In the current study, we found a U-shaped relationship between the LDL-C/HDL-C ratio and all-cause mortality in the elderly hypertensive population in China. Both lower and higher LDL-C/HDL-C ratios were associated with higher all-cause mortality. The optimal range of LDL-C/HDL-C ratio was 1.67–2.10. Compare with lower and higher LDL-C/HDL-C ratio groups, patients with LDL-C/HDL-C ratio of 1.67–2.10 had a significant higher survival probability.
Considering that CVD is one of the leading causes of death in elderly patients with hypertension, and studies on the relationship between LDL-C/HDL-C ratio and all-cause mortality are limited. Therefore, we also discussed the study of LDL-C/HDL-C ratio and CVD, in order to comprehensively and deeply examine the predictive value of LDL-C/HDL-C ratio. In retrospect of previous studies, there are inconsistent conclusions regarding the predictive value of the LDL-C/HDL-C ratio. Matsumoto I et al.[6] included 687 patients who underwent PCI (mean age 67.7 ± 9.9, mean follow-up years = 2.75) for analysis and found a positive association between the LDL-C/HDL-C ratio and CVD, and considered that the LDL-C/HDL-C ratio should be controlled below 1.5. Zhong et al.[7] enrolled 1,937 acute coronary syndromes (ACS) patients (mean age 64.0 ± 10.8, mean follow-up years = 1.00) for analysis, found that high LDL-C/HDL-C ratio increased the risk of CVD, and considered that LDL-C/HDL-C ratio should be controlled below 2.7. Yokokawa et al.[5] included 8,714 male patients (mean age 63.7 ± 11.5, mean follow-up years = 2.7 ± 0.9) in the analysis and found that compared with LDL-C/HDL-C ratio < 2.6, > 2.6 patients had a higher risk of CVD. The above studies suggest that there may be a positive association between LDL-C/HDL-C ratio and CVD, and different studies have proposed different reference ranges for LDL-C/HDL-C ratio. By contrast, You et al[9] included 356 patients with intracranial hemorrhage (mean age 64.1 ± 13.7, mean follow-up years = 0.22) and found that LDL-C/HDL-C ratio was negatively correlated with all-cause mortality, and considered that LDL-C/HDL-C ratio should be controlled at more than 2.96. Liu et al.[10] recruited 3,250 stroke patients (mean age 63.72 ± 11.33, mean follow-up years = 1.00) for analysis and found a negative relationship between LDL-C/HDL-C ratio and all-cause mortality, mortality was lowest as LDL-C/HDL-C ratio was between 2.23 and 2.88. The above studies suggest that the relationship between LDL-C/HDL-C ratio and all-cause mortality may be negative, and propose an inconsistent optimal range of LDL-C/HDL-C ratio. These conflicting results can be attributed to differences in the study population, follow-up, and end-point events. Meanwhile, it also reminds us that the exact relationship between LDL-C/HDL-C ratio and all-cause mortality and the optimal level of LDL-C/HDL-C ratio are still unclear. The above factors prompted us to conduct the current study.
The mechanism driving this association is still unclear. Several possible mechanisms could explain this finding. When LDL-C/HDL-C ratio was at a low level, there was a negative correlation between LDL-C/HDL-C ratio and all-cause mortality. In Table S1, we found that all deaths caused by respiratory diseases occurred in the LDL-C/HDL-C ratio < 1.67 group. It may be that lower LDL-C/HDL-C ratio is associated with inflammation and increases energy demand for breathing, which in turn aggravates respiratory failure[14]. Of note, lower LDL-C/HDL-C ratio was caused by higher HDL-C levels[15]. Consistent with our study, Bowe et al.[16] included 1,764,986 American adults (mean follow-up years = 9.1) for analysis and found a U-shaped relationship between HDL-C and all-cause mortality, suggested that higher HDL-C was associated with increased mortality. C.M. Madsen et al.[17] included 116,508 patients (mean follow-up years = 6.0) for analysis and found that higher HDL-C levels increased all-cause mortality. It might be that higher HDL-C is associated with gene variability, including mutations in ABCA1, LIPC, and SCARB1, which in turn promote the occurrence and progression of CVD[18, 19]. On the other hand, HDL-C loses its protective effect when it is at a higher level, paradoxically enhancing senescence and impairing endothelial progenitor cell vascularization and angiogenesis[20, 21]. These mechanisms suggest that the conformational and functional properties of HDL particles may change when HDL-C is at a high level, which may lead to harmful effects. When the LDL-C/HDL-C ratio was at a high level, the LDL-C/HDL-C ratio was positively correlated with all-cause mortality. On the one hand, higher LDL-C/HDL-C ratio may promote coronary inflammation[22]. On the other hand, higher LDL-C/HDL-C ratio may also increase the vulnerability and rupture of coronary plaque[23, 24]. In addition, higher LDL-C/HDL-C ratio was associated with increased LDL-C levels[15]. Higher LDL-C was associated with increased mortality, mainly through oxidative stress and inflammatory response[25, 26]. It must be admitted that all-cause mortality contains a variety of factors, so it is difficult for us to fully explain the mechanism of the relationship between LDL-C/HDL-C ratio and all-cause mortality. Therefore, further basic experiments are needed to fully elucidate the specific biological mechanism behind this connection.
Compared with the normal population, elderly hypertensive patients have a higher mortality [27]. Therefore, for such patients, it is more important to find a valuable predictor of mortality. LDL-C/HDL-C ratio might be the useful indicators for prognosis prediction as it simultaneously evaluates the levels of both LDL-C and HDL-C. More important, the finding of U-shaped relationships has a greater clinical significance than linear relationships[28], because it suggests that LDL-C/HDL-C ratio may have an optimal range, so we can control it better.