The epidemiology of salt and hypertension

The relationship between salt intake and blood pressure was once highly controversial. Early comparisons of populations did support the salt hypothesis but were difficult to interpret because of unmeasured confounding variables. However, the INTERSALT project, published in 1988 over-came many of these problems and reported within - and between population gradients of sodium excretion and blood pressure or the rise in blood pressure with advancing age. Of the five long-term follow-up studies of salt intake and cardiovascular disease, two suggested that a low salt intake might be harmful. These studies are however open to much criticism. The three other studies reported positive relationships between salt intake and stroke or heart attack. Whilst there is little doubt that salt restriction lowers blood pressure in hypertensive patients, there remains some uncertainty whether any effect is seen in normotensives. The effects, if present, are small (reducing systolic pressure by about 2 mmHg) and not clinically useful, although if applied on a population-wide basis they might reduce the prevalence of hypertension by about 17%. In many countries food producers and food standards agencies are beginning to respond by cutting the salt content of processed foods.