Salt
The first suggestion that a high salt intake gives rise to high blood pressure goes back 4000 years to the ancient Chinese yellow Emperor, Huang Ti.
He suggested that people who ate too much salt developed harder pulses. More recent epidemiological evidence is so impressive that the salt hypothesis can no longer be considered controversial.
Primitive rural societies in Africa and also the South Pacific islands consume very little salt (below 50 mmol/day) and have hardly any hypertension and no rise in blood pressure with age.
Conversely, European, American and Japanese populations consume a lot of salt (200 – 300 mmol/day) and have high average blood pressures.
The very incidence of strokes amongst the northern Japanese may well be related to their very high salt intake and the recent impressive reduction in stroke rates may well be attributable to the reduction in salt intake that has occurred over the last 20 years.
The INTERSALT project was a major international collaborative study in which directly comparable data were obtained form 52 different populations in 32 countries. All the important confounding differences between urban or westernized population and primitive groups were taken into account.
The study showed unequivocally that the rise in blood pressure seen with advancing age in urban but no rural populations was related to the amount of salt in the diet.
Similarly meta-analysis of all the reliable individual population surveys of blood pressure in relation to salt intake confirmed the close relationship between salt intake and the height of the blood pressure and demonstrated that this effect was independent of the degree of urbanization.
There is evidence that over the last 50 years dietary salt consumption in the USA and Europe has fallen and this has been paralleled by a fall in stroke incidence. The fall in stroke incidence since the Second World War is only partly due to the more frequent use of antihypertensive medication.
Also, there is now evidence that high salt intake may cause strokes, partly through a direct effect on cerebral vessels and partly by a concomitant high prevalence of hypertension.
Salt
