The impact of obesity on the pathogenesis of non-insulin-dependent diabetes mellitus: a review of current hypotheses

Increased β-Cell Workload Modulates Proinsulin-to-Insulin Ratio in Humans

Increased proinsulin secretion, which characterizes type 2 diabetes and insulin resistance, may be due to an intrinsic, primitive defect in proinsulin processing or be secondary to increased demand on β-cells (hyperinsulinemia secondary to insulin resistance). An alternative way to investigate the relation between relative hyperproinsulinemia and increased secretory demand is to study the dynamic changes in the proinsulin-to-insulin ratio after partial pancreatectomy, a model of acute increased β-cell workload on the remaining pancreas. To pursue this aim, patients without diabetes, scheduled for partial pancreatectomy, underwent 4-h mixed-meal tests and hyperinsulinemic-euglycemic clamps before and after surgery. After acute β-cell mass reduction, no changes were observed in the fasting proinsulin-to-insulin ratio, whereas the fold change in the proinsulin-to-insulin ratio significantly increased over time after the meal. Further, our data demonstrate that whole-body insulin resistance is associated with underlying defects in proinsulin secretion, which become detectable only in the presence of increased insulin secretion demand.

Different associations of diabetes with β-cell dysfunction and insulin resistance among obese and nonobese Chinese women with prior gestational diabetes mellitus

OBJECTIVE

To examine the relative contributions of β-cell dysfunction and insulin resistance to postpartum diabetes risk among obese and nonobese women with prior gestational diabetes mellitus (GDM).

RESEARCH DESIGN AND METHODS

We performed a cross-sectional survey 1-5 years after 1,263 women who had GDM gave birth. Polytomous logistic regression models were used to assess the associations of β-cell dysfunction (the lower quartile of HOMA-%β), insulin resistance (the upper quartile of HOMA-IR), decreased insulin sensitivity (the lower quartile of HOMA-%S), and different categories of BMI with prediabetes and diabetes risk.

RESULTS

β-Cell dysfunction, insulin resistance, and decreased insulin sensitivity all were significantly associated with hyperglycemic status across normal weight, overweight, and obese groups, and the patterns of insulin resistance and decreased insulin sensitivity were similar. BMI was inversely associated with β-cell dysfunction and positively associated with insulin resistance across normal glucose, prediabetes, and diabetes categories. Compared with women with normal glucose and weight, obese women with normal glucose had increased β-cell secretory function (odds ratio [OR] 0.09 [95% CI 0.02-0.37]) and insulin resistance (OR 17.4 [95% CI 9.47-31.9]). Normal weight diabetic women displayed the most β-cell dysfunction (OR 13.6 [95% CI 4.06-45.3]), whereas obese diabetic women displayed the highest insulin resistance (OR 45.8 [95% CI 18.5-113]).

CONCLUSIONS

For women with prior GDM, β-cell dysfunction had more pronounced contribution to postpartum diabetes among nonobese subjects, whereas insulin resistance contributed more to postpartum hyperglycemia among obese subjects.

Diversity of Insulin Resistance in Monkeys with Normal Glucose Tolerance

Insulin resistance has been proposed as a critical factor in the development of Type II diabetes, hypertension, dyslipidemia, and coronary artery disease. However, even in normal healthy individuals, a wide range of in vivo insulin action has been found. In the present study we sought to examine this heterogeneity in insulin action in both normal and spontaneously obese nonhuman primates. Maximal insulin responsiveness as measured by a hyperinsulinemic euglycemic clamp, fasting plasma glucose, and insulin levels, beta-cell insulin response to glucose, glucose tolerance, and adiposity were measured in 22 male rhesus monkeys. Results showed that lean animals (body fat < or = 22%) had higher insulin-stimulated glucose uptake (M rate: 14.42+/-1.8 mg/kg FFM/min) compared to obese (8.08+/-0.8). The obese monkeys, with 23-49% body fat, had a wide range of M values (5.32-14.29 mg/kg FFM/min) which showed no relationship to degree of adiposity. In all monkeys, M values had a strong inverse correlation with fasting plasma insulin levels (r=-0.76; p<0.001), but not with fasting glucose or glucose disappearance rate. We conclude that neither degree of obesity above a critical threshold nor range of glucose tolerance is related to insulin resistance; however, in individuals with normal glucose tolerance an early reliable indicator of defective insulin action appears to be fasting insulin concentration. Longitudinal determination of basal insulin levels obtained under standardized conditions so as to minimize extraneous variability is likely to strengthen the ability to predict insulin resistance and possible later development of overt Type II diabetes.

The insulin gene VNTR is associated with fasting insulin levels and development of juvenile obesity

In millions of people, obesity leads to type 2 diabetes (T2D; also known as non-insulin-dependent diabetes mellitus). During the early stages of juvenile obesity, the increase of insulin secretion in proportion to accumulated fat balances insulin resistance and protects patients from hyperglycaemia. After several decades, however,beta-cell function deteriorates and T2D develops in approximately 20% of obese patients. In modern societies, obesity has thus become the leading risk factor for T2D (ref. 5). The factors that predispose obese patients to alteration of insulin secretion upon gaining weight remain unknown. To determine which genetic factors predispose obese patients to beta-cell dysfunction, and possibly T2D, we studied single-nucleotide polymorphisms (SNPs) in the region of the insulin gene (INS) among 615 obese children. We found that, in the early phase of obesity, alleles of the INS variable number of tandem repeat (VNTR) locus are associated with different effects of body fatness on insulin secretion. Young obese patients homozygous for class I VNTR alleles secrete more insulin than those with other genotypes.

Past and current obesity in Koreans with non-insulin-dependent diabetes mellitus

To determine the prevalence of past and current obesity among patients with non-insulin-dependent diabetes mellitus (NIDDM) and to define the clinical characteristics of non-obese NIDDM patients in South Korea, we studied a cross-section of 749 NIDDM patients and a group of age- and sex-matched control subjects. Current height, weight and waist-to-hip ratio (WHR), the history of weight changes and the family history of diabetes were recorded. Obesity was defined as body mass index (BMI) > 25 kg/m2. The maximum lifetime BMI of diabetic patients was significantly higher than that of control subjects (P < 0.001). Compared with control subjects, current BMI was higher in diabetic women (P < 0.001) but not in diabetic men. In contrast, WHR of both diabetic men and women were significantly higher than those of controls (P < 0.05). BMI and WHR correlated significantly with fasting C-peptide levels and log-triglyceride levels in NIDDM patients. As a whole, 72% of the South Korean NIDDM patients had a history of past obesity as assessed by their maximum weight, while only 38% of them were currently obese. Compared with obese patients, non-obese patients were characterized by lower fasting serum C-peptide levels (P < 0.001), a higher percentage of insulin treatment (P < 0.05), lower maximum BMI (P < 0.001) and more pronounced weight loss from the time at their maximum weight (P < 0.001). In summary, increased upper body adiposity and a history of past obesity were associated with NIDDM in South Korea. Although most South Korean NIDDM patients were previously obese, many of them were currently not obese. Lower maximum BMI, lower serum C-peptide levels and a higher percentage of insulin treatment in non-obese NIDDM patients suggest that the capacity to increase insulin secretion in response to increasing weight gain is rather limited in these patients.

Alterations of plasma lactate and glucose metabolism in obese children

Using a double stable isotope infusion method, we quantified plasma glucose and lactate inter-relationships in five recently obese children. Compared with five age-matched controls, obese children had an approximately 50% increase of total body lactate turnover [167 +/- 20 vs. 111 +/- 20 (SE) mg/min, P < 0.05]. The rate of lactate conversion to glucose was double the normal rate (96 +/- 21 vs. 46 +/- 10 mg/min, P < 0.05). Increased gluconeogenesis from plasma lactate correlated with total glucose production (r = 0.74), with plasma lactate contributing to 58% of glucose production in obese children vs. 38% in normal children (P < 0.05). Conversion into glucose correlated with the rate of lactate release in the circulation (r = 0.76). In turn, the obese children converted a larger fraction (35 +/- 2 vs. 27 +/- 2%, P < 0.02) and amount (58 +/- 10 vs. 34 +/- 5 mg/min, P < 0.05) of glucose into plasma lactate. The amount of lactate originating from plasma glucose correlated (r = 0.70) with lipid oxidation, which was increased in the obese children (58 +/- 4 vs. 23 +/- 5 mg/min, P < 0.02). Our data suggest that increased gluconeogenesis from lactate is associated with increased lipid oxidation and could contribute to the progressive development of insulin resistance and glucose intolerance in juvenile obesity.

Relationships between diabetes duration, metabolic control and beta-cell function in a representative population of type 2 diabetic patients in Sweden

To clarify whether metabolic control and beta-cell function deteriorate with increasing duration of diabetes, we investigated in a cross-sectional study Type 2 diabetic patients in an area-based population. Type 2 diabetic patients (n = 231: 112 males, 119 females) were identified by age at onset > or = 35 years, fasting levels of C-peptide > 0.04 nmol l-1, and absence of islet cell antibodies. Body weight was slightly elevated (BMI 26.8 +/- 0.3 kg m-2), however 76/210 (36%), had normal weight (BMI < 25 kg m-2). Fasting blood glucose rose significantly during the first 10 years of known diabetes from 8.2 +/- 0.3 mmol l-1 in patients with 0-5 years of duration to 9.9 +/- 0.7 mmol l-1 in those with 5-10 years of duration, p < 0.01 and HbA1c from 6.4 +/- 0.2 to 7.4 +/- 0.4%, p < 0.05. Fasting C-peptide levels decreased after 10 years duration from 0.90 +/- 0.06 nmol l-1 during 5-10 to 0.69 +/- 0.08 nmol l-1 during 10-15 years of diabetes, p < 0.05. The proportion of insulin treated patients increased from 13% (12/94) with 0-5 years of duration to 33% (13/39) with 10-15 years and 60% (18/30) with more than 15 years of duration. In conclusion in Type 2 diabetic patients without signs of autoimmunity, metabolic control, and beta-cell function deteriorate with increasing duration of diabetes, leading to common but not inevitable occurrence of 'secondary failure'.

Pre- and posttranslational upregulation of muscle-specific glycogen synthase in athletes

Expression of muscle-specific glycogen synthase (GS) and phosphofructokinase (PFK) was analyzed in seven athletes and eight control subjects who were characterized using the euglycemic, hyperinsulinemic (2 mU.kg-1.min-1) clamp technique in combination with indirect calorimetry and biopsy sampling of vastus lateralis muscle. In the basal state, total GS activity and half-maximal GS activation by glucose 6-phosphate (G-6-P) were respectively 34% (P < 0.03) and 50% (P < 0.005) higher in athletes than in control subjects. In parallel, GS mRNA/microgram total RNA in athletes was 40% (P < 0.005) higher. No difference in GS immunoreactive protein abundance was found between the groups. PFK activity and protein levels were respectively 15% (P < 0.05) and 20% (P < 0.02) lower in athletes, whereas no differences was found in the level of PFK mRNA. After 4 h of hyperinsulinemia, total glucose disposal rate (P < 0.005) and both nonoxidative (P < 0.02) and oxidative (P < 0.03) glucose metabolism were significantly higher in athletes. In parallel, after hyperinsulinemia, the relative activation of GS by G-6-P was significantly higher in athletes, whereas total activity and gene expression of both GS and PFK were unaffected by insulin. We conclude that athletes have increased whole body insulin-stimulated nonoxidative glucose metabolism associated with both pretranslational (mRNA) and posttranslational (enzyme activity) upregulation of GS. However, the immunoreactive mass of GS is normal, emphasizing that posttranslational regulation of the GS protein activity is important for the increased glycogen synthesis rate of muscle in endurance-trained individuals.

Increased serum N-acetyl-beta-D-glucosaminidase and alpha-D-mannosidase activities in obese subjects

We have studied N-acetyl-beta-D-glucosaminidase and alpha-D-mannosidase activities in human sera from 35 control subjects, 47 normo- and hyperinsulinemic obese persons, and 12 diabetic patients after a fasting period of 12 h and at 30, 60, 90, and 120 min after an oral glucose overload. The results show a significantly higher activity of these 2 enzymes in obese subjects and diabetic patients, of similar magnitude, especially in those obese persons with a higher grade of obesity. Moreover, the activity of these glycosidases decreases in a similar way in all these 3 groups after the oral glucose overload.

Effects of high fat-feeding to rats on the interrelationship of body weight, plasma insulin, and fatty acyl-coenzyme A esters in liver and skeletal muscle

Rats fed a high-saturated fat diet consumed more energy, gained more weight, and displayed hyperinsulinemia (P less than .05) without an elevation in the fasting plasma glucose level, compared with animals on two different high-carbohydrate diets. The total fatty acyl-coenzyme A (CoA) concentration was 18% (P less than .0001) and 46% (P less than .0001) higher in liver and skeletal muscle, respectively, from rats fed the high-fat diet compared with each of the other diet groups. Major long-chain fatty acyl-CoA molecular species of both tissues in high fat-fed rats reflected the fatty acid profile of the diet. Approximately 29%, 21%, and 16% of total liver and skeletal muscle fatty acyl-CoAs were comprised of oleoyl-CoA, palmitoyl-CoA, and stearoyl-CoA, respectively. The amounts of these three fatty acyl-CoA esters were significantly higher in liver and skeletal muscle after high-fat feeding than with the other diet treatments (P less than .0001). In contrast, the concentration of linoleoyl-CoA was lower in both tissues after high-fat feeding (P less than .0001). In rats fed the high-fat diet, plasma insulin levels were significantly correlated with gain in body weight or body weight (r = .80, P less than .001 for insulin and gain in body weight; r = .73, P less than .001 for insulin and body weight). Total fatty acyl-CoA ester content in liver and skeletal muscle was also strongly correlated with the plasma insulin concentration in high fat-fed rats (r = .80, P less than .001 for liver; r = .78, P less than .001 for skeletal muscle).(ABSTRACT TRUNCATED AT 250 WORDS)

Different aetiologies of type 2 (non-insulin-dependent) diabetes mellitus in obese and non-obese subjects

Insulin responses to intravenous glucose infusion and glucose utilization during hyperinsulinaemic euglycaemic clamp were determined in a large homogeneous group of 65-year-old male subjects. Twenty-eight had untreated Type 2 (non-insulin-dependent) diabetes mellitus and the remaining 44 control subjects had a normal glucose tolerance. Diabetic patients with abdominal obesity displayed peripheral insulin resistance in combination with defective insulin secretion, whereas non-obese diabetic patients showed only a secretory defect. Thus, Type 2 diabetes in obese and non-obese elderly male subjects may take two forms where the cause of hyperglycaemia differs.