Action on Salt response to Lancet publication
12 August 2018
Action on Salt's response to the latest publication in the Lancet by Mente A, O’Donnell M et al: “Urinary sodium excretion, blood pressure, cardiovascular disease, and mortality: a community-level prospective epidemiological cohort study”
This paper, similar to a 2016 publication by Mente, O'Donnell and others, has several issues, including:
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 7-11 24 hour urines to accurately reflect an individual’s intake. Our recent publication in the Journal of International Epidemiology demonstrates that estimated salt intake from spot urine by the Kawasaki formula (as used in the Lancet study) finds a risk associated with low sodium intake, but accurate measurements find that lower intakes of sodium lead to a lower risk of death. Our paper can be found here.
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.
Professor Graham MacGregor, Professor of Cardiovascular Medicine at Queen Mary University of London and Chair of Action on Salt says “This publication is simply a repeat of a study issued by the authors in 2016, using a slightly modified study population and analysis, however the authors have not addressed any of the serious criticisms from the wider scientific community of their 2016 study in this subsequent publication. These criticisms include the use of ill participants in the study, leading to reverse causality (i.e. those suffering with heart disease don't eat much food, and consequently eat less salt, but it is the illness that leads to death rather than lower salt intake), and the use of spot urine measurements, which, as our paper in the International Journal of Epidemiology demonstrates, is an inaccurate measurement and leads to incorrect findings on the relationship between salt intake and health.
"The totality of evidence shows that lowering salt intake leads to a fall in cardiovascular disease events, the commonest cause of death and disability worldwide. In our view, papers of poor scientific quality should not be considered as part of the evidence base.”