Gender differences in the association between body mass index and self-rated health among patients with hypertension: evidence from Rizhao City, China

Objective: This study is designed to examine the association between body mass index (BMI) and self-related health (SRH), and to explore gender differences in BMI-SRH association among hypertensive patients further. Methods: The status of SRH in 1219 hypertensive patients over 18 years old was analyzed by the multi-item self-rated health measurement scale (SRHMS). Chi-square test and t-test were used to analyze BMI and SRHMS status of hypertensive patients, the logistic regression analysis was used to explore the association between BMI and SRHMS of male and female separately after controlling for potential confounders, and to assess gender differences on the relationship between BMI and SRHMS. Results: The prevalence of underweight in hypertensive patients was 1.6%, normal weight was 33.0%, overweight was 45.9%, and obese was 19.5%. And the SRHMS of very good, good, fair, bad, and very bad were 20.2, 15.4, 29.1, 12.5, and 22.8%, respectively. After controlling potential confounders, we found that underweight, overweight and obese had a negative impact on female's SRHMS. Underweight had a negative impact on male, but obesity had a positive impact on male's SRHMS. Besides, this study did not nd that overweight had a statistically signicant effect on male SRHMS. Conclusion: In this cross-sectional study, we found a gender differences on association between BMI and SRHMS among hypertensive patients. Overweight and obesity had a negative impact on female's SRHMS after adjusting for other inuential factors, but not for male. Gender differences should be considered when developing weight and hypertension management programs, especially in women.


Introduction
Hypertension is one of the most common noncommunicable chronic diseases [1], which is becoming a major public health problem that challenges the whole world [2]. The global prevalence rate of hypertension is expected to rise to 29%, with 1.56 billion people suffering from hypertension by the year 2025 [3]. Hypertension also causes a massive burden in China, and it has been identi ed as the second leading health risk factor in China [4,5]. There are a large number of hypertensive patients in China, and uncontrolled hypertension is a powerful risk factor of cardiovascular disease, efforts to detect and control hypertension has been a continuously important issue to public health.
Concurrently, obesity has been increasing rapidly, which is recognized as the major and modi able risk factor for hypertension, being responsible for 20-30% of disease cases [6]. Obesity is a universal disease of increasing prevalence with alarming proportions worldwide, especially in the developing nations [7][8][9][10].
Prevalence of obesity and hypertension can predispose one to various other health complications such as diabetes, renal failure, and cardiovascular disease [11]. Body mass index (BMI) as an anthropometric index is used most frequently to de ne different obesity categories among various populations [12], and it has been widely used in the study of obesity-related hypertension [13,14].
Self-rated health (SRH) is a subjective assessment of individual health status, also known as subjective health or self-assessed health [15]. SRH is considered to be an appropriate and effective indicator of service needs and intervention outcomes [16,17], and can predict individual health outcomes [18].
However, studies have shown that single SRH as a dependent variable is not speci c enough [19,20]. WHO de nes health as a multidimensional concept that includes physical health, mental health and social well-being [20], this concept is created out of many factors subjectively that include health behaviors, general physical functioning and speci c health situations. Thus, it is impractical for a singleitem question to capture the characteristics of health conditions fully. Based on this, some researchers have developed the multiple-item self-rated health measurement scale (SRHMS) according to the health characteristics of Chinese people and applied it to SRH practice [21,22].
Prior studies exploring the association between BMI and SRH, some have found U shaped relationship between them [23,24], some studies have proved that BMI and SRH varies signi cantly across ages and sexes [25]. However, Chinese research in this eld is insu cient. So far, some studies have explored the association between them, most of them have been conducted in western or other high-income countries.
Furthermore, most of the studies are aimed at healthy people, and few of them are aimed at hypertensive patients.
A better understanding of the BMI and SRH mechanisms will help in the design of more targeted and appropriate interventions. To do so, we have the following speci c objectives. Firstly, we will analyze the SRHMS and BMI status of hypertensive patients. Secondly, we will examine the gender difference in the association between BMI and SRHMS.

Study site
This study was conducted in Rizhao City, Shandong Province, China. Rizhao City is located in the southeast of Shandong Province, and it's a coastal city in the east near the Paci c Ocean. It mainly includes 4 districts, Donggang, Lanshan, Juxian and Wulian, with altogether 55 townships. In this study, according to the principle of multi-stage strati ed random sampling, two districts of Lanshan and Wulian were selected randomly considering the GDP level (high and low) and location (coastal and inland). Then, 4 communities and 4 villages were selected from each sampling district. Thus, we selected 8 urban communities and 8 rural villages in total. Figure 1 shows the location of the two study sites.

Data collection
The survey was conducted from March to April in 2019, a set of standardized questionnaires was used to collect data in the community health service institutions where the respondents were located. The questionnaires included sociodemographic information, SRHMS status, health-related behaviors, and blood pressure values. All participants (over 18 years of age) were interviewed face to face by trained nurses in isolated rooms. To ensure the quality, the completed questionnaires were carefully checked by the quality supervisor after the interview. In total, 4390 eligible participants were included in this study.
Trained nurses performed blood pressure measurements, and the participants measured the blood pressure of the right arm using a sphygmomanometer after sitting for 15 minutes. Each subject's blood pressure was measured three times, and the average of the last two times was taken as the blood pressure value. Hypertension was de ned based on the JNC 7 guidelines (SBP ≥ 140 or DBP ≥ 90 mmHg). Considering other diseases that may affect SRH (cancer, mental illness, metabolic disorders, etc.), nurses asked respondents about their past disease history and excluded them from the study.

Variables
Body mass index Body mass index (BMI) was calculated as weight in kilograms divided by height in centimeters squared, height and weight were measured by trained nurses. The interviewees were asked to remove their shoes for the measurement of height. Further, it was categorized as < 18.5 kg/m 2 (underweight), 18.5-25 kg/m 2 (normal weight), 25.0-30 kg/m 2 (overweight), ≥ 30.0 + kg/m 2 (obesity)[26].

Self-rate health
In this study, self-rated health (SRH) was measured by the multi-item self-rated health measurement scale (SRHMS) developed by Xu Jun et al [21,22]. The scale is designed according to WHO's de nition of health, health is regarded as a multidimensional concept including physical health, mental health and social well-being. The scale re ects the change of health measurement from a single dimension to multidimension and from group to individual. Previous studies have shown that SRHMS could accurately and comprehensively re ect the individual's health status, reliability and validity are acceptable, and have strong adaptability in China [27,28].The SRHMS consisted of 48 items, the items are divided into 9 dimensions: physical symptom and organic function, daily physical activities, physical mobility, psychosocial symptom and negative emotion, positive emotion, cognitive function, role activity and social adaptability, social resource and social contact, and social support. The 9 dimensions were also categorized into 3 subscales: physical health, mental health and social health. Each of the 48 items has a maximum possible score of 10 and a minimum possible score of 0, the subscales score and total score maximum possible score of 100, and the minimum possible score of 0.

Data analysis
Descriptive statistic methods were used to analyze the characteristics of the study subjects; Chi-square test or t-test were used to compare the difference in categorical and continuous variables; Logistic regression analysis was used to explore the association between BMI and SRHMS among gender differences in hypertensive patients. The data were double entered and veri ed using EPI Data 6.04, with all data analyzed using SPSS version 16.0 (SPSS, Chicago, Illinois, USA). The level of signi cance was set at P-values < 0.05.
BMI and SRHMS status in hypertensive patients Table 2 shows the BMI and SRHMS status in hypertensive patients, the average BMI was 26.83 kg/m² (SD=3.77), the prevalence of underweight in hypertensive patients was 1.6%, normal weight was 33.0%, overweight was 45.9%, and obese was 19.5%, the proportion of underweight, overweight and obese male was higher than that of female. And the SRHMS of very good, good, fair, bad, and very bad was 20.2, 15.4, 29.1, 12.5, and 22.8%, respectively. The statistical signi cances were found for SRHMS and physical health in gender, male's SRHMS and physical health was better than female, but not for BMI. Table 3, the frequency distribution of SRHMS according to BMI groups, there were signi cant differences among different body weight groups of SRHMS (P=0.009), SRHMS in hypertensive patients were better in normal weight groups than the underweight, overweight and obesity groups. Furthermore, there was statistical signi cance between mental health, physical health and BMI groups.

Association between BMI and SRHMS and its gender difference
We presented our results in two logistic regression models so that we could better understand the association between BMI and SRHMS among gender differences in hypertensive patients (Table 4, Table   5). We also found that residence and reduce salt intake were signi cantly associated with SRHMS for both male and female, the SRHMS of urban residents was better than that of rural residents, and hypertensive patients with reduced salt intake had better SRHMS both male and female.

Discussion
Gender differences in the association between BMI and SRHMS among hypertensive patients were investigated explicitly in our study. To our knowledge, this is the rst study that reports the BMI-SRHMS association among Chinese hypertensive patients. The results showed the different associations between BMI and SRHMS among male and female hypertensive patients. Speci cally, this study has the following valuable ndings.
Prior studies have found U shaped relationship between BMI and SRH, it's different from what we found [24,29]. After controlling potential confounders, we found that underweight, overweight and obese had a negative impact on female's SRHMS. Underweight had a negative impact on male, but obesity had a positive impact on male's SRHMS. In addition, this study did not nd that overweight had a statistically signi cant effect on male SRHMS.
In recent years, many studies have investigated the obesity-SRH across the globe, and the relationship between underweight and SRH is likely to be ignored. Nevertheless, the impact of lower BMI on health is stronger in developing countries [30,31].Our study found that underweight had a negative effect on SRH which was consistent with prior literature [32][33][34], it was known that underweight was linked to eating disorders such as anorexia nervosa, or consumptive diseases such as malignant tumors [35].
Generally, overweight and obesity were negatively associated with SRH, previous studies have shown this [23,[36][37][38]. Moreover, hypertensive patients often have other complications affecting their quality of life and leading to poor overall health, such as diabetes, stroke, cardiovascular diseases [39,40]. A surprising but interesting nding in our study was that overweight had a negative impact on female's SRHMS after controlling other sociodemographic and health-related variables, but not for male. And obesity had different effects on gender, which had a positive impact on male's SRHMS. After analyzing the causes, female might be more susceptible to body image or weight problems than men, it can be interpreted as the sociocultural context of female's body image, an inherent concept of the personal body weight, such as more discrimination against female with excess body weight at work or in life [41]. From the perspective of Chinese cultural factors, male obesity is a symbol of wealth and status, because only the rich people can afford to eat more and gain weight [42]. It's the opposite of the stigma of female obesity in Chinese society. Moreover, considering the components of SRHMS, mental health is an important component of SRHMS, the "Jolly Fat" hypothesis explains this phenomenon in mental health, it suggests that obese people may have lower levels of depression [43]. The hypothesis of "Jolly Fat" was also supported by another study, they found that those who were overweight or obese were more jolly than those who were thinner [44,45], which was consistent with our research. Accordingly, more targeted in hypertension management and weight management should be considered, including gender-speci c intervention strategies.
We also found an association between SRHMS and reduced salt intake in male and female. Rizhao City is near the sea, although most of the residents have received propaganda to reduce the salt intake, due to cultural traditions and habits, the residents consume more salted seafood. The local CDC distributed quantitative salt spoons to residents free of charge and recommended a standard of 6 grams of salt per person per day. We found that hypertensive patients with reduced salt intake had better SRHMS both male and female. Salt intake plays a vital role in regulating blood pressure, moderate and long-term reduction of salt intake in population, which will lower population blood pressure and therefore reduce cardiovascular mortality [46]. This study has several limitations. Firstly, this study was a cross-sectional design, and it may be di cult to con rm the causal relationships of SRHMS and its determina nts. Secondly, SRHMS and hypertensionrelated information were self-reported, leading to the possibility of subjective bias. Thirdly, the relationship between SRHMS and BMI were complex, other diseases may affect SRHMS (cancer, mental illness, metabolic disorders, etc.), nurses asked respondents about their past disease history and excluded them from the study. We considered as many confounders as possible, however, there were some unknown factors. In the follow-up study, we will consider more potential factors to correct possible deviations.

Conclusion
In this cross-sectional study, we found that the SRHMS of male hypertensive patients was better than female. We also found a gender differences in the association between BMI and SRHMS among patients with hypertension, overweight and obesity had a negative impact on female's SRHMS after adjusting for other in uential factors, but not for male. Reducing salt intake can effectively improve SRHMS of hypertensive patients. According to the gender differences in patients with hypertension, more targeted hypertension and weight management should be taken into consideration, including gender-speci c intervention strategies.
Abbreviations BMI: Body mass index; SRH: Self-rated health; SRHMS: The multiple-item self-rated health measurement scale;

Ethics approval
The Ethical Committee of Weifang Medical University School of Public Health and Management approved the study protocol. The investigation was conducted after the informed consents of all participants were obtained.

Funding
This study was funded by the National Natural Science Foundation of China (71804131 and 71373182) and Technical Innovation Team Support Program of Weifang Medical University.

Availability of data and materials
The data generated during this study are not publicly available, due to the reason that su cient information is contained to enable readers to identify sampled community health service institutions and contains a large amount of private information of the participants, but a de-identi ed analytical le is available from the corresponding author on reasonable request.

Competing interests
None declared.

Authors' contributions
Zhiqiang Feng and Wenqiang Yin developed the study design. Zhiqiang Feng, Longde Zhou and Zixuan Zhao participated in the analysis and interpretation of the data and drafted the manuscript. All authors contributed to the editing of the manuscript. All authors read and approved the nal manuscript. 4 . He FJ, MacGregor GA: Effect of modest salt reduction on blood pressure: a meta-analysis of randomized trials. Implications for public health., 16(11):761-770.   Figure 1 Location of the study sites in Rizhao City, Shandong Province. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

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