Renal function impairment is often asymptomatic. Many metabolic abnormalities may promote the development of CKD. Preclinical examination has great advantages in preventing CKD from progressing to end-stage renal disease (ESRD) [11]. The present study aimed to describe distribution characteristics of four renal function indicators in the population, including CysC, UA, Cr and UN. A total of 14,716 health examination population from Chengdu city and its vicinity in Sichuan province from 2012 to 2018 were enrolled in this study. Results revealed that UA levels were increased year by year from 2012 to 2018 both in males and females. However, the other three indicators’ levels were declined in years 2012 to 2018 both in two genders. Increased concentrations were found in males compared with those in females every year. When dividing participants into 7 groups by age, we found that UA level was generally increased from 2012 to 2018 in every group. On the contrary, the concentrations of the other three indexes decreased year by year in all subgroups. Notably, the elderly have higher levels of all four kidney indicators than those in the young.
Previous research reported higher levels of serum uric acid (SUA) in males compared with those in females [15]. In 2019, the prevalence of hyperuricemia (HUA) in the whole elderly population was 22.2%, in which it was significantly higher in elderly men than that of elderly women [16]. Similarly, there was a correlation between HUA and age [17]. Elevated SUA concentrations in the older population were found by Chang et al [18]. Increased SUA levels in women aged more than 50 were observed. However, the increase disappeared after excluding menopause and other age-related factors, suggesting a strong association between elevated uric acid and age [19]. It is accepted that a higher alcohol consumption in male and the promoting effect of estrogens on UA secretion in females may contribute to this phenomenon. In addition, postmenopausal women presented higher levels of SUA, which could explain increased UA expression in our older females [15]. Clinical studies have demonstrated that decreased serum UA is of great significance in protecting renal function, improving endothelial dysfunction and reducing cardiovascular risk [20]. Emerging researches detected an increased risk of CKD associated with SUA levels [21]. Elevated SUA concentrations have potential to independently predict the development of CKD. In addition, increased UA levels in rats may lead to glomerular hypertension and kidney disease. The process is mediated by arteriolosclerosis, glomerular injury, and tubulointerstitial fibrosis [22]. Collectively, these findings indicated that there was a gender difference in UA levels and it was higher in elderly population than that in young population, which is consistent with our result.
GFR is an important index to evaluate kidney function. It is widely accepted that both Cr and CysC could be used to estimate GFR. Cr in the blood is mainly produced as a metabolite of muscle activity and is excreted daily by the kidney with urine. When renal function is impaired, Cr products cannot be completely excreted, resulting in elevated serum Cr concentration [23]. Bertille et al. reported increased levels of Cr and CysC in males when compared with those in females, and they were higher in subjects over 55 years old [24]. Similar to these findings, increased Cr and CysC levels in males and the elderly population were also detected in our study. In addition, A strong correlation of CKD and high Cr levels was reported [25]. Thus, monitoring the expression of Cr could effectively prevent the occurrence of CKD.
The level of blood urea nitrogen (BUN) may associate with decreased renal function [26]. Several external factors will influence BUN level, including protein intake, catabolic status, heparin synthesis. Increased UN may indicate renal hypoperfusion due to hypovolemia, nephrovascular disease, or reduced cardiac output [25]. In CKD patients, the decrease of renal function was accompanied by the elevation of BUN. Moreover, higher BUN level may be related to poor renal prognosis in moderately and severely CKD patients [27]. A recent survey showed that BUN was an independent risk factor for CKD in patients with maintenance of renal function [28]. Chang et al. found that elevated UN may accelerate the progression of CKD to dialysis and death [29]. In brief, higher UN expression was not a good signal. In our study, high UN concentrations were detected in men and the elderly, reminding us to monitor the renal function indexes of high-risk groups regularly to prevent or alleviate CKD.
In fact, many previous studies have reported that the prevalence of CKD goes up with age. In view of the high prevalence of CKD in the elderly and the growing elderly population, CKD has become an increasingly serious public health problem [30]. In our study, four renal function indicators showed similar changes with age. According to the global burden of disease study 2013, many risk factors of CKD have been identified, such as diabetes, hypertension, glomerulonephritis [31]. Prevention and control of these related diseases may reduce the risk of CKD. In addition, an observational study lasting over 10 years showed that the risk of CKD was significantly increased in non-diabetic individuals who ate foods with higher carbohydrate concentrations, suggesting that dietary adjustment may be beneficial to control the occurrence and development of CKD [32]. Dietary interventions have been used to prevent or slow down the adverse prognosis of CKD in ways that affect kidney and risk factors for CKD. A 23-year follow-up study reported that eating nuts and beans may reduce the risk of kidney disease progression in the population. These two foods are rich in magnesium, which can reduce the secretion of inflammatory and atherogenic cytokines by endothelial cells and protect renal function [33–35]. Studies on the association between alcohol intake and CKD risk have been inconsistent. A meta-analysis reported that healthy adult men with high alcohol intake had a 0.72-fold lower risk of chronic kidney disease than those who drank less frequently [36].
There are some limitations in our study. First, due to the limitations of cross-section design, the present study failed to analysis the causal relationship between four renal indicators and risk factors. Second, participants were only collected from Chengdu and its vicinity in Sichuan Province, thus, the conclusion could not be extended to the whole country or even to a wider area. Third, our study failed to divide physical examination populations into abnormal and normal group, therefore, the specific distribution of indicators is unclear. However, our research also has some advantages. For instance, our study owns a large sample, and the data were collected through years of follow-up, which made the conclusion more credible and convincing.