Japan : In the late 1950s the Japanese became aware that certain parts of Japan , particularly the north, had a high salt consumption and deaths from stroke were amongst the highest in the world. It was then found that the numbers of strokes in different parts of Japan were directly related to the salt intake. In view of these findings, there was a Government campaign to reduce salt intake, which was successful in reducing salt intake over the following decade from an average of 13.5 grams per day to 12.1 grams per day. However, in the north of Japan the salt intake fell from 18 grams per day to 14 grams per day. At the same time, there was a fall in blood pressure both in adults and children, and an 80% reduction in stroke mortality (1). At the time of this fall and the reduction in stroke mortality, there were large population increases in fat intake, cigarette smoking, alcohol consumption and an increase in weight. It would appear that the Western influence which was rapidly overtaking Japan seemed to have little effect on blood pressure, provided salt intake was reduced, and overall the reduction in salt intake appeared to be associated with large falls in deaths from stroke.
North Karelia, Finland : In this study, 14,000 men aged 35-64 years were given advice about reducing fat and salt intake, increasing fruit and vegetable consumption and decreasing smoking. In co-operation with the food industry in Finland , they were given processed foods with less salt. Over the 20 years of the study which started in 1972, cholesterol was reduced by 13%, diastolic pressure by 9% and smoking by 15%. At the same time, there was a reduction in stoke mortality by 66% and coronary heart disease mortality by 55%. The reduction in the three major risk factors explained four-fifths of the fall in coronary heart disease mortality and two-thirds of the fall in stroke mortality (2). It has been estimated that about half of these changes were due to the change in diet and that the reduction in salt intake played an important role in the fall in blood pressure and the reduction in strokes that occurred.
Both the Japanese and North Karelia experience suggest that where the right advice is given to the public and where processed foods are provided with less salt, it is relatively easy for the whole population to reduce their salt intake.
A well controlled migration study of a rural tribe in Kenya showed that on migration to an urban environment, there was an increase in salt intake and a reduction in potassium intake and blood pressure rose compared to those in a similar control group who remained in the rural environment (3). The increase in salt intake was an important factor in the increase in blood pressure and furthermore, the stress of the urban environment may have caused additional retention of sodium above that which would have occurred with the increase in salt intake.
(1) Iso H, Shimamoto T, Yokota K, et al. [Changes in 24-hour urinary excretion of sodium and potassium in a community-based heath education program on salt reduction]. Nippon Koshu Eisei Zasshi. 1999;46:894-903.
(2) Vartiainen E, Sarti C, Tuomilehto J, Kuulasmaa K. Do changes in cardiovascular risk factors explain changes in mortality from stroke in Finland ? Bmj. 1995;310:901-4.