Community-Based Education Classes for Hypertension Control
A 1.5-Year Randomized Controlled Trial
Abstract Community-based hypertension control is important for primary prevention of cardiovascular disease. In this study, untreated men and women aged 35 to 69 years were randomly assigned to an intervention (n=56) or control (n=55) group in a 1.5-year community-based education program. Subjects had no evidence of hypertensive end-organ defects and had screening blood pressures of 140 to 179 mm Hg systolic and/or 90 to 109 mm Hg diastolic, with no difference in mean blood pressure between groups (148 to 150 mm Hg for mean systolic and 83 to 84 mm Hg for mean diastolic pressures). The intervention group took four education classes in the first 6 months and four classes during the next year, and the control group took two classes. Health education focused on reduced dietary sodium and increased milk intake, brisk walking, and, if necessary, reduction of alcohol and sugar intakes. Antihypertensive medication was started less often in the intervention than in the control group at 1.5 years (9% versus 24%, P<.05). Mean systolic pressure was 5 to 6 mm Hg less in the intervention than in the control group at both 6 months and 1.5 years (P<.05), with or without inclusion of those subjects who began antihypertensive medication. Diastolic pressure and body mass index did not change significantly between groups. Urinary sodium excretion declined in the intervention but not in the control group (differences between groups: P=.04 at 6 months and P=.07 at 1.5 years). According to a behavioral questionnaire, sodium reduction and milk increase were greater in the intervention than the control group (sodium: P<.01 at 6 months and P=.08 at 1.5 years; milk: P<.001 at 6 months and P<.01 at 1.5 years). Mean ethanol intake was reduced in the intervention but not the control group (P=.04 at 1.5 years). This community-based hypertension control program was effective in reducing systolic pressure levels by nonpharmacological means during the first 6 months and maintaining the reduction for 1.5 years.
Hypertension is the major cause of stroke,1 the most common cardiovascular disease in Japan2 ; therefore, hypertension control is important for prevention of stroke. Evidence has accumulated from trials of patients in clinics or work site populations that less-severe hypertension can be controlled by nonpharmacological means, such as reduction of salt intake,3 4 alcohol intake,5 6 and weight.7 8 Reduction of salt intake is thought to be important because the average salt intake has been higher in Japan than in other developed countries.9 10
A national program of BP screening and health education for residents aged 40 years and older in communities has been conducted since 1983 to control hypertension and prevent cardiovascular disease.11 However, there are few trials in communities to test the feasibility and effectiveness of health education in controlling hypertension. The present trial examines whether a 1.5-year community-based education program improves lifestyle and reduces BP levels in middle-aged hypertensive individuals.
Screening for Participants
To identify hypertensive men and women aged 35 to 69 years without evidence of hypertensive end-organ defects, we held two screenings in Kyowa town (census population: N=17 217), a rural community in Ibaraki Prefecture, Japan. In this community, about 1000 hypertensive people were screened and registered between 1981 and 1989 under a community-based program for stroke prevention.12 Approximately 60% of hypertensive individuals were taking antihypertensive medication.12 For the present trial, a first screening was conducted in November 1990 to identify untreated people with SBP of 140 to 179 mm Hg and/or DBP of 90 to 109 mm Hg on the basis of the second of two measurements with a standard sphygmomanometer. We excluded people with major electrocardiographic abnormalities; history of stroke, coronary heart disease, or kidney diseases; or elevated serum creatinine (≥124 μmol/L).
The second screening was done 6 months later in May 1991 to identify men and women who remained untreated and had SBP ≥140 mm Hg and/or DBP ≥90 mm Hg. Three BP measurements were conducted with subjects in a sitting position, after a 5-minute rest, by a trained observer using a random-zero mercury manometer (Hawksley). BP measurements were taken at least 30 seconds apart. The average of the three measurements was used for eligibility. We repeated these screenings in November 1991 and May 1992 to recruit a second cohort. One hundred and eleven people were eligible and agreed to participate in the program. For program evaluation, BP measurement with the random-zero mercury manometer was conducted for both intervention and control groups 6 months and 1.5 years after randomization. Intervention or control class assignment was not known to the BP observer. The observer was trained for BP measurement according to the procedure of the Hypertension Detection and Follow-up Program13 before the baseline measurement and was recertified before the two follow-up measurements at 6 months and 1.5 years.
Group Assignment and Education Program
A 1.5-year education program was conducted between May 1991 and November 1992 for the first cohort and between May 1992 and November 1993 for the second cohort. A total of 111 men and women (70 in the first batch and 41 in the second batch) were invited to the program. After the first class in May, they were randomized into intervention or control groups by the permuted block method14 stratified by two categories of BP level at the second screening: one group of people with SBP in the range of 140 to 159 mm Hg and DBP in the range of 90 to 94 mm Hg, and another group with SBP ≥160 mm Hg or DBP ≥95 mm Hg. The proportion of hypertension type among the 111 participants was 72% for isolated systolic hypertension (SBP ≥140 mm Hg and DBP <90 mm Hg), 8% for isolated diastolic hypertension (SBP <140 mm Hg and DBP ≥90 mm Hg), and 20% for the combined type. Signed consent was not obtained from individuals because people in the rural community were not familiar with this procedure. Instead, at the first class, we explained to the participants that half of them would be assigned to have intensive education and the other half would receive usual education. Consent was implied by subsequent participation of almost all participants. This trial was conducted as part of an ongoing community-based program for stroke prevention in Kyowa.12 Implementation of the trial was approved by the program committee composed of the representatives of the municipal government, local physicians’ association, and University of Tsukuba.
Individual goals for the intervention were to reduce daily sodium intake and excretion to 137 mmol or less (8.0 g salt), walk briskly 30 minutes or more daily, control weight by losing 3 kg or more of body weight (applied only when BMI was ≥25.5 kg/m2 in men and ≥26.6 kg/m2 in women), and reduce alcohol intake to no more than 35 g ethanol per day. No individual goal for caloric intake was established.
A class was held for all participants in each year of recruitment shortly before randomization. For the intervention group, three more classes in the first 6 months and four classes in the next year were held, whereas one more class was held 8 months after randomization for the control group. Education classes were held at a public health center and two local community centers to which participants had easy access.
Classes were composed of a half-hour lecture by physicians on how to control hypertension for prevention of stroke and practical sessions directed by public health nurses and a nutritionist on how to reduce salt intake; increase intake of dairy foods, beans, and vegetables; reduce alcohol intake; and increase physical activity. Individual counseling was conducted at the end of each class. According to baseline urinary sodium excretion, ethanol intake, BMI, and responses to a lifestyle questionnaire, three individual goals were set up in counseling. Individual goals were translated into concrete messages, such as one or less bowl of miso soup, three drinks or fewer per day, two alcohol-free days a week, and a brisk walk 30 minutes or more a day. For the intervention group, the participation rates were high: 80% to 100% in the first three classes and 70% to 90% in the later classes. The participation rates were 100% in the first class and 60% in the second class for the control group. For the intervention group, a textbook was mailed to the nonparticipants in each class, and a phone call or home visit was conducted by a public health nurse to provide counseling and encourage them to participate in the next classes.
Measurement of Urinary Sodium, Potassium, and Creatinine
The participants in both intervention and control groups were asked to collect 24-hour urine samples with a 3-L plastic bottle at baseline, 6 months, and 1.5 years. Total urine volume and concentrations of sodium, potassium, and creatinine were examined. Urine aliquots were stored at −80°C for 1 month until analysis. Sodium and potassium were analyzed by an electrolyte analyzer (Hitachi). Creatinine was analyzed by the Jaffe alkaline picrate method with a spectrophotometer (Hitachi Autoanalyzer H-736-60E). The participants were asked to record the time of each excretion during the 24 hours, whether they collected urine properly at each void, and the number of voids missed. Incomplete urine samplings based on the record and/or creatinine excretion <3.5 or ≥22.1 mmol/d were excluded from the analyses.
Measurement of Alcohol Intake, Height, Weight, and BP-Related Lifestyles
Alcohol intake was obtained by interview as usual weekly intake of alcohol in go (a traditional Japanese unit of volume corresponding to 23 g ethanol) and converted to grams of ethanol per day. One go is 180 mL of sake and corresponds to one bottle (663 mL) of beer, two shots (75 mL) of whisky, or two glasses (180 mL) of wine.
Height with subjects in stocking feet and weight in light clothing were measured. BMI was calculated as weight (in kilograms) divided by the square of height (in meters). BMI of ≥25.5 kg/m2 for men and ≥26.6 kg/m2 for women was regarded as overweight. These BMI cut points correspond approximately to 120% of medians of body weight distributions by height and sex for 5086 healthy Japanese aged 20 years and older in 1960-1962.15
Selected lifestyles relating to sodium reduction, other dietary behaviors, and regular walking were queried with the use of a standard questionnaire (items listed in Table 1⇓). An individual score for sodium reduction was calculated by adding one point for each of 10 sodium reduction behaviors listed in Table 1⇓. We also asked the average frequency of milk intake (200-mL bottle as the unit): two or more bottles per day, a bottle per day, a bottle in 2 days, one or two bottles per week, and rarely. Individual milk intake (grams per day) was estimated with weights of 2.5, 1.0, 0.50, 0.21, and 0, respectively.
Mean values of BP and other continuous variables at baseline were compared by t test between the intervention and control groups. Proportions at baseline were tested by χ2 test. Changes in continuous variables from baseline to the two follow-up points were analyzed with paired t test to compare the intervention and control groups. Changes in categorical variables were examined with χ2 test for 2×2 tables, stratified by baseline status. One-tailed tests were used on the basis of an a priori hypothesis of direction of change.
To examine the relation between changes in lifestyle and BP levels among individuals, we examined mean values of changes in BP by categories of sodium excretion, potassium excretion, milk intake, ethanol intake, and BMI, each split at its median, by paired t test for intervention and control groups combined. For the analysis of lifestyle-BP associations, people who started antihypertensive medication (n=20) were excluded from the analysis. Individual values of sodium excretion, potassium excretion, milk intake, BMI, ethanol intake, and BP at 6 months and 1.5 years were averaged to reduce variability. We also used multiple linear regression analysis to examine associations of changes in sodium excretion, potassium excretion, milk intake, ethanol intake, and BMI with BP changes, controlling for baseline BP levels (with one-tailed tests). In the linear regression analysis, the interactions of sex by sodium excretion change for BP change and of baseline sodium excretion by milk intake change for BP change were also tested (with two-tailed t tests).
Table 2⇓ shows baseline variables for the intervention and control groups. There were no statistically significant differences in age and sex distribution or other baseline variables between the two groups.
Table 1⇑ shows sodium reduction behaviors and other lifestyle preferences and behaviors at baseline and the two follow-up points stratified by baseline preference or behavior. There were no statistically significant differences between the intervention and control groups at baseline. During follow-up, there were only small, statistically nonsignificant differences in preference for less-salty food, use of less-salty seasoning, and reduced consumption of salt-preserved food as well as trying to reduce salt intake, increasing vegetable consumption, and avoiding sweets. Infrequent consumption of miso soup and addition of soy sauce to pickles tended to be maintained at both 6 months and 1.5 years in those subjects who had reduced consumption at baseline. Several sodium reduction behaviors increased at both 6 months and 1.5 years among those not performing the behaviors at baseline: eating salt-preserved pickles no more than once a day, eating less-salty pickles, not putting soy sauce on dishes, and not eating salty noodle soup. Milk drinking and consumption of soybean products (other than miso soup and soy sauce) increased substantially among those who were infrequent consumers at baseline. The habit of taking daily brisk walks also increased among those who did not have this habit at baseline.
Mean urinary sodium excretion declined 12% in the intervention group and increased 5% in the control group at 6 months (Table 3⇓); the group difference was statistically significant. The percent decline in the intervention group was 5% and the percent increase in the control group was 5% at 1.5 years, with borderline significance in the group difference. Mean urinary potassium output did not differ significantly between the two groups at 6 months or 1.5 years. The sodium-potassium ratio declined in the intervention group and did not change in the control group; the group difference was of borderline significance at 6 months but not at 1.5 years. Mean sodium reduction score increased in the intervention group, and there was a small increase in the control group. The score difference between the two groups was significant at 6 months and of borderline significance at 1.5 years. Mean calcium intake from milk increased in the intervention group but not in the control group; the group difference was significant at both 6 months and 1.5 years. Mean BMI did not differ significantly between the two groups at either time. Mean ethanol intake declined in the intervention group and decreased only a little in the control group. The difference in mean ethanol intake between the two groups was not statistically significant at 6 months but was significant at 1.5 years.
Mean SBP declined in the intervention group but not in the control group at 6 months, and the BP difference between the two groups was statistically significant (Table 4⇓). Mean SBP declined in both groups, but the group difference was still significant at 1.5 years. Mean DBP levels declined in both groups, and no group difference was found 6 months or 1.5 years after baseline.
Because some of the participants were taking antihypertensive medication during the program, BP changes were stratified by antihypertensive medication use at 6 months and 1.5 years (Table 5⇓). The results stratified by medication use at 6 months were essentially the same as the results for all subjects because there were only 5 people (2 intervention and 3 control subjects) who started medication use. Intervention subjects were less likely than control subjects to begin medication use at 1.5 years (9% versus 24%, P=.02). When stratified by medication use at 1.5 years, a significant difference in mean SBP was found between the intervention and control groups for both no-medication and medication subgroups.
To examine further the relation between lifestyle changes and SBP changes among individuals, we compared changes in BP levels between two subgroups split by the median change of sodium excretion, potassium excretion, milk intake, ethanol intake, and BMI (Table 6⇓). Median sodium excretion change was 0 mmol/d. Mean SBP decline was 2.8 mm Hg greater in the half with reduced sodium excretion than in the complementary half (difference in BP change: P=.10). Median potassium excretion change was 3.0 mmol/d. Mean SBP decline was 1.5 mm Hg larger in the half with increased potassium excretion than in the complementary half (P=.25). Median milk change was 0 g/d. Mean SBP decline was 1.8 mm Hg greater in the half with increased milk intake than in the complementary half (P=.20). Median alcohol change was 0 g/d among the 43 drinkers. Mean SBP decline was 6.3 mm Hg greater in the half reducing alcohol intake than in the complementary half (P=.02). Associations of individual changes in sodium excretion, potassium excretion, milk intake, and alcohol intake with DBP changes were weak and not statistically significant. Median BMI change was 0 kg/m2. Mean SBP declined 3.6 mm Hg (P=.05) and DBP declined 2.4 mm Hg (P=.03) more in the half with decreased BMI than in the complementary half.
Intercorrelations between lifestyle change variables, ie, changes of sodium excretion, potassium excretion, milk intake, alcohol intake, and BMI, were generally weak and insignificant except for the correlations between changes in sodium and potassium excretions (r=.54, P<.001) and between changes in alcohol intake and BMI (r=.30, P<.01). According to a multiple regression analysis for the drinkers and nondrinkers combined, a positive relation with SBP change was found for sodium excretion change (P=.06) and BMI change (P=.04), and an inverse relation with SBP change was found for potassium excretion change (P=.08) and milk intake change (P=.15). A positive relation with DBP changes was found for BMI change (P=.07) but not for other covariates. A decrease of 22.7 mmol/d sodium excretion, an increase of 9.1 mmol/d potassium excretion, a decrease of 0.44 kg/m2 BMI, and an increase of 96 mL milk (2.4 mmol calcium) were each associated with a decrease of 1.0 mm Hg SBP. A decrease of 0.93 kg/m2 BMI was associated with a change of 1.0 mm Hg DBP. In an analysis of drinkers and nondrinkers combined, alcohol intake was unrelated to SBP (P=.25) or DBP (P=.43). Regression analyses were repeated for the 43 drinkers at baseline; a decrease of 4 g/d ethanol was significantly associated with a decrease of 1.0 mm Hg SBP (P=.02) but not with a change of DBP (P=.44). The association between sodium excretion change and SBP change did not vary significantly by sex (P=.78 for the interaction term). The association between milk intake change and SBP change did not vary significantly by sodium excretion at baseline (P=.33).
The present trial indicated that intensive education during the first 6 months with maintenance intervention for the next year was effective in reducing SBP level for 1.5 years. A larger reduction of average sodium excretion and alcohol intake was found in the intervention group than in the control group. An increase of average milk intake was observed in the intervention group but not in the control group. Moreover, individual SBP reduction was associated with individual reduction of alcohol intake and sodium excretion and with increased milk intake; probability values in multiple regression, adjusting each lifestyle factor for the others, ranged between .02 and .15. Thus, SBP decline was in part due to reduction of sodium intake, reduction of alcohol intake, and possibly increase of calcium intake. Several trials in clinical and occupational settings have demonstrated effects of nonpharmacological intervention on reduction of BP3 4 7 or primary prevention of hypertension.16 17 To the best of our knowledge, the present study is the first community-based trial to show the effectiveness of nutritional intervention for BP reduction. In Japanese rural communities, adult classes have proved to be an effective strategy in health education because people are eager to attend classes if the program is attractive.
Antihypertensive medication use 1.5 years after baseline was more common in the control group than in the intervention group. The initiation of medication use was at the discretion of personal physicians, who likely responded to the higher BP values in the control group by prescribing medication. It is possible that people in the intervention group were told more frequently that BP was controllable by nutritional means, and they may have attended education classes rather than seeing their personal physicians as often. As a result, intervention group participants would have been less likely to be prescribed medication. A subgroup analysis by medication use indicated that both no-medication and medication subgroups showed a larger reduction of SBP in the intervention group than in the control group. All these factors suggest a continuing effect of intervention 1.5 years after baseline.
Cutler et al18 reviewed 23 randomized clinical trials on sodium reduction and BP and showed that a 17.5 mmol/d sodium reduction contributed to an average decline of 1.0 mm Hg SBP for hypertensive subjects. Law et al19 reviewed 78 trials and estimated that a 10 mmol/d sodium reduction would result in approximately 1.0 mm Hg decline in SBP; this estimate may be too large because it included 68 crossover trials, many with nonrandom allocation and/or comparing salt-sensitive and nonsensitive patients. In the present trial, an individual reduction of 22.7 mmol/d sodium was estimated to correspond to a 1.0 mm Hg decline in SBP level. Because the mean difference in sodium excretion between the intervention and control groups was 27.0 mmol/d at 6 months and 15.6 mmol/d after 1.5 years, the contribution to SBP reduction associated with reduced sodium intake was estimated to be 1.5 mm Hg at 6 months and 0.9 mm Hg at 1.5 years according to the Cutler review data and 1.2 and 0.7 mm Hg, respectively, according to the present trial data. The actual difference in mean SBP was 5.6 mm Hg at 6 months and 4.7 mm Hg at 1.5 years, so one fifth to one fourth of the differences could be explained by sodium reduction.
Other factors related to a greater reduction of SBP in the intervention group than in the control group included reduction of alcohol intake and increase of milk intake. A reduction of approximately 10 g/d ethanol leads to a 1.0 mm Hg decline in mean SBP according to clinical trials.5 6 Among the drinkers in the present trial, an individual decrease of 4.0 g/d ethanol was associated with a 1.0 mm Hg SBP decline. Because the difference in ethanol intake between the intervention and control groups was 2.7 g/d at 6 months and 5.7 g/d at 1.5 years, the contribution to SBP reduction associated with reduced ethanol intake was estimated to be 0.3 mm Hg at 6 months and 0.7 mm Hg at 1.5 years according to the previous trial data5 6 and 0.7 and 1.4 mm Hg, respectively, according to the present trial data.
Clinical trials suggest that calcium supplementation reduces BP levels.20 21 Our cross-sectional study in seven Japanese populations suggested that a 4.6 mmol/d higher intake of total calcium was associated with a 1.0 mm Hg lower SBP and that a 2.2 mmol/d higher intake of dairy calcium was associated with a 1.0 mm Hg lower SBP.22 In the present trial, an individual increase of 2.4 mmol/d (96 mg/d) calcium intake from milk was associated with a 1.0 mm Hg decline in SBP level. Mean calcium intake from milk was about 1.0 to 2.0 mmol/d higher in the intervention group than in the control group. If a hypotensive effect of dietary calcium is accepted, the contribution to SBP reduction associated with an increased intake of dairy calcium was estimated to be 0.8 to 0.9 mm Hg at 6 months and 0.4 to 0.5 mm Hg at 1.5 years according to both cross-sectional data and the present trial data.22 These estimations suggest that another one fourth to one third of the difference in SBP level between the intervention and control groups was attributable to reduced ethanol intake and increased calcium intake.
We found no significant change in mean BMI in either the intervention or control groups, unlike previous trials in Caucasians.7 8 Nevertheless, individual changes in BMI were positively associated with changes in both SBP and DBP levels. A probable reason for no intervention effect on weight reduction in the present study was that only about 25% of our participants were deemed overweight, even with the use of a BMI cut point for overweight less than the average BMI in Americans.23 Our intervention staff were not as enthusiastic about controlling “overweight” as about reducing sodium and alcohol intakes and increasing calcium intake.
This study examined a program that combined a variety of educational interventions to reduce BP levels; because each component was successfully delivered, the study has little power to examine the effectiveness of individual educational components. Each class was led by a physician and followed by individual counseling. Class attendance was excellent in the intervention group: 59% of the participants attended the classes 7 or 8 times, 30% attended 5 or 6 times, and only 11% attended 3 or 4 times. In linear regression analyses controlling for baseline BP levels, the number of classes attended was not associated with either SBP (P=.97, two-tailed) or DBP (P=.33) change. We believe that the use of textbooks and phone call follow-up contributed to the high class attendance rate.
The participants in the present trial were “healthy” and active people recruited by community-based screening, and the education classes were conducted at public places in the community in the context of a community-wide effort to prevent stroke. The results in our trial are likely to be generalizable to middle-aged Japanese men and women in other communities. Since 1983, every municipal government in Japan has been required to conduct health screenings and education for residents aged 40 years and older to control hypertension and prevent cardiovascular disease. This trial showed that nonpharmacological intervention in classes connected to screening is effective in the control of hypertension in Japanese communities.
Selected Abbreviations and Acronyms
|BMI||=||body mass index|
|DBP||=||diastolic blood pressure|
|SBP||=||systolic blood pressure|
This study was supported in part by the Ministry of Health and Welfare, Japan, and Tsukuba Research Project. The authors thank Michiko Harada, Yoko Wakabayashi, Mieko Inagawa, and Mikiko Ohki for their assistance in conducting education classes.
Reprint requests to Hiroyasu Iso, Institute of Community Medicine, University of Tsukuba, 1-1-1, Tennodai, Tsukuba-shi, Ibaraki-ken 305, Japan.
- Received June 7, 1995.
- Revision received July 19, 1995.
- Accepted December 21, 1995.
Tanaka H, Ueda Y, Hayashi M, Date C, Baba T, Yamashita H, Shoji H, Tanaka Y, Owada K, Detels R. Risk factors for cerebral hemorrhage and cerebral infarction in a Japanese rural community. Stroke. 1982;13:62-73.
Health and Welfare Association. Trends in national hygiene [in Japanese]. Kosei no Shihyo. 1994;41(suppl):402-403.
Ueshima H, Mikawa K, Baba S, Sasaki S, Ozawa H, Tsushima M, Kawaguchi A, Omae T, Katayama Y, Kayamori Y, Ito K. Effect of reduced alcohol consumption on blood pressure in untreated hypertensive men. Hypertension. 1993;21:248-252.
Reisin E, Abel R, Modan M, Silverberg DS, Eliahou HE, Mondan B. Effect of weight loss without salt restriction on the reduction of blood pressure in overweight hypertensive patients. N Engl J Med. 1978;298:1-6.
MacMahon S, Cutler J, Brittain E, Higgins M. Obesity and hypertension: epidemiological and clinical issues. Eur Heart J. 1987;8(suppl B):57-70.
Dahl L. Possible role of salt intake in the development of hypertension. In: Cottier P, Bock KD, eds. Essential Hypertension: An International Symposium. Berlin, FRG: Springer-Verlag; 1960:53-65.
INTERSALT Cooperative Research Group. INTERSALT: an international study of electrolyte excretion and blood pressure: results for 24 hour urinary sodium and potassium excretion. BMJ. 1988;297:319-329.
The Ministry of Health and Welfare. Manual for Screening Under an Act on Health and Medical Care. Tokyo, Japan: Japan Public Health Association; 1987.
Iso H, Yokota K, Shimamoto T, Sankai T, Miyagaki T, Fukuuchi K, Kitamura A, Sato S, Harada T, Wakabayashi Y, Ohtani K, Inagawa M, Ohki K, Komachi Y. Effectiveness of a community-based education program on blood pressure reduction for cardiovascular disease prevention [in Japanese, English abstract]. Jpn J Public Health. 1993;40:147-158.
Pocock SJ. Clinical Trials: A Practical Approach. New York, NY: John Wiley & Sons; 1987:66-89.
Minowa S. Study on standard weights for Japanese adults: charts of Minowa’s relative weight index [in Japanese]. Jpn J Med. 1962;1988:24-28.
Cutler JA, Follmann D, Elliot P, Suh I. An overview of randomized trials of sodium reduction and blood pressure. Hypertension. 1991;17(suppl I):I-27-I-33.
Law MR, Frost CD, Wald NJ. By how much does dietary salt reduction lower blood pressure? III: analysis of data from trials of salt reduction. BMJ. 1991;302:819-824.
Cutler JA, Brittain E. Calcium and blood pressure: an epidemiologic perspective. Am J Hypertens. 1990;3(suppl 2):137S-146S.
McCarron DA. Is calcium more important than sodium in the pathogenesis of essential hypertension? Hypertension. 1985;7:607-627.
Iso H, Terao A, Kitamura A, Sato S, Naito Y, Kiyama M, Tanigaki M, Konishi M, Shimamoto T, Komachi Y. Calcium intake and blood pressure in seven Japanese populations. Am J Epidemiol. 1991;425:776-782.