Cardiovascular disease is the leading cause of death in patients with diabetes (12), therefore aiming cardiovascular risk reduction is a priority in these individuals. The LDL-c control is recommended as a primary target to reduce CV risk by the 2016 and 2019 European Society of Cardiology /European Atherosclerosis Society (ESC/EAS) Guidelines (3). Nonetheless, even with LDL-c levels in the considered target range, T2DM patients still suffer CVD events, indicating there is a residual CV risk that is not detectable by LDL-c assessment. Accordingly, the 2019 guidelines update have also highlighted the importance of ApoB and non-HDL-c measurements in patients with diabetes. In fact, ApoB is also recommended to assess CV risk, particularly in individuals with hypertriglyceridemia, diabetes, obesity, metabolic syndrome and very low LDL-c levels and it can be used as an alternative to LDL-c for screening, diagnosis and management (6, 13). The present study shows that in type 2 diabetes individuals there are several atherosclerotic lipoproteins that remain elevated even when patients present an optimal LDL-c level. We found that 25% and 22.2% of type 2 diabetes individuals had high ApoB levels and non-HDL-c, respectively, regardless of having an LDL-c below the cut off for which treatment initiation is recommended. Moreover, when updating the patients’ targets accordingly to the 2019 guidelines, 38.8% and 50% of patients presented ApoB and non-HDL-c above target. Thus, in patients with diabetes, targeting only LDL-c seems to be insufficient to estimate the real CV risk and accordingly to the new 2019 guidelines it is clearly insufficient to estimate the real CV risk.
ApoB and non-HDL
ApoB and non-HDL were not routinely evaluated, since they are considered secondary targets(3). However, several studies have demonstrated an important role of non-HDL and ApoB when compared with LDL-c regarding the occurrence of CHD events(14, 15). There is evidence that the non-HDL cholesterol and ApoB may be better predictors of CVD incidence among diabetic men than LDL-c(16). This could occur because all major atherogenic particles originated in liver (very-low density lipoproteins (VLDL) and intermediate-density lipoproteins and LDL have a apolipoprotein B100 (ApoB) molecule (17). Therefore, the ApoB evaluation could work as a direct proxy when assessing the number of atherogenic particles (LDL and non-LDL). Non-HDL-c has also been suggested as a better marker of CVD risk and coronary atherosclerosis(18) and non-HDL lipoproteins, were associated with atherogenic dyslipidemia, insulin resistance, portal hyperinsulinemia and the metabolic syndrome phenotype(15, 17, 19-23). Patients with higher levels of triglycerides were shown to have lower LDL-c levels for any given ApoB concentration compared with subjects with lower levels of triglycerides by Leroux et al (24). Thus, patients with hypertriglyceridemia, such as T2DM patients, may present lower LDL-c concentrations since these values are falsely diminished by elevated triglycerides levels. Nevertheless, these patients still have an high atherogenicity risk since ApoB levels remain elevated, even when there is a decrease of LDL-c levels(24). In our study, we found that 22% and 25% of T2DM individuals had high non-HDL-c and ApoB levels, respectively, regardless of having an LDL-c considered as within target. When updating our results accordingly to the 2019 guidelines, where atherogenic lipoproteins targets are even lower, the proportion of patients with elevated ApoB and non-HDL-c, despite LDL-c within target increased. Friedewald et al. recognized in their original publication that at lower LDL-c levels, even small errors in very low-density lipoprotein cholesterol estimation resulted in significant errors in LDL-C estimation[(11) thus in patients with LDL-c considered within target by the recent guidelines and who have low LDL-c levels, the atherogenicity could be underestimated if only LDL-c is considered. To overcome this, the 2019 guidelines reinforce that in patients with very low LDL-c levels, ApoB can be a better alternative to LDL-c in order to assess CV risk and guide treatment. Our results suggest that in T2DM patients when only LDL-c is considered for deciding treatment, some patients may not be identified, causing a missing oportunity to adjust therapy and thus reduce CV risk. As guidelines have established even lower cutoffs for LDL-c levels, the mismatch LDL-c/ApoB increases and thus, ApoB should also be included obligatory in the evaluation of an individual with T2DM. Of note, in this study even in patients with ApoB within target according to the 2019 guidelines, some patients (16.7%) still present elevated LDL-c.
HDL-c and ApoA1
HDL-C has a cardioprotective role that has been attributed to its role in reverse cholesterol transport, its effects on endothelial cells, and antioxidant activity(25). Similarly, HDL molecules also present anti-inflammatory anti-apoptotic, vasodilatory, antithrombotic, and anti-infectious roles and can modulate directly the glucose metabolism[(26). Although an association between low HDL-c concentrations and an increased risk of type 2 diabetes (T2D) has been shown at an epidemiological level (27, 28), the benefits of raising HDL-C are still not a primary target for CV risk reduction therapy and by them self, serum HDL-C levels are also not sufficient to predict cardiometabolic risk, especially in type 2 diabetes(29). In our study, we did not find any differences in HDL-c levels between patients with LDL-c within and above target. Low levels of ApoA1 also seem to independently associated with new T2DM(30). Nonetheless, according to the ESC/EAS guidelines, our patients evidenced normal ApoA1 concentrations and we did not find any differences in ApoA1 levels between patients with LDL-c within and above target.
ApoB/ApoA1 ratio is a simple and accurate risk factor for CVD (31, 32). Carnevale et al. (2011) describe that a low ApoB/apoA1 ratio reflects a less atherogenic lipid profile, regardless of LDL-c (32). Several studies have also suggested that an elevated ApoB/ApoA1 ratio is a more powerful predictor than other lipid fractions for metabolic disorders, including type 2 diabetes(33-38). Recently, it has been demonstrated that ApoB/ApoA1 ratio is independently associated with carotid atherosclerosis in T2DM with well-controlled LDL cholesterol levels(39). In this study, both groups presented a mean ApoB/ApoA1 ratio within the reference range. Patients with LDL-c levels above target according to the 2016 guidelines showed a significantly higher ratio than those within target, which was not seen when analyzing patient’s status accordingly to the 2019 guidelines, indicating that the ApoB/ApoA1 ratio is not a good indicator of atherogenicity in these patients.
In our study, 44.4% of patients with type 2 diabetes and LDL-c within target had oxidized LDL-c above the reference range. When updating the results according the 2019 guidelines, 43.8% of patients remained elevated with oxidized LDL-c. Oxidized LDL-c has been associated with progression of atherosclerosis and CHD(40-42). Holvoet et al. suggested that the predictive value of oxidized LDL-c seems to be additive to that of the Framingham global risk assessment score for CV risk (40). Chronic hyperglycemia triggers the production of excess free radicals causing peroxidation of lipid molecules in a chain reaction fashion(43). Malondialdephyde (MDA), oxidized LDL, oxidized LDL/LDL and oxidized LDL/HDL-c levels are significantly elevated in type 2 diabetes patients versus controls in several studies(44-46). MDA is a recognized marker of end products of lipid peroxidation and is reported to modify ApoB, increasing the susceptibility of LDL-c to oxidation and production of oxidized LDL-c (41, 47). In our study, patients presented elevated oxidized LDL-c levels, even patients with LDL-c within target, suggesting a high risk of progression of atherosclerotic CVD in these patients. The results were maintained even when lowering the LDL-c cutoffs in the 2019 guidelines, suggesting that the oxidized LDL-c levels are not affected by LDL-c levels and could be a reliable index of atherogenicity in T2DM individuals, regardless of the LDL-c status.
Lipid parameters correlations
In T2DM patients dyslipidemia presents itself with an high flux of free fatty acids, hypertriglyceridemia, low HDL-c values, increased sd-LDL particles and high ApoB (48). This study correlated LDL-c levels with other lipid parameters according to LDL-c status in patients with T2DM, and presents an positive correlation between LDL-c and total cholesterol and non-HDL-c, with non-HDL-c and ApoB being the most strongly correlated to LDL-c. Non-HDL considered a better risk marker for coronary heart disease had also a marked correlation with ApoB and with oxidized LDL-c, Although these correlations were present for LDL-c, they were weaker than in non-HDL-c. While ApoB and oxidized LDL-c assessment may not be routinely available, non-HDL-c levels could an important tool for atherogenic profile characterization. Since, non-HDL-c levels are inexpensive and easy to obtain and may represent an appropriate index of CV risk better than LDL-c, patients with T2DM included(22). Additionally, non-HDL cholesterol seems also able to predict CVD over a wider range of triglyceride concentrations(22) than LDL-c, where calculation trough either Friedewald’s formula or direct measurement are affected by triglycerides concentrations.
Our study presents some methodological limitations relatable to his cross-sectional approach, as the non-assessment of long-term outcomes, such as the occurrence of CVD according to lipid levels parameters. Thus, a prospective follow-up approach is required to evaluate medical interventions and lipid goal attainment in relation to mortality in T2DM patients.