TÉCNICAS DE PROCESAMIENTO Y ANÁLISIS DE DATOS

The relationship between insulin resistance and urinary albumin excretion has been a matter of debate with positive [37-44] and negative results [45-48]. In this cross-sectional study of 155 type 2 diabetic patients with different degrees of albuminuria, we have found convincing evidence that those with microalbuminuria were more insulin resistant than those with a normal urinary albumin excretion. More than forty percent of patients with microalbuminuria had a HOMA-IR value in the fourth quartile of the total studied population of type 2 diabetics. The magnitude of insulin resistance was independently associated with microalbuminuria. There was also a clear association between more severe insulin resistance and microalbuminuria in the subgroup of patients with normal blood pressure considered separately from those with arterial hypertension. Moreover, normotensive patients with microalbuminuria were more insulin resistant than hypertensive patients with normoalbuminuria. Similarly, association between insulin resistance and microalbuminuria was highly significant in groups of patients using or not using metformin. Relationship between insulin resistance and microalbuminuria was highly significant and independent of possible confounders affecting both variables like age, BMI, waist circumference, duration of diabetes, triglycerides, cholesterol, and smoking, anti-diabetic and anti-hypertensive drug therapy.

This studied population included all consecutive, eligible patients reporting in the diabetes clinic, representative of poor and middle class families from different regions of the country, so they represent the average population of type 2 subjects of our country. Any change in their concomitant treatments were intentionally avoided to keep from interfering with their routine care. It would have been reasonably difficult to get a statistically significant sample of a selected group of type 2

diabetic subjects, on diet control and without hypertension. Moreover, it was important to study the relationship of insulin resistance and microalbuminuria in a large number of patients in the presence of all the influencing factors as described above, so to avoid introducing systematic bias in data interpretation. . Thus, our present findings can be reasonably generalized to the average population of patients with type 2 diabetes in our country.

There was no significant difference noted when subjects with normoalbuminuria and microalbuminuria were compared with regard to age, gender, duration of disease, presence of hypertension, serum cholesterol and triglycerides level, fasting plasma glucose and HbA1c. Only significant difference was noted in case of BMI and waist circumference, which were significantly higher in subjects with microalbuminuria, both supporting the presence of higher insulin resistance.

These findings, combined with the evidence of an independent association between insulin resistance and microalbuminuria at multivariate analyses, can reasonably exclude the possibility of a systematic confounding effect of the above factors on the study findings. Consistent with results of multivariate analyses, the above findings confirm that reduced insulin sensitivity is independently associated with microalbuminuria in people with type 2 diabetes, as already documented in type 2 diabetic subjects and nondiabetic populations [270].

We have found in our study that correlation between insulin resistance and urine albumin excretion was highly significant in women as compared to men, though a similar trend was noted in the later group also. This finding is inconsistent with a few studies from other parts of the world [44,271,272] which showed more significant association of insulin resistance in men as compared to women but a few studies did not

find any significant gender difference in this association [273]. There was no significant difference regarding age, duration of diabetes, hypertension, fasting plasma glucose, HbA1c and serum triglycerides level, but women had higher BMI and waist circumference and higher serum insulin level and HOMA-IR values.

In our sample, Insulin resistance (HOMA-IR) was significantly correlated with waist circumference, BMI, serum triglycerides, fasting plasma glucose, HbA1c levels and presence of metabolic syndrome. High triglyceride levels, a typical feature of insulin resistance, represents an independent predictor of renal damage in type 2 diabetes [274]. Increased formation of small dense LDL particles, which has increased susceptibility to oxidation, is a likely mechanism through which triglycerides could initiate endothelial damage and eventually induce renal disease [274]. Another study on type 2 diabetic patients from Karachi region [275] noted 75th percentile of HOMA as 5.4, which is slightly more than 4.17 value in our studied population, but they found poor correlation with clinical parameters of metabolic syndrome in these patients, in contrast to our findings. They did not study urine albumin excretion in these subjects. In a study carried out on non-diabetic population, 75th percentile value of HOMA was 2.6 [276].

Patients with insulin resistance are expected to have significantly poor glycaemic control (i.e., higher HbA1c levels), which in fact turned out to be the case in the cohort we studied, HOMA-IR showed highly significant correlation with HbA1c (r = 0.25, p = 0.01 in men and r = 0.42, p = < 0.0001 in women). Though mean HbA1c for all 155 patients was 7.01 ± 1.56, but patients with HOMA-IR in the 1st quartile had a mean HbA1c of 6.39 ± 1.14, and patient with HOMA-IR in the 4th quartile had a mean HbA1c of 7.63 ± 1.70 This difference may have also played a role in determining increased urinary albumin excretion.

Microalbuminuria is a marker for diabetic nephropathy and predictor of cardiovascular disease in patients of type 2 diabetes mellitus [182-184]. Therefore, the presence of microalbuminuria in type 2 DM may be more reflective of a generalized vascular disease than diabetic glomerulopathy.

People of South Asian background (from India, Pakistan, Bangladesh, and Sri Lanka) have a three times higher risk of developing diabetic nephropathy [255]. A population survey in the U.K. showed more microalbuminuria in South Asians when compared with Europeans [222]. The risk to develop renal injury appears to occur earlier in the course of the disease. Central obesity reflected by a high waist-to-hip ratio (WHR) is considered a risk factor for renal disease in nondiabetic subjects [223,224]. The pathogenesis is unclear and could be mediated primarily by adipogenic inflammation and endothelial dysfunction giving microalbuminuria or secondarily by hypertension and hyperglycemia, which accompany central obesity, which is known to be more common in South Asians compared with Caucasians [225]. Moreover, at the same level of WHR, South Asians seem to have increased abdominal visceral fat and greater insulin resistance compared with Caucasians [226]. We have also noticed that waist circumference and BMI had highly significant association with microalbuminuria.

Association of microalbuminuria with insulin resistance and other cardiovascular risk factors such as increased waist circumference, high BMI, dyslipidaemia and presence of metabolic syndrome can explain the well-established association between increased UAE and cardiovascular morbidity and mortality [36] shown in type 2 diabetes. Hussein noticed a high incidence of silent myocardial infarctions in type 2 diabetic patients with microalbuminuria as compared to normoalbuminuric patients [277].

In our 155 type 2 diabetic patients we did not observe statistically significant association of microalbuminuria with age, duration of diabetes, presence of hypertension. A large scale multicentre study involving more than 2000 patients of type 2 diabetes, in Karachi region noticed a 34 % prevalence of microalbuminuria, which was significantly related to age, duration of diabetes, diastolic blood pressure, serum LDL and retinopathy[278].

Different mechanisms may link insulin resistance to abnormal albuminuria. In response to defective responsiveness of peripheral tissues and vasculature, plasma insulin may rise to supranormal concentrations that may sustain glomerular hyperfiltration [33], endothelial dysfunction [34], and increased vascular permeability [35], effects that eventually result in increased albumin ultrafiltration and leakage into the urine. Furthermore, impaired insulin sensitivity is associated with altered renal cellular metabolism and electrolyte composition, mesangial hyperplasia, and renal hypertrophy and increased endothelial cell proliferation and lipid and hyaluronate deposition in the renal matrix and inner medulla, effects that may directly contribute to progressive kidney damage, even independently of hyperglycemia [279]. Finally, central actions of insulin stimulating the sympathetic nervous system activity and renal effects enhancing renal sodium reabsorption may contribute to the aetiology of arterial hypertension, a frequent concomitant of the insulin-resistant state that may further contribute to renal damage by increasing glomerular capillary pressure and protein traffic [279].

Hyperfiltration and the degree of microalbuminuria are indicators of established diabetic nephropathy [243]. Any reduction of microalbuminuria may be beneficial for kidney function. In several studies [244] of diabetic nephropathy, retardation of the progression of nephropathy toward end-stage renal failure has been found associated

with preceding reduction of proteinuria. Two major trials, Irbesartan diabetic nephropathy trial (IDNT) [250] and Reduction of Endpoints in NIDDM with the Angiotensin II Antagonist Losartan (RENAAL) study [251,252] have demonstrated a clear benefit in terms of renoprotection with angiotensin receptor blockers (ARBs) in patients with nephropathy due to type 2 diabetes. Angiotensin converting enzyme (ACE) inhibitors have also been shown to prevent the development and progression of proteinuria and nephropathy in diabetes patients [253,254].

PPAR-gamma agonists, such as the thiazolidinediones (eg, pioglitazone and rosiglitazone) are insulin sensitizers that appear to have a variety of beneficial effects in animal models of diabetic nephropathy, such as reductions in fibrosis, mesangial cell proliferation, and inflammation [49-51]. While the human data on outcomes are limited, PPAR-gamma agonists reduce urinary albumin excretion at various stages of nephropathy [52-54]. Pistrosch, 2004 [263] and Natali, 2004[264] showed that rosiglitazone corrected endothelial dysfunction of forearm resistance vessels in diabetic patients. One aspect of endothelial dysfunction is increased leakiness of the endothelium. Hence, any improvement of glomerular endothelial function might translate into decreased microalbuminuria. It has also been demonstrated that insulin resistance [207] and low-grade inflammation [265] is associated with microalbuminuria and diabetic nephropathy. Thiazolidinediones are able to improve insulin resistance and reduce systemic inflammation, thereby improving microalbuminuria and diabetic nephropathy. Recent studies of renal biopsies in patients with type 2 diabetes demonstrated an increased width of the glomerular basement membrane in association with the degree of inflammation [265]. Furthermore, there are experimental studies indicating changes of extracellular matrix proteins within the

glomerular mesangium after treatment with thiazolidinediones [266], a mechanism that is able to influence glomerular hemodynamics [267].

In UKPDS, metformin has also been shown to reduce microvascular and macrovascular complications independent of tight glycaemic control [268].Marked decreases in proteinuria have also been observed in obese diabetics who lose weight [257].

Larger studies of longer duration are required to study and thoroughly establish the renoprotective effects of lifestyle measures and drugs that target insulin resistance.

SUMMARY

Type 2 diabetes mellitus is a worldwide public health concern and an important cause of morbidity and mortality. Microalbuminuria is a major risk factor for renal and cardiovascular events, and the early identification and treatment of patients at increased risk for microalbuminuria may be instrumental to limit the excess renal and cardiovascular disease, associated with type 2 diabetes. Insulin resistance precedes and probably contributes to the development of microalbuminuria in type 1 and type 2 diabetics, as well as in non-diabetic subjects. Treating insulin resistance therefore, suggests a possibility of prevention and delay in progression of microalbuminuria in patients of type 2 diabetes mellitus. This may reduce renal and cardiovascular disease risk in these patients.

Considering the possibility that treating and modifying insulin resistance in patients of type 2 diabetes would improve urine albumin excretion, this study was done to investigate the relationship of insulin resistance with microalbuminuria in patients of type 2 DM and observe any difference between men and women in this association.

One hundred and fifty five (155) patients of type 2 DM were selected, who were either having normoalbuminuria or microalbuminuria. Homeostatic model assessment of insulin resistance (HOMA-IR) was used as a measurement of insulin resistance. Urine albumin excretion was measured as a ratio of urine albumin and creatinine on an early morning urine sample. Other variables studied and compared in both groups were, body mass index, waist circumference, blood pressure, fasting plasma glucose (FPG), serum insulin, total and HDL cholesterol, triglycerides, creatinine and HbA1c.

This study has revealed that patients with microalbuminuria were more insulin resistant than those with a normal urinary albumin excretion.

In contrary to few previous studies, the correlation between insulin resistance and urine albumin excretion was highly significant in women as compared to men, though a similar trend was noted in the later group also. More than forty percent of patients with microalbuminuria had a HOMA-IR value in the fourth quartile of the total studied population of type 2 diabetics. The magnitude of insulin resistance was independently associated with microalbuminuria. There was also a clear association between more severe insulin resistance and microalbuminuria in the subgroup of patients with normal blood pressure considered separately from those with arterial hypertension. Moreover, normotensive patients with microalbuminuria were more insulin resistant than hypertensive patients with normoalbuminuria. Similarly, the association between insulin resistance and microalbuminuria was highly significant in groups of patients using or not using metformin.

Relationship between insulin resistance and microalbuminuria was highly significant and independent of possible confounders affecting both variables like age, BMI, waist circumference, duration of diabetes, triglycerides, cholesterol, and smoking, anti-diabetic and anti- hypertensive drug therapy. It also suggested that insulin resistance (HOMA-IR) was significantly correlated with waist circumference, BMI, serum triglycerides, fasting plasma glucose, HbA1c levels and presence of metabolic syndrome.

The study concludes that urine albumin excretion in patients of type 2 diabetes is strongly associated with insulin resistance and related cardiovascular risk factors. This association appears to be stronger in women than men, in our population.