The interaction between serum uric acid and triglycerides level on blood pressure for middle-aged and elderly individuals in China


 Background The purpose of the research is to explore the extent of interaction between triglycerides (TG) and serum uric acid (SUA) level with blood pressure (BP) in middle-aged and elderly individuals in China.Methods Data were selected from the CHARLS, a cross-sectional study. A total of 7119 individuals aged 45 to 96 in our study was included. Differences between gender, or between categories of blood pressure levels were evaluated by t-test or chi-square test. The adjusted associations between various characteristics and BP status were first compared using linear regression models, as appropriate. Then, general linear models adjusting for related potential confounders were used to examine the synergistic effect of SUA and TG level on blood pressure (BP) for middle-aged and elderly individuals in China.Results Age-adjusted partial Pearson’s correlation coefficient showed that SUA and TG level positively correlated with both SBP and DBP in men and women. Multiple linear regression analysis showedTG levels were significantly and positively associated with systolic blood pressure (SBP) and diastolic blood pressure (DBP) in both males (SBP: β =0.068, P =0.001; DBP: β =0.064, P =0.002) and females (SBP: β =0.061, P =0.002; DBP: β =0.084, P =0.000), but serum uric acid (SUA) were significantly and positively associated with systolic blood pressure (SBP) in both males (SBP: β =0.047, P =0.013) and females (SBP: β =0.040, P =0.028), regardless of other confounding factors. A general linear model adjusted for confounding factors was used to assessed the statistical significance of the synergistic relationship between serum uric acid (SUA) and triglycerides (TG) level. Evidence of interaction between serum uric acid (SUA) and triglycerides (TG) level on SBP (men: β =-1.090, P =0.726; women: β =-0.692, P =0.861) and DBP (men: β=-1.026, P =0.572; women: β =-0.794, P =0.842) was not observed.Conclusion The interaction effect of serum uric acid (SUA) and triglycerides (TG) level on blood pressure (BP) was not observed in our study.

blood pressure (BP) was not observed in our study.

Background
As well known, according to the relationship between prehypertension and cardiovascular disease [1][2][3] and the etiologies of cardiovascular disease (CVD) [4][5][6], the hypertension is defined with the standards as follows: ①diastolic blood pressure (BP) of >90 mmHg; and/or ②systolic blood pressure (BP) of >140 mmHg. Hypertension is a cluster of risk factors[7-14] associated with aging, central obesity, overweight, the household heredity factors, unhealthy behavior and life styles(cigarette smoking, alcohol consumption, and lack of physical activities), diabetes, dyslipidemia, low levels of high-density lipoprotein(HDL) cholesterol, high levels of low-density lipoprotein (LDL) cholesterol, elevated fasting glucose levels, and elevated triglycerides(TG). Because hypertension is more complex and patients suffer from physical, psychosocial, and economical burden, it has become a serious public health in the worldwide [15].
Recently, prevalence and incidence of hypertension remained higher up in China [16,17].
For the aging population increases, it is disproportionately high among middle-aged and elderly individuals in China [18][19][20]. Hypertension is a multi-factor caused disease, and patients suffer a lot from economic burden, psychological and physical distress. Recently, hypertension became most important public health challenges worldwide. In sum, hypertension prevention and treatment strategies, and its risks should be carefully studied. Exploring the its timely associated risks and their interaction of hypertension may provide the insight in public health implications for prevention and management of hypertension in future.
Serum uric acid (SUA) is an endogenous end product and is involved in the production of reactive-oxygen species. It is important to evaluate their status in advance of chronic disease development [21]. In recent years, as a key mark, systemic measured by serum uric acid (SUA) has become an important marker for chronic disease development. Studies have conduct that serum uric acid (SUA) is associated with various diseases, such as cardiovascular disease (CVD) [22][23][24][25], prehypertension [26][27][28][29], metabolic syndrome [30][31][32], and hypertension [33][34][35]. However, despite the association between SUA levels and these risk conditions, SUA levels may not be regarded as an independent risk factor. Since SUA level is highly associated with overweight, obesity and other risk factors [36][37][38], which is in turn associated with risk of hypertension, a causal condition may exist between triglycerides (TG) and risk of hypertension. Therefore, the association between SUA levels and risk of hypertension and the effects of triglycerides (TG) on this association are of considerable interest, and a modulating effect between triglycerides (TG) and serum uric acid (SUA) levels on blood pressure (BP) may also be fully considered.
To date, few studies on the association and interaction analysis between serum uric acid (SUA) and triglycerides (TG) level and blood pressure (BP) were conducted in individuals aged ≥45 years. The study has[39] explored the association and/or interaction analysis between serum uric acid (SUA) and triglycerides (TG) and blood pressure (BP), but the relationships lack of the related research in china culture background. Thus, this study aimed to determine the prevalence of normotension and hypertension and their association with serum uric acid (SUA), triglycerides (TG) level, and other confounding factors based on gender using the cross-sectional study data(CHARLS) from individuals aged ≥45 years in China.

Statistical analysis
Data were analyzed by using SPSS17.0 software forWindows10 (IBM Corp., Armonk, NY, USA) and expressed as the mean SD or frequency, as appropriate. Differences between gender, or between categories of blood pressure levels were evaluated by t-test or chisquare test. The adjusted associations between various characteristics and BP status were first compared using linear regression models, as appropriate. Then, general linear models adjusting for related potential confounders were used to examine the synergistic effect of serum uric acid and triglycerides level on blood pressure for middle-aged and elderly individuals in China. 2-tailed, and a value of P of 0.05 were considered significant.

Results
In total, 7119 participants who effectively completed the questionnaires were included in our research. The baseline of demographic variables is shown in table 1, and most variables based on our previous studies [41][42][43]. Overall, 3345(46.99%) of the participants were male, and 3374 (53.01%) of the participants were female. The average ages of the male and female participants were 60.24 ± 9.24 and 59.91 ± 9.95years, respectively. In male, the mean and standard deviation of SUA levels were 4.87±1.24 mg/dl in normotension group, and 5.20±1.33 mg/dl in hypertension group, respectively. In male, the mean and standard deviation of TG levels were 120.58±100.65 mg/dl in normotension group, and 133.51±111.81 mg/dl, respectively. In female, the mean and standard deviation of SUA levels were 3.93±1.04 mg/dl in normotension group, and 4.24±1.16 mg/dl in hypertension group, respectively. In female, the mean and standard deviation of TG levels were 129.26±82.88 mg/dl in normotension group, and 154.49±112.79 mg/dl, respectively. Table1 show the relationship of various characteristics and CRP levels in the participants. Significant differences in distribution were observed between blood pressure status in male in all of the variables, except cigarette smoking, alcohol consumption, eating habit, social events, physical exercise, history of liver diseases, anti-diabetic medication, and HDL cholesterol. CRP, HbA1c, FPG, LDL-C, BMI, TG, and SBP and DBP were significantly higher in the hypertension than those in the normotension, whereas, eGFR levels was lower in the hypertension than that in the normotension. Significant differences in distribution were observed between blood pressure status in female in all of the variables, except residence, cigarette smoking, alcohol consumption, eating habit, social events, accidental injury, regular physical exercises, and history of liver diseases. CRP, HbA1c, FPG, LDL-C, TG, SBP, SUA level and DBP were significantly higher in in the hypertension than those in the normotension, but HDL-C and eGFR level were lower in the hypertension than that in the normotension.   suggests that SUA may play an important role in hypertension, and gender-specific factors may also be crucial. The SUA levels were higher in males than in females, which can partially explain the underlying mechanisms that possibly account for gender differences, such as alcohol consumption, whose prevalence is usually higher in males. Additionally, body fat and steroid hormones, and their interaction in middle-aged and older adults may also be associated with hypertension.
This study has three limitations. First, the association and interaction between serum uric acid (SUA) and triglycerides (TG) level and BP becomes seriously more complex. We only considered the identified confounders; however, some unknown factors still exist. Second, the relationship should be studied prospectively; however, our study investigated the interaction between serum uric acid (SUA) and triglycerides (TG) level and blood pressure (BP) in participants aged ≥45 years in a cross-sectional study. Follow-up study was relatively short to comprehensively observe changes in the next phase. Finally, more research is needed to confirm the results. However, this study has several strengths as well. First, this study was conducted based on a nationwide survey, and second, the analyses were conducted based on gender.

Conclusion
An interaction between SUA and TG level and BP was not observed in both men and women in our study.

Competing interests
The authors declare that they have no competing interests.