In the current large population-based cohort study, we investigated the association between TRL and LDL particle concentrations, determined with a novel NMR-based algorithm, with HOMA-β. In subjects without T2D at baseline, very large TRLP, large LDLP, and LDL size were inversely associate with HOMA-β, whereas large and medium TRLP and TRL size were positively associated with HOMA-β when taking account of HOMA-IR. During a median follow-up of 7.3 years, very large, large and very small TRLP and TRL size were positively associated, whereas large LDLP and LDL size were inversely associated, with incident T2D after adjustment for multiple T2DM risk factors, including BMI, HDL-C, or alternatively HDL size, HOMA-β, and HOMA-IR. In secondary analyses, higher concentrations of total TRLP and LDLP were particularly associated with a higher risk of incident T2D in non-statin users. In addition, higher levels of large TRLP, and TRL size were associated with an increased risk of T2D in participants who did not consume high amounts of alcohol.
It is well appreciated that insulin resistance plays a key role in the development of lipoprotein abnormalities featured by elevated triglycerides, higher large VLDLP and increased VLDL size, as well as a shift from larger towards smaller LDLP and decreased LDL size [7–12]. Using a novel NMR platform-derived algorithm which captures five different TRL subfractions and three LDL subfractions such relationships with HOMA-IR were confirmed both in non-diabetic and diabetic individuals, independent of BMI, HOMA-β and other relevant covariates. In addition, a modest, but hitherto unappreciated, positive relationship of HOMA-β with TRL and an inverse relationship with LDL size was observed independent of HOMA-IR, particularly in subjects without T2D.
Regarding the associations of various TRL and LDL subfrctions with incident T2D, the current results are generally consistent with prior studies, showing that NMR-measured VLDLP concentrations and size, and small LDLP concentrations were positively but large LDLP concentrations and LDL size were inversely related to incident T2D [10, 12, 23, 25, 32]. For TRL particles, we demonstrated for the first time that very large, large and very small TRLP had a relatively similar association with incident T2D. For LDL particles, higher concentration of large LDL and greater LDL size were suggested to be inversely associated, whereas higher concentrations of small LDL were positively associated with incident T2D in previous studies [9, 23, 25]. Although we found that LDL size and large LDL particles were inversely associated with risk of T2D development, we did not find a strong association between small LDL particles and incident T2D. Hence, a smaller LDL size may be associated more robustly with T2D development [9, 10, 23, 25, 33] than absolute concentrations of small LDLP. In comparison, of conventional (apo)lipoprotein and lipid measures, the plasma triglyceride concentration was the only variable that was associated with incident T2D in multivariable analysis in the whole study population at risk. Notably, since statin use is associated with an increased risk of incident T2D [34–36], we adjusted for statin use in multivariable models. Taken together, the present findings highlight the relevance of lipoprotein subfraction measurement in assessing the association of lipoprotein characteristics with the risk of T2D development. HDL particles are able to affect β-cell function [20, 37], and HDL-C makes part of established diabetic risk models [38, 39]. Recently, HDL size was found to be associated with incident T2D even independent of HDL-C [26]. In the current study, very large, large, and very small TRLP and TRL size, as well as large LDLP and LDL size, were associated with incident T2D independent of HDL-C or alternatively of HDL size.
Remarkably, the associations of TRL and LDL particle characteristics with incident T2D as documented in our study were independent of, and only modestly diminished after, adjustment for HOMA-IR despite the presently reiterated strong association of HOMA-IR with T2D development [40]. Moreover, such associations were independent of HOMA-β. In the interpretation of these results, it should be recognized that HOMA-IR is a measure of insulin resistance on glucose metabolism and hence does not not necessarily represents a strong proxy of insulin resistance on free fatty acid metabolism in adipose and liver tissue [41–43]. Furthermore, HOMA-β is a static measure of insulin secretion with only has a modest relationship with dynmic tests of insulin secretory capacity by pancreatic β cells [29, 44, 45].
Evidence is mounting that cholesterol accumulation in pancreatic ß-cells may give rise to ß-cell dysfunction [17, 18, 46, 47]. Of further interest, VLDL and LDL particles are able to modulate ß-cell function [16], possibly in part attributable to pancreatic steatosis [48]. While oxidized LDL induces ß-cell apoptosis [49], both LDL and VLDL particles may decrease ß-cell proliferation by reducing cyclin B1 expression [50]. Furthermore, addition of human LDL to cultured islets impairs glucose-stimulated insulin secretion, mediated by the LDL receptor [51]. Interestingly, proprotein convertase subtilisin/kexin type 9 (PCSK9) deficiency results in increased accumulation of cholesteryl esters in pancreatic islets, coinciding with increased intracellular insulin but decreased circulating insulin [17]. In humans, circulating PCSK9 is associated with intermediate density lipoproteins [52], and high PCSK9 plasma concentrations may associate with increased risk of T2D development [18]. However, the mechanisms responsible for the association of specific TRL and LDL particle characteristics with T2D develepoment are still unprecisely known and should await further studies.
Reasoning that associations of TRL and LDL particles characteristics with incident T2D could in part be dependent on cholesterol accumulation in pancreatic β-cells [46, 47], and that inhibition of 3-hydroxy-3-methylglutaryl-CoA reductase could affect glucose tolerance and insulin secretion [53–55], secondary analysis were performed according to statin use. Statin users had lower levels of TC, LDL-C, non-HDL-C, apoB [56] and a slightly lower LDL particle size [57] as expected. In agreement with the possibility that exposure to higher circulating levels of lipoprotein-associated cholesterol would increase diabetes risk, higher total TRLP and LDLP concentrations were associated with incident T2D in crude and in fully adjusted analysis in non-statin users, but not in statin users. In comparison, using NMR spectroscopy (LP3 algorithm), Mackey et al. could not find a potential effect modification by use of statin on the association of NMR- measured lipoprotein particles with incident T2D [25].
Of further relevance, alcohol consumption has been included in risk scores for T2D prediction [58–60]. Although low and moderate alcohol consumption was found to be inversely associated with T2D development risk [61–64], high alcohol intake is associated with metabolic disorders including glucose, lipid and lipoprotein abnormalities, which may contribute to pancreatic toxicity [65–68]. Alcohol-induced hypertriglyceridemia is due to increased VLDL secretion, impaired lipolysis and increased free fatty acid fluxes from adipose tissue to the liver [66]. As expected, LDL-C, triglycerides, TRL subfractions, TRL size, and small LDLP were higher in subjects with high alcohol intake compared to the subjects who did not consume high amounts of alcohol [56]. Nonetheless, large TRLP and a greater TRL size were not associated with incident T2D in individuals with high alcohol consumption.
The current study has several strengths and limitations. A strength of this study is that it includes a large number of participants with a large age range, and the long-term follow-up, from the general population. Given the design of our longitudinal observation study, cause-effect relationships cannot be ascertained with certainty. Although the majority of the PREVEND participants were of north European descent, our findings are in line with previous studies which were performed in the population with different ethnicities [24, 25]. Moreover, alcohol intake was collected by participants’ self-report, making that the possibility of underestimation of alcohol consumption cannot be excluded.