2022 Volume 21 Issue 2 Pages 64-81
In recent years, lowering the sodium-to-potassium ratio (Na/K ratio) in the diet has been linked to an antihypertensive effect. However, very few studies have identified regular dietary patterns with a low Na/K ratio. Therefore, the purpose of this study was to identify the dietary patterns of adult men and women and to clarify the relationship between these patterns and the Na/K ratio in the diet. The study included 639 (240 men and 399 women) subjects aged 20 years or older from the 2015 Niigata prefectural health and nutrition survey. Dietary patterns were extracted from 24 food groups by principal component analysis. To compare the characteristics of each dietary pattern, the principal component scores were divided into tertiles, and analysis of covariance was conducted on the intake of nutrients and foods, with sex, age, and energy as adjustment variables. To examine the relationship between dietary patterns and the dietary Na/K ratio, we conducted multiple regression analysis using the Na/K ratio as the dependent variable and a total of 16 items (10 basic attributes and six principal component scores, for each dietary pattern) as independent variables for each sex. Six dietary patterns were identified by principal component analysis. In both men and women, subjects with higher scores for the “vegetable/rice,” “milk/bread/fruit,” and “legumes” patterns had a significantly decreased Na/K ratio, and subjects with a higher score for the “noodles” pattern had a significantly increased Na/K ratio. In women, subjects with higher scores for the “meat/fat” and “fish and seafood” patterns had significantly increased Na/K ratio. It is suggested that the dietary patterns recommended to lower the Na/K ratio for hypertension prevention are the “vegetable/rice,” “milk/bread/fruit,” and “legumes” patterns.
In Japan, hypertension is the greatest risk factor for cerebrovascular disease and ischemic heart disease [1]; cerebrovascular disease is the second leading cause of certification for requiring nursing care [2]. Therefore, prevention of hypertension is important to extend a healthy life span. It has been reported that excessive salt (sodium) intake and insufficient potassium intake are associated with increased blood pressure [3–5]. In recent years, the ratio of sodium to potassium (Na/K ratio) has gained attention as an indicator of salt (sodium) and potassium intake. Several intervention studies have shown that lowering the Na/K ratio has a greater antihypertensive effect than reducing sodium intake or increasing potassium intake alone [6], and a recent systematic review has also reported that lowering the Na/K ratio has an antihypertensive effect [7]. Furthermore, a large cohort study in Japan showed that a low dietary Na/K ratio was associated with a lower mortality rate from cerebrovascular and cardiovascular diseases, which are thought to be caused by hypertension [8].
The World Health Organization (WHO) recommends a sodium intake of less than 2,000 mg/day and a potassium intake of over 3,500 mg/day [9, 10]. Dietary Reference Intakes for Japanese [11] also requires a salt (equivalent to sodium chloride) intake of less than 7.5 g/day for men and less than 6.5 g/day for women and a potassium intake of over 3,000 mg/day for men and over 2,600 mg/day for women. However, the average salt intake of Japanese people aged 20 years and older in 2019 was 11.0 g/day for men and 9.3 g/day for women, with an average potassium intake of 2,454 mg/day for men and 2,282 mg/day for women [12]. This is far from the targets set by the WHO and Japanese dietary guidelines. Health Japan 21 (second stage) [13], which started in 2013, sets targets for the improvement of hypertension, nutrition, and dietary habits; the targets include reduced salt intake and increased intake of vegetables and fruits rich in potassium.
In Niigata Prefecture, the mortality rate from cerebrovascular diseases as well as the salt intake are higher than the national average. Thus, since 2009, the prefecture has implemented a 10-year plan known as the “Niigata Salt Reduction Renaissance Movement” to reduce salt intake. During this period, the salt intake of Niigata residents decreased, although potassium intake, which was targeted along with salt reduction, did not increase, but instead decreased [14, 15]. The mortality rate for cerebrovascular disease in Niigata Prefecture has been on a downward trend for the past 10 years, similar to the national rate, although it is still high, ranking fourth to eighth worst in Japan [16]. The average salt intake of people aged 20 years and older in Niigata Prefecture in 2019 was 11.0 g/day for men and 9.7 g/day for women, whereas the average potassium intake was 2,509 mg/day for men and 2,366 mg/day for women—both of which were higher than the national average [12, 17]. However, potassium intake remains below the levels recommended by the Dietary Reference Intakes for Japanese [11]. The next challenge is to reduce the Na/K ratio in the diet by reducing salt and increasing potassium intake.
In Niigata Prefecture, as in many other prefectures, a health and nutrition survey is routinely conducted to obtain basic data for nutritional policies, and a dietary assessment is conducted based on the intake of nutrients and different foods. However, since people consume a variety of nutrients and foods as meals in their daily lives, many studies on dietary patterns that comprehensively capture a combination of these nutrients and foods have been reported in Japan and abroad. There have been several reports on dietary patterns and cardiovascular diseases in Japanese individuals [18–23]. By contrast, there has been only one report on the association between dietary patterns and the urinary Na/K ratio [24], although the subjects were employees of a medical welfare facility and not representative of the general population. At present, no studies have examined the relationship between dietary patterns and the dietary Na/K ratio in the general population. Therefore, both residents and health workers are largely unaware of the dietary patterns eliciting a low Na/K ratio, which could be useful for preventing hypertension.
Health Japan 21 (second stage) [13] set a national goal of reducing salt intake and increasing the intake of vegetables and fruits rich in potassium. However, it is essential for each prefecture to create its own measures based on its dietary habits. Prefectures, which are focusing on changing dietary patterns for disease management, such as Niigata Prefecture (which is focusing on salt reduction as a countermeasure against cerebrovascular diseases), can provide health and nutrition survey data for examining the relationship between dietary patterns and the Na/K ratio. This would help to solve national nutrition issues.
In this study, we analyzed the relationship between dietary patterns and the dietary Na/K ratios of adult men and women and identified dietary patterns with low Na/K ratios that are useful for preventing hypertension, using Niigata Prefecture's population as an example. This study also aimed to obtain basic data for examining nutritional measures for the prevention of hypertension in Niigata Prefecture.
1. Target of the survey
Data from the 2015 Niigata prefectural health and nutrition survey were used. This survey is conducted every three to four years by Niigata Prefecture with the aim of obtaining basic data for the comprehensive promotion of the health of the prefecture's residents. The survey targets a sample of approximately 900 households in Niigata Prefecture, according to the prefectural health and nutrition survey manual [25]. To avoid regional bias, each public health center sets the survey area according to the population in its jurisdiction. In fiscal year 2015, there were 881 household members in 23 districts in the prefecture and 2,503 persons aged one year or older. The cooperation rate for the nutrition intake status survey was 61.5% (1,540/2,503), and 1,075 participants aged 20 years and over (excluding those who were subject to the national health and nutrition survey) were included in this study. Of those included in the study, 426 subjects had deficiencies in items of the nutrition intake status survey, physical status survey, and lifestyle survey; four had energy intake of more than 4,000 kcal/day or less than 600 kcal/day based on previous studies [26], and six were deleted listwise using the outlier test for the Na/K ratios. Thus, 639 subjects (230 men and 399 women) were included in the analysis.
2. Survey items
In this study, we used items from the nutrition intake status survey, physical status survey, and lifestyle survey. The nutrition intake survey was conducted by a representative of the household or a person in charge of meal preparation for one day (excluding Saturday and Sunday) in November, using the meal recording method (weighing method or reference amount method). The physical condition survey was conducted by a physician, a dietitian, and a public health nurse at the measurement site. The items of the nutrition intake status survey and physical status survey were in the same format as in the national health and nutrition survey. In the lifestyle survey, a questionnaire survey was conducted using a placement method to ascertain overall lifestyle habits.
(1) Nutrition intake survey
The energy and nutrient intakes calculated from the daily food intake status and food weight by food group were based on the data compiled by “Dietary Shirabe 2015” used in the national health and nutrition survey. Nutrients were defined as protein energy ratio, fat energy ratio, saturated fatty acid energy ratio, dietary fiber, salt equivalent, sodium, and potassium, which are set as target amounts among the indicators in the Dietary Reference Intakes for Japanese [11]. For the food intake status, all food categories were used (major and medium categories in the appendix of the national health and nutrition examination survey food groups).
(2) Physical condition survey
Height, weight (body mass index [BMI] calculation), blood pressure (systolic and diastolic), exercise habits (the number of days of exercise in a week, average number of hours exercised per day, and exercise intensity), and medication status (blood pressure, dyslipidemia, and diabetes) were used.
(3) Lifestyle survey
Items related to smoking and drinking habits were used in the survey. The smoking-related survey questions were “Have you ever smoked cigarettes habitually?” and “Do you currently (in the past month) smoke cigarettes?” The survey items to assess drinking-related habits were “How many days a week do you drink alcohol?” and “On the days you drink alcohol, how much alcohol do you drink in one day?”
3. Statistical analyses
The basic attributes of the subjects and their food intake by food group are presented as descriptive statistics. Next, principal component analysis was used to examine dietary patterns. The food groups were based on the major categories in the table of food groups in the national health and nutrition examination survey, although the middle category was used for foods considered to have an effect on salt intake, referring to previous studies [27]. Specifically, cereals were classified into “rice,” “bread,” “noodles,” and “other cereals;” vegetables into “green and yellow vegetables,” “other vegetables,” and “salted or pickled vegetables;” fish and shellfish into “fish and seafood” and “processed fish;” and beverages into “alcoholic beverages” and “other beverages.” Principal component analysis was conducted using the food group-specific intakes of all 24 items as variables, and dietary patterns with eigenvalues greater than 1, for which the principal component loadings by food group could be interpreted, were identified. To clarify the characteristics of each dietary pattern with reference to previous studies [28], the principal component scores in each dietary pattern were divided into three quartiles (low, medium, and high scores) and compared with respect to energy and major nutrients, as well as with respect to food and to the intake of vegetables and fruits, which are the target values of Health Japan 21 (second stage) [13]. For comparison of characteristics among the three groups, χ2 test was used for sex, one-way analysis of variance for age, and analysis of covariance for sex and age as adjusted variables for energy intake and energy ratio of protein, fat, saturated fatty acid, and Na/K ratio; analysis of covariance for sex, age, and energy as adjusted variables for intake by nutrient and food group; and the Bonferroni test was used for multiple comparisons. The Na/K ratio in the diet was calculated by dividing the sodium intake (mg) and potassium intake (mg) by the Na atomic weight (23) and K atomic weight (39.1), respectively.
Multiple regression analysis were performed for each sex, with the dependent variable being the Na/K ratio in the diet and the independent variables being 10 basic attributes mainly related to blood pressure (age, BMI, systolic blood pressure, diastolic blood pressure, presence of blood pressure medication, presence of diabetes medication, presence of cholesterol medication, presence of exercise habit, presence of drinking habit, and presence of smoking habit) and six principal component scores for each dietary pattern. The variables were selected using the multiplicative method, and the input criterion was set at Pin = Pout = 0.15. The independent variables were age, BMI, systolic blood pressure, and diastolic blood pressure measurements; whether medication was being taken to lower blood pressure (0: no, 1: yes), blood sugar (0: no, 1: yes), or cholesterol (0: no, 1: yes); whether the subject was exercising at least 30 min consecutively at least twice a week for at least one year (0: yes, 1: no); whether the patient ever smoked and still smokes (0: no, 1: yes); and consumption of ≧ 20 g of pure alcohol per serving at least three times a week (0: no, 1: yes).
For statistical analyses, Excel Statistics 2016 Ver. 3.21 from BellCurve was used. The significance level was set at a risk rate of less than 5%.
4. Ethical considerations
This study analyzed data from the 2015 prefectural health and nutrition survey owned by the health policy division, welfare and health department, Niigata Prefecture, with the aim of utilizing the data for nutritional policy in Niigata Prefecture. The request for the data was approved in September 2020. In April 2021, approval was obtained from the ethics committee of Niigata University of Health and Welfare (Approval No. 18601-210405).
1. Attributes of the subjects
Basic attributes are shown in Table 1. The total number of subjects analyzed was 639 (240 men and 399 women). The age structure of the studied population was as follows 13.0% in the 20–30 years age group, 25.3% in the 40–50 years age group, and 61.7% in the 60 years and older age group. The percentage of the 60 years and older age group was approximately 13 percentage points higher than the national value [29]. The intake by food groups is shown in Table 2. Compared with the national value [29], the average intake was higher for rice, vegetables, and seafood and lower for meat.
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2. Classification by dietary pattern and basic attributes
Six dietary patterns were identified by principal component analysis. The principal component loadings by food group for each dietary pattern are shown in Table 3. The names of the food groups with principal component loadings of 0.4 or higher were used for each dietary pattern.
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The first dietary pattern was named “vegetable/rice;” the second dietary pattern, “milk/bread/fruit;” the third dietary pattern, “meat/fat;” the fourth dietary pattern, “noodles;” the fifth dietary pattern, “fish and seafood;” and the sixth dietary pattern, “legumes.” The first, second, and fourth dietary patterns are characterized by staple foods, whereas the third, fifth, and sixth dietary patterns are characterized by main dishes. The contribution rate of each dietary pattern was 8.6%, 7.3%, 6.9%, 5.9%, 5.2%, and 4.7% for the first, second, third, fourth, fifth, and sixth dietary patterns, respectively, resulting in a total cumulative contribution rate of 38.6%.
The characteristics of tertiles of scores by dietary pattern are shown in Table 4. The proportion of men in the high-score group was high for the “vegetable/rice” and “fish and seafood” patterns, whereas the proportion of women in the highscore group was high for the “bread/milk/fruit” pattern. In terms of age, subjects in the high-scoring group for the “vegetable/rice” and “milk/bread/fruit” patterns were significantly older than those in the low-scoring group (p<0.001), and subjects in the high-scoring group for the “meat/fat” pattern were significantly younger than those in the low-scoring group (p<0.001). There was no significant difference between the three groups in terms of sex or age for the “noodles” and “legumes” patterns.
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3. Dietary patterns and nutrient intake
Table 5 shows the energy and nutrient intakes of the three groups according to their dietary patterns. In the “vegetable/rice” pattern, sodium and potassium intakes were significantly higher in the high-score group than in the low-score group (p<0.001), and the Na/K ratio was significantly lower (p<0.001). The intake of vegetables and fruits was also significantly higher in the highscore group than in the low-score group (p<0.001 and p<0.01, respectively). In the “milk/bread/fruit” pattern, sodium intake was not significantly different, although potassium intake was significantly higher (p<0.001), and the Na/K ratio was significantly lower (p<0.001) in the high-score group than in the low-score group. Fruit intake was significantly higher in the high-score group than in the low-score group (p<0.001), although there was no significant difference in vegetable intake. In the “meat/fat” pattern, sodium intake was not significantly different among the scoring groups; potassium intake was significantly lower (p<0.001) in the high-scoring group than in the low-scoring group, especially fruit intake, which was the lowest among the six dietary patterns. In the “noodles” pattern, sodium and the Na/K ratio were significantly higher in the high-score group than in the low-score group (p<0.001), although there was no significant difference in potassium. There was no significant difference in the intake of vegetables and fruits among the groups. In the “fish and seafood” pattern, sodium was significantly higher in the high-score group than in the low-score group (p<0.001), although there was no significant difference in potassium or the Na/K ratio. Vegetable intake was significantly lower in the high-score group than in the low-score group (p<0.001), and fruit intake was significantly higher in the high-score group than in the low-score group (p<0.05). In the “legumes” pattern, sodium and the Na/K ratio were significantly lower in the high-score group than in the low-score group (p<0.001), although there was no significant difference in potassium. The intake of vegetables and fruits was significantly higher in the high-score group than in the low-score group (p<0.05 and p<0.001, respectively).
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4. Association between dietary patterns and the dietary Na/K ratio
To clarify the relationship between the six dietary patterns and the Na/K ratio in the diet, multiple regression analysis was conducted for each sex. The results are shown in Tables 6 and 7. In terms of basic attributes, men with higher BMI had a significantly increased Na/K ratio (p=0.035), whereas women taking antihypertensive medication had a significantly increased Na/K ratio than those who did not take it (p=0.042). In terms of dietary patterns, subjects with higher scores for the “vegetables/rice,” “milk/bread/fruit,” and “legumes” patterns had a significantly decreased Na/K ratio in both men and women (“vegetable/rice” pattern: men and women p<0.001, “milk/bread/fruit” pattern: men, p<0.01; women, p<0.001, “legumes” pattern: men and women p<0.001). Subjects with a higher score for the “noodles” pattern had a significantly increased Na/K ratio in both men and women (men, p<0.05; women, p<0.001). Subjects with higher scores for the “meat/fat” and “fish/seafood” patterns had a significantly increased Na/K ratio in women (p<0.001 and p<0.05 for “meat/fat” and “fish and seafood,” respectively). Although not significant, subjects with higher scores for the “meat/fat” and “fish/seafood” patterns had an increased Na/K ratio in men (p=0.074 and p=0.061 for “meat/fat” and “fish and seafood,” respectively).
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This is the first study to identify dietary patterns associated with a decrease in the Na/K ratio in the general population. We found that the high-scoring groups of “vegetable/rice,” “milk/bread/fruit,” and “legumes” patterns had lower Na/K ratios.
The “vegetable/rice” pattern was similar to the “Japanese food,” “vegetable,” and “healthy food” patterns that were identified as the first dietary patterns in previous studies of Japanese subjects [18–24]. A diet with rice as the staple food, as well as a large amount of vegetable, potato, mushroom, and bean plant foods, with the use of many seasonings for stewing and other seasonings is considered. Therefore, this dietary pattern had the highest salt (sodium) intake of the high-scoring group among the six dietary patterns, but the lowest dietary Na/K ratio because it also had the highest potassium intake. This dietary pattern has been reported to be associated with lower blood pressure [18, 20] and lower cardiovascular and cerebrovascular disease mortality rates [21–23] in Japan. The “vegetable/rice” pattern is recommended because of its low Na/K ratio; however, because of its high salt (sodium) intake, the use of low-sodium seasonings is recommended. In this pattern, there is one report wherein rice appears [20] and another in which it does not [18]. In the current study, the main ingredient load of rice was high, which may reflect the regional characteristics of Niigata Prefecture. The “milk/bread/fruit” pattern, which is a staple food, contains salt (sodium) unlike rice; hence, there was no significant difference in salt (sodium) intake among the scoring groups, although the Na/K ratio was low because of the high potassium content of dairy products and fruits. It was similar to the “Western,” “bread and dairy,” and “bread and breakfast” patterns of previous studies [18–22, 24]. This dietary pattern has been reported to be influenced by the antihypertensive effects of potassium and calcium, which are abundant in dairy products [19, 22]. The “milk/bread/fruit” pattern has a low Na/K ratio and is recommended, although the high-scoring group has the highest saturated fatty acid energy ratio of the six dietary patterns; thus, the quality and amount of fat should be adjusted, for example, by using low-fat milk. There are no previous studies that have identified the “legumes” pattern as a single dietary pattern, and it is thought to be a characteristic of Niigata Prefecture. The Na/K ratio is low because the intake of soybean products, such as tofu and natto, which are high in potassium, is high and the salt (sodium) intake is the lowest among all the dietary patterns. While meat is increasing and seafood is decreasing as a main dish in Japan [12], the fact that a diet with soybean products as a main dish results in a low Na/K ratio is a viewpoint that should be focused on when providing nutrition guidance and information in the future.
Conversely, subjects with higher scores for the “meat/fat,” “noodles,” and “fish/seafood” patterns had increased Na/K ratio. The “meat/fat” pattern was similar to the dietary “meat and fat” and “animal products” patterns in previous studies [18–22, 24]. In this study, the high-scoring group of this dietary pattern was less affected by fish and seafood, and the saturated fatty acid energy ratio exceeded that in the Dietary Reference Intakes for Japanese [11]. Since this dietary pattern has been increasing among Japanese people in recent years [30], it is considered to be a way of eating that requires attention in the future. The “noodles” pattern was similar to the “noodles” pattern of previous studies [20, 24] and had the highest Na/K ratio among the six dietary patterns. The reason for this is that noodles contain a lot of salt in the soup [27], and the results of this study were consistent with this. It is considered necessary to use low-sodium seasoning or to omit the soup. In the “fish and seafood” pattern, fish and seafood were extracted alone, followed by salted or pickled vegetables (main component load: 0.284) and alcoholic beverages (main component load: 0.270). This dietary pattern had the highest salt (sodium) intake but the lowest vegetable intake (excluding salted or pickled vegetables) among the six dietary patterns, resulting in a high Na/K ratio. Although there are few previous studies on this dietary pattern, it was similar to the “traditional Japanese diet” pattern [23]. In general, processed fish products, such as salted salmon and fish roe, are known to be associated with high salt intake [27, 31, 32], although it was raw fish and seafood that were extracted in this study. Some previous studies have reported no association between raw fish and seafood intake and urinary Na excretion [31], although fish and seafood are frequently used in traditional Japanese cooking and are generally prepared with a lot of soy sauce and sugar [33]. In the case of Niigata Prefecture, the intake of raw fish and seafood is slightly higher than the national average [29, 34], and many of the people in the high scoring group for this dietary pattern are older, suggesting that consumption of traditional fish dishes, such as boiled fish, grilled fish, and sashimi, result in higher salt intake. The high-scoring group of this dietary pattern had the lowest saturated fatty acid energy ratio among the six dietary patterns, and since fish and seafood are recommended for the prevention of cardiovascular disease [35], the use of low-sodium seasonings and other methods is recommended when cooking fish and seafood.
In a study on urinary Na/K ratio and dietary patterns in Japanese subjects [24], four dietary patterns were extracted: “fish/vegetables,” “meat/vegetables/oil,” “noodles,” and “bread/candy.” patterns. In the “fish/vegetables” pattern, which is similar to the “vegetables/rice” pattern in this study, the Na/K ratio was lower for men in the higher-scoring group, whereas in the “noodles” pattern, the Na/K ratio was higher for men in the higher-scoring group, consistent with the results of the current study. However, for women, none of the dietary patterns were associated with the Na/K ratio. This may be due to differences in the dietary survey methods and food categories, as well as differences between urinary and dietary Na/K ratios, although further research is needed. The results of multiple regression analysis in this study showed that the Na/K ratio was higher in men with higher BMI and in women taking blood pressure medication. Tabata et al. [36] found that the urinary Na/K ratio was affected by sex and high BMI, which is consistent with the results of our study. However, the reason why the Na/K ratio is higher only in women taking blood pressure medication is unclear, and it may be due to the diuretic influence of drugs [37]. This needs to be examined in the future.
In the national health and nutrition examination survey of 2015 [29], which was conducted in the same year as this study, the average daily sodium intake per person aged 20 years or older was 3,937 mg, and the average daily potassium intake was 2,356 mg, with a Na/K ratio of 2.84. Similarly, in this study, the sodium intake and potassium intake of the residents of Niigata Prefecture were 4,136 mg and 2,539 mg, respectively, and the Na/K ratio was 2.92. At present, there is no standard value for the Na/K ratio in Japan or any other country, although a review by Iwahori et al. [7] advocated that the urinary Na/K ratio should be less than 1 or less than 2, with the goal of reducing the risk of hypertension and cardiovascular diseases. The Dietary Reference Intakes for Japanese [11] also indicates that lowering the Na/K ratio is effective for antihypertensive effect and needs to be studied in the future. However, in this study, the Na/K ratio was 2.62 even in the high-scoring group of the “vegetable/rice” pattern with the lowest Na/K ratio, which deviates significantly from these goals. In addition, for the same Na/K ratio, both sodium and potassium may be high or low. A recent report [38] has shown that blood pressure does not decrease any further when both are high than when they are low, which indicates the importance of decreasing sodium intake. In this study, salt (sodium) intake was higher in all dietary patterns than that recommended in the Dietary Reference Intakes for Japanese [11]; therefore, salt reduction measures will continue to be important.
Niigata Prefecture has been focusing on salt reduction measures for many years, and the salt intake of the prefecture's residents has been decreasing; however, in recent years, there has been no decrease [17]. As for future measures to reduce salt intake, it is difficult to promote only further salt reduction in the current diet, and it is even more important to combine such measures with efforts to increase potassium intake. Thus, the Na/K ratio can be used as an objective indicator, and it is necessary to consider effective methods of providing nutritional guidance and information. For example, the high-scoring group in the first dietary pattern of this study, “vegetable/rice” pattern had the lowest Na/K ratio and a high main ingredient load of various food groups, such as soybeans, potatoes, mushrooms, and fruits, which is considered to be close to the key components of a typical Japanese-style diet recommended by the Japanese government and Niigata Prefecture “diet including a well-balanced meal with staple food as well as main and side dishes” [13, 39, 40]. Nakamura et al. [35] listed vegetables, soybean products, seaweed, seafood, mushrooms, and fruits as the constituent foods of a dietary pattern that is preventive against cardiovascular diseases, although in the dietary survey data from which the dietary pattern was extracted, the method of analysis and pattern name given differed in dietary pattern studies targeting Japanese people. Since this dietary pattern is declining nationwide in all age groups [30], we believe that it is necessary to raise awareness regarding this in cooperation with school and occupational health systems in the future. In addition, since salt intake increases with the number of meals that include staple foods, a main dish, and a side dish [32], it is necessary to provide nutritional guidance and information on salt reduction when recommending this dietary pattern. Furthermore, dairy products extracted from the “milk/bread/fruit” pattern, which also have a lower Na/K ratio, are rich in potassium and calcium, which have an antihypertensive effect [19, 22]. Therefore, it is necessary to raise awareness of dairy products (preferably low-fat) as a food to be taken to prevent hypertension, along with vegetables and fruits [13, 40] recommended by the government and Niigata Prefecture. The results of this study will be useful as basic data for considering nutritional measures, including nutritional guidance and information provision focusing on new dietary patterns that lower the Na/K ratio, and for improving the dietary environment. We believe that this study will contribute to efforts toward the prevention of hypertension in Niigata Prefecture.
This study has some limitations. First, as the analysis was based on the daily dietary record method, it may not reflect the habitual way of eating. Although a multi-day dietary survey or a food intake frequency survey would be desirable, the results of this study suggest that these would yield results similar to those of other dietary survey methods. Second, our dataset may have selection bias. The cooperation rate for individual units in the prefectural health and nutrition survey is 61.5% for the nutrition intake status survey, and the age structure of the study population leans toward the elderly than the actual age structure of Niigata Prefecture residents. A previous study [41] reported that the cooperation rate for the 2003 to 2007 national health and nutrition survey was 61.3% and that there were differences in the survey cooperation rate depending on the sex and age of the target population. Although this survey was conducted by random sampling, it is not necessarily representative of all prefectural residents. We believe that by increasing the rate of survey cooperation in the future, we will get closer to a representative group in this prefecture. Third, the survey was conducted in November, as was the case with the national health and nutrition survey, and there is a possibility of seasonal variations affecting the results.
Despite the limitations described above, this is the first study to identify dietary patterns associated with a lower dietary Na/K ratio in the general population. These results will contribute to organizing public nutrition activities for the prevention of hypertension in Niigata Prefecture. Furthermore, conducting similar studies on health and nutrition surveys in each prefecture in the future would be a useful resource for salt reduction measures in each prefecture, as well as to study regional differences, which would help to solve national nutrition issues.
We revealed that the consumption of “vegetable/rice,” “milk/bread/fruit,” and “legumes” patterns may be recommended to lower the dietary Na/K ratio and contribute to hypertension prevention. A Japanese-style diet with a complete well-balanced set of staple food, main dishes, and side dishes is recommended for the effective prevention of hypertension. We also emphasize the importance of raising awareness about the consumption of dairy products, which are high in potassium.
We would like to express our sincere gratitude to all the respondents who cooperated with the 2015 prefectural health and nutrition survey and to all the people involved with conducting this study. We would also like to express our deepest gratitude to all those involved in the health measures division of the welfare and health department, Niigata Prefecture.
There is no conflict of interest in this study.
