pressure

20 May 2016

Mente A, O’Donnell M et al “Associations of urinary sodium excretion with cardiovascular events in individuals with and without hypertension: a pooled analysis of data from four studies” http://dx.doi.org/10.1016/S0140-6736(16)30467-6

This paper in the Lancet by Salim Yusuf and colleagues claims that reducing salt intake to recommended levels would be harmful. Before coming to this conclusion, it is important to realise that there are two major problems with this analysis.

1. A single spot urine in one individual does not reflect their salt intake. This is primarily because spot urines are an inaccurate way of calculating 24 hour urinary sodium excretion, which is the gold standard for measuring salt intake. However salt intake varies widely from day to day, so that even one 24-hour urine does not reflect an individual’s intake, and can therefore only be used to calculate a population intake when done from a large randomised sample of the population. Indeed careful metabolic studies show that you need to collect 7 to 11 24-hour urines to accurately reflect an individual’s intake.

2. Furthermore, there is no proper discussion of the problems relating to reverse causality. That is when people are dying they eat little or no food and as a result their salt intake is low. It is not the low salt intake that is causing their death, but rather their illness that is causing their low salt intake.

The evidence for salt causing high blood pressure and that reducing salt intake lowers blood pressure and prevents strokes and heart attacks, is overwhelming.  This current study, because of the weakness of the design, does not in any way overturn public health messages to reduce salt intake, both in the UK and worldwide.