Cardiovascular disease is a major cause of morbidity and mortality in East Asian countries as well as in western countries.1-3 However, it is a characteristic specific to East Asia that stroke is more prevalent than coronary heart disease (CHD). Strokes cause disability in around 20% of cases.4 A Japanese cohort study showed that 50% of cases of bedridden elderly men 65 years of age and older and 25% of cases of elderly women were caused by stroke.5 Therefore, the prevention of stroke is an important issue among elderly people in East Asian countries.
Japan now has the highest longevity in the world,6 a position it has held since the mid-1980s after overcoming the highest stroke mortality rate in the world in 19652,7 and also preventing an increase in coronary artery disease (CAD).1-3 This article will address how this has been accomplished.8
Trends in Stroke and Heart Disease Mortality in Japan
The age-adjusted all-stroke mortality rate in Japan increased after World War Two until 1965 and then showed a significant decline until 1990.2 In fact, an approximate 80% reduction in the age-adjusted all-stroke mortality rate occurred between 1965 and 1990.7 The age-adjusted mortality rate from all heart disease and CHD increased until 1970 and then gradually declined.1-3,7-9 Even in 1970, the age-adjusted mortality rate from all heart disease, CHD and acute myocardial infarction (AMI) was far lower than that from stroke.1,2,7,8 The age-adjusted all-stroke mortality rate in Japan in 1965 was the highest in the world,1,2 whereas the age-adjusted CHD and/or AMI mortality rate around 1970 was one of the lowest among industrialised countries, and as low as in some Mediterranean countries.3,10
The high stroke rate and low CHD mortality rate is a specific feature of Japan and other East Asian countries among industrialised countries and it continues to the present day,10 although the stroke mortality rate was greatly reduced between 1965 and 2000.
Trends in the Incidence of Stroke and Myocardial Infarction
The incidence of stroke, either thrombotic or haemorrhagic, has declined in correlation with the stroke mortality rate.11-17 Epidemiological studies have revealed that more than half the reduction in the stroke mortality rate can be explained by the decline in the incidence of stroke.11-17 It is also true that the reduction in the CHD mortality rate since 1970 is responsible for the decline in the incidence of AMI.11-18
Risk Factors for Stroke and Coronary Heart Disease in Japan
The most important risk factor for stroke, whether cerebral haemorrhage or cerebral infarction, is high blood pressure,19-23 although hypertension is more specific to cerebral haemorrhage than to cerebral infarction. The higher the blood pressure, the higher the risk ratio. There is no threshold between blood pressure and stroke occurrence,20-22 and this holds true for both the young and the old.23
In the past, smoking was not found to be a risk factor for stroke.24,25 However, recent large cohort studies in Japan such as NIPPON DATA80 show a clear graded relationship between smoking and stroke,26-28 as has also been found in western countries.29 One explanation for this relationship is that the magnitude of hypertension as a strong risk factor weakened due to the decline in the population blood pressure level.2,3,7 Since the smoking rate for Japanese men is still around 40% in spite of a substantial reduction, the population smoking risk contributable to stroke in men is around 30%; this means that 30% of strokes in men could be prevented by smoking cessation.19 In addition, based on Japanese cohort studies it is estimated that a 1% reduction in the smoking rate in Japan would result in about a 1.3% reduction in the incidence of stroke.19 Therefore, it is reasonable to conclude that the recent reduction in the smoking rate in men contributed to the decline in stroke mortality and incidence.
Serum total cholesterol is not a risk factor for stroke, whether cerebral haemorrhage or cerebral infarction, in Japan27,30-33 because most incidents of cerebral infarction in Japan are caused by hypertension and smoking.13,19-28,30 Although atherosclerosis of large vessels in the brain is caused by hypercholesterolaemia, the proportion of strokes caused by hypercholesterolaemia is quite low in Japan.30
The most important risk factors for CHD are hypertension, hypercholesterolaemia, smoking and diabetes.19,27 These findings are no different from those in the US and Europe.34
Possible Explanations for the Reduction in the Incidence of Stroke and Coronary Heart Disease
Hypertension and smoking are potent risk factors for CHD and stroke.13,19-34 The higher the population blood pressure, the higher the risk of CHD and stroke.19-22,27 As estimated in Health Japan 21, if the population SBP was lowered by 2mmHg, the estimated reduction in the incidence of CHD and stroke would be 4.8 and 6.4%, respectively, based on Japanese cohort studies.19 Similarly, it is estimated that a 1% reduction in the smoking rate would result in a 1.3% decrease in the incidence of CHD and stroke.19
Therefore, it is reasonable to conclude that a reduction in the population blood pressure level and a substantial reduction in the prevalence of severe hypertension contributed greatly to the decline in the CHD mortality rate and the incidence of stroke.2,3,7 Since the average blood pressure reduction in Japanese men 30-69 years of age was around 7.4mmHg and there was an approximate 20% smoking rate reduction between 1965 and 1990, it would be expected that the reduction in CHD and stroke mortality would be 44 and 50%, respectively; however, the actual reduction was 51 and 79%, respectively. Therefore, more than 80% of the observed reduction in CHD mortality for men 30-69 years of age can be explained by the decrease in the population blood pressure level and the reduced smoking rate, while 63% of the reduction in stroke mortality can be explained by the same factors.
Application of the Japanese Experience to East Asian Countries
The experience of Japan is applicable to most East Asian countries because in the latter the stroke mortality and morbidity rates are high, as are the leading causes of cardiovascular diseases.10 Blood pressure level and smoking rate are generally high in East Asian countries, as they used to be in Japan.35 Therefore, measures for lowering population blood pressure, treating hypertension and promoting smoking cessation are key to reducing the stroke burden in the elderly.36
It is well known that salt consumption is positively correlated with population blood pressure level and is a causative factor for hypertension. INTERSALT, an international study on salt and blood pressure, found that salt consumption was higher in China, Korea and Japan in the 1980s than in other countries.36 However, it is noteworthy that the Japanese people reduced their daily salt consumption by at least 10-15g between the 1950s and the 1980s.2,8,37
The INTERSALT study estimated a blood pressure rise with age based on the results of blood pressure and 24-hour urine collection of over 10,000 people in 32 countries.36 Based on this finding, it is estimated that SBP has been reduced by 15mmHg compared with 30 years ago. This estimated value is similar to that observed in Japan.
It is also estimated, based on Japanese cohort studies, that a 1% reduction in the smoking rate causes a 1.3% fall in stroke morbidity and mortality.3 Therefore, it is important to reduce the smoking rate in East Asian countries, especially in men. The 20% reduction in the smoking rate in Japanese men between the 1960s and the 1990s is estimated to have led to a 26% reduction in the incidence of stroke.3
These two measures are applicable to and feasible for other East Asian countries. It should be noted that it is easier to carry out antismoking campaigns than to reduce the population's salt intake.
Lowering blood pressure by reducing salt intake, promoting smoking cessation and preventing diabetes are key strategies for preventing stroke and disability in the elderly. Between the 1950s and the 1980s, Japanese people reduced their daily salt consumption by 10-15g, and among men the smoking rate fell by 20%. This experience is applicable for reducing the stroke burden in the elderly population of East Asian countries.