Whose data are they anyway?

The design and updated results of Intersalt may still fail to convince sceptics of a causal relation between salt intake and blood pressure, and some of the difficulties in demonstrating an association should therefore be emphasised.These include the measurement of salt intake, which is notoriously inaccurate for individuals, and the range of variation, which may be too narrow within a population compared with the large variation between individuals.The Intersalt study does not have the perfect design to overcome these difficulties, and on its own it cannot answer the question as to whether high salt intake causes high blood pressure.But until someone sets up a 30 year longitudinal study to monitor sodium chloride intake and blood pressure prospectively in a sufficiently large population, this hybrid cross sectional, within population and cross population, ecological study is likely to be the only feasible epidemiological design.The updated version of Intersalt provides robust results that are in concert with other studies, including experiments on animals and clinical trials.7A recent study on chimpanzees showed that adding 100 mmol of sodium a day to their food increased their systolic blood pressure by 12 mm Hg.Blood pressure rose further with further increases in sodium intake and fell when sodium supplementation was stopped.8The Intersalt results must be viewed in the context of such existing evidence suggesting a causal relation between salt intake and blood pressure.Whether the evidence is strong enough to war- rant the reductions in salt recommended by the authors is, as always, a question of judgment.But useful clinical and public health actions have been undertaken on much weaker evidence.