Title, Date of Publication & Journal

Salt reduction in England from 2003 to 2011: its relationship to blood pressure, stroke and ischaemic heart disease mortality.

14 April 2014 BMJ Open.

Claim supported by evidence?

The paper does not provide any new evidence for controlling stroke and ischaemic heart disease through population wide reduction in salt intake.

Its conclusion that “salt intake is likely to be an important contributor to the falls in blood pressure from 2003 to 2011” is not based on any new direct evidence, although it may possibly be true.

Summary

• Data extracted from Health survey for England in 2003, 2006, 2008 and 2011 show a steady reduction in blood pressure.
• Data extracted from the national Diet and Nutrition Survey for 2000/2001, 2005/2006, 2008 and 2011 show a decline in salt intake as measured from urine samples.
• Data from Office of national Statistics over the period 2003, 2006, 2008 and 2011 show a decline in the death rate from ischaemic heart disease and Stroke.

• Although there are parallel trends for these across time, there is no evidence in this study to suggest a causal link between these three observations.

Study Conclusions

• The argument in this paper is simply based on the fact that these three trends continue at a roughly constantly rate in England across the first decade of the 21^st century.
• However, parallel trends across time are a notoriously low level of evidence.
• A secondary analysis, which excluded those on therapy to control blood pressure, does not “investigate the role of salt in reduction in the changes of BP” as claimed.  It does not address salt intake at all.
• The impact of smoking history is not discussed in much detail. There has been a reduction in smoking prevalence in the past few decades and it is possible this might have been driving some of the changes over time.
• The fall in systolic blood pressure “was larger than that which might have been predicted from the meta-analysis of randomised salt reduction trials” by He, Li & MacGregor (2013). One obvious alternative conclusion is that other factors are driving the observed population reduction in blood pressure.

Strengths/Limitations

• There is very little new quantitative information.
• There is no direct linkage between the three sources of information.
• It is well known that salt intake alters blood pressure and that controlling blood pressure reduces mortality from heart disease. But this paper adds no new evidence about the size of or veracity of such an assertion.