In this cross-sectional analysis of over 2000 KEEP participants with DM and CKD stages 3–5, we report that poor glycemic control is associated with micro-albuminuria and macro-albuminuria. These same associations were not apparent for any measured lipid as abnormalities were not present. Given that abnormal UAE is felt to be a marker of significantly high-risk kidney disease for any level of GFR, including a much greater risk for both rapid progression to end stage renal disease and premature cardiovascular morbidity and mortality, these data corroborate focusing on glycemic control in CKD patients with diabetes.
Several reports have identified the presence of CKD in DM patients with normoalbuminuria
[
14,
15], but development of microalbuminuria has been considered to be one of the first detectable signs of the classic course of diabetic nephropathy that leads to CKD and, eventually, CVD and end-stage renal disease (ESRD). Consequently, there is a pressing need to further explore the role of risk factors for abnormal urinary albumin excretion, which conceivably “gets the ball rolling” in the pathogenesis of diabetic kidney disease. Glycemic control and dyslipidemia are two leading candidates for such risk factors.
Glycemic control, as evidenced by HbA1c, has previously been shown to be an independent risk factor for the development of microalbuminuria in type 1 and 2 diabetes mellitus in prospective study of normoalbuminuric patients
[
16]. However, our understanding of the relationship between proteinuria and glycemic control is largely derived from investigations targeting glycemic reductions and using proteinuria as a measure of kidney function in populations with preserved or only mildly impaired renal function. Several large randomized controlled trials, including the Diabetes Control and Complications Trial in type 1 diabetes
[
17], the UK Prospective Diabetes Study
[
18], and the Kumamoto Study
[
19], indicate that tighter glycemic control can decrease the risk of nephropathy. Recently, the ADVANCE trial
[
20] documented in subjects with DM that strict glycemic control (mean HbA1
c: 6.5%), in comparison with the standard control (mean HbA1
c: 7.3%), is associated with a significant reduction in renal events, including onset of or worsening of nephropathy [hazard ratio (HR) 0.79;
p
=

0.006], new-onset microalbuminuria (HR 0.91;
p
=

0.02) and, in particular, development of macroalbuminuria (HR 0.70;
p
<

0.001).
However, information about the effect of strict glycemic control on outcome in diabetic patients with established CKD is very limited
[
21], making it difficult to extrapolate that these positive effects of intensive glycemic control are indeed present in DM patients with moderate to advanced CKD. Therefore, our finding that poor overall glycemic control as evident from HbA1c values is significantly associated with proteinuria strengthens our understanding of the relationship between diverse metabolic risks and CKD progression. This relationship remained significant even after adjusting for co-variables (e.g. hypertension, smoking status) that are known to influence albuminuria. The strength of association of HbA1c with ACR was reduced when compared between micro-albuminuria and macro-albuminuria groups in our study. This finding could be reflection of the fact that HbA1c levels might underestimate mean blood glucose levels in patients with CKD and especially in diabetic subjects with severe nephropathy as evident from macro-albuminuria
[
22,
23].
Dyslipidemia has been shown to be independently associated with micro
[
24] and macrovascular
[
25] diseases. A link between dyslipidemia and nephropathy has been demonstrated by prospective studies in the past
[
26,
27]. However, understanding lipid fractions measured in these studies were limited due to the lack of information obtained regarding lipid-lowering therapies as well as fasting status and the lack of association with kidney disease is likely driven by the relatively normal measures. However, the importance of lipid abnormalities in kidney disease can not be understated. Other studies on patients with type 1 diabetes suggest a differential association between lipid variables and kidney disease depending upon the duration of diabetes
[
28] or the stage of renal impairment
[
29]. More recently, serum Apo(B) and Lp(a) increases have been reported at the stages of microalbuminuria and macroalbuminuria, respectively, in a cohort of type 2 diabetic patients but triglyceridemia was significant throughout the three stages of albuminuria
[
30]. In addition, decreased eGFR has been shown to be independently associated with greater odds of having a low HDL level
[
31]. Despite all these statistical associations between dyslipidemia and albuminuria, it is uncertain whether impaired lipid metabolism in CKD patients contributes to the progression of kidney disease
[
32] or, instead, progression of CKD and onset of significant albuminuria itself causes dyslipidemia
[
33].
In our study, neither microalbuminuria, nor macroalbuminuria was associated with lipids in screened KEEP participants. This finding partly may be explained by only 6% of the study population having overt proteinuria. Our understanding of dyslipidemia in CKD is in more advanced stages of CKD (i.e. stages 4–5) or with more significant proteinuria. Interestingly, we also found that HDL is negatively associated with ACR when compared between normo-albuminuria and micro-albuminuria. This association was not sustained when compared between micro-albuminuria and macro-albuminuria groups, perhaps underscoring other metabolic adverse effects that may contribute to low HDL in patients with advanced CKD or macroalbuminuria
[
31].
Our study does have several limitations that should be noted. The cross-sectional nature prohibits inference on causation. Secondly, we rely on single measurements of all laboratory parameters as screening program. While HbA1c values should not significantly change on repeated measurements (assuming glycemic control remains constant), conceivably both urinary ACR values and, more noticeably, lipid measurements can be affected by time of testing and pre-testing diet (e.g. fasting status). In addition, HbA1c values may underestimate mean blood glucose levels in CKD patients, particularly those with CKD-associated anemia, although this should be a non-directional bias towards the null in this cohort in which all participants had CKD. Finally, we do not have medication data on these participants including lipid-lowering therapies, which may influence the interpretation of the presented data. For example, if we knew that all patients in this cohort were on cholesterol-lowering medications such as statins, then we could theoretically explain the lack of association between rising lipid abnormalities and urinary ACR by a protective effect of statins.