Patients with type 2 diabetes and impaired glucose tolerance (IGT) have increased incidence of coronary heart disease (CHD) (1,2). The risk of CHD appears to be similar in patients with type 2 diabetes and IGT (3,4). Thus, factors other than the level of glycemia seem to accelerate the development of CHD in type 2 diabetes. This idea is supported by the notion that the duration of diabetes and the level of glycemia are risk factors for microvascular disease but not for CHD in type 2 diabetic patients (5,6). Alternatively, it may be hypothesized that the relation between type 2 diabetes and CHD may not be causal and that the events preceding the onset of diabetes (i.e., the prediabetic state) may contribute to CHD.
Whereas insulin resistance is considered to be associated with CHD, Kugiyama et al. (7) showed that fasting remnant lipoprotein levels predict coronary events in patients with CHD. Ai et al. (8) and our group (9) recently disclosed that remnant lipoprotein is associated with insulin resistance in IGT subjects and type 2 diabetic patients. Thus, remnant lipoprotein, in conjunction with insulin resistance, is considered one of the important risk factors for the development of CHD in type 2 diabetes.
Although glucose-tolerant relatives of type 2 diabetic patients are considered to be insulin resistant (10,11,12), it is unclear whether remnant lipoprotein is increased in glucose-tolerant relatives of type 2 diabetic patients. Only one study contains data on the lipid profile in nondiabetic relatives of type 2 diabetic patients. Laws et al. (13) documented that nondiabetic relatives of patients with type 2 diabetes had insulin resistance and hypertriglyceridemia. However, they did not study remnant lipoprotein, and the patients studied were all obese. It is well known that the degree of being overweight per se affects insulin resistance and lipid profile (14). In addition, hypertension itself is associated with insulin resistance and lipid abnormalities in humans (15). We therefore recruited nonobese nonhypertensive glucose-tolerant relatives of type 2 diabetic patients who were carefully matched for BMI, blood pressure, and fasting glucose to glucose-tolerant subjects without any family history of type 2 diabetes. This is the first report showing that nonobese nonhypertensive Japanese glucose-tolerant relatives of type 2 diabetic patients have already high remnant lipoprotein and diminished insulin sensitivity before the onset of diabetes.
A total of 45 healthy glucose-tolerant relatives with type 2 diabetes (offspring) participated in the study (34 men and 11 women). They all had a parent who developed type 2 diabetes. The control group consisted of 65 healthy glucose-tolerant subjects without a family history of type 2 diabetes (52 men and 13 women). All subjects studied had a BMI <25 kg/m2 and a blood pressure measurement <140/90 mmHg. They all had normal glucose tolerance on the basis of at least two fasting plasma glucose concentrations <110 mg/dl (16,17).
All subjects studied were Japanese and had ingested at least 150 g carbohydrate for the 3 days preceding the study. They did not perform heavy exercise for at least 1 week before the study. None of the subjects had significant renal, hepatic, or cardiovascular disease or were taking any medication. Blood was drawn the morning after a 12-h fast. Plasma glucose was measured with the glucose oxidase method, and serum insulin was measured using a two-site immunoradiometric assay (Insulin Riabead II; Dainabot, Osaka, Japan). Coefficients of variation were 4% for insulin >25 μU/ml and 7% for insulin <25 μU/ml, respectively. The total cholesterol, HDL cholesterol, LDL cholesterol, triglycerides, and remnant-like particle cholesterol (RLP-C) were also measured. The LDL cholesterol level was calculated using the Friedewald formula (18). The RLP-C level was measured by the method reported by Nakajima et al. (19). The estimate of insulin resistance by homeostasis model assessment (HOMA-IR) was calculated with the following formula: fasting serum insulin (μU/ml) × fasting plasma glucose (mmol/l)/22.5, as described by Matthews et al. (20).
The statistical analysis was performed with the StatView 5.0 system (Statview, Berkeley, CA). The differences of the means were determined by Student’s t test. Data are expressed as means ± SEM.
The clinical characteristics and clinical profile between offspring (n = 45) and control subjects (n = 65) were compared. No significant difference was observed in age (36.5 ± 1.3 vs. 37.6 ± 1.0 years, P = 0.281), systolic (117 ± 2 vs. 114 ± 1 mmHg, P = 0.158) and diastolic (68 ± 1 vs. 66 ± 1 mmHg, P = 0.226) blood pressure, and fasting glucose (91 ± 1 vs. 89 ± 1 mg/dl, P = 0.493) levels between the two groups. The offspring subjects had higher BMI (22.2 ± 0.3 vs. 22.0 ± 0.2 kg/m2, P = 0.252) and total (190 ± 4 vs. 184 ± 2 mg/dl, P = 0.092) cholesterol levels than normal control subjects, but the difference was not statistically significant. In contrast, the offspring subjects had significantly higher serum triglyceride (98 ± 9 vs. 76 ± 3 mg/dl, P = 0.005), RLP-C (4.0 ± 0.3 vs. 2.8 ± 0.1, P = 0.006), serum insulin (5.6 ± 0.3 vs. 4.7 ± 0.1 μU/ml, P = 0.008), and HOMA-IR (1.25 ± 0.07 vs. 1.05 ± 0.03, P = 0.008) levels than normal control subjects. No significant difference was observed in HDL (60 ± 2 vs. 59 ± 1 mg/dl, P = 0.359) and LDL (110 ± 4 vs. 110 ± 2 mg/dl, P = 0.455) cholesterol levels between the two groups.
In the present study, we demonstrated that nonobese healthy glucose-tolerant relatives of type 2 diabetic patients had higher HOMA-IR levels compared with normal control subjects. This is compatible with some previous reports that showed insulin insensitivity in relatives of type 2 diabetic patients (10,11,12). However, which factor contributes to insulin resistance in diabetic offspring subjects has yet to be clarified. BMI, blood pressure, and fasting glucose levels are known to be associated with insulin resistance in humans (14). Hence, we recruited nonobese nonhypertensive glucose-tolerant relatives of type 2 diabetic patients and normal control subjects who had no family history of diabetes, taking into account BMI, blood pressure, and fasting glucose levels. All subjects studied had a BMI <25 kg/m2, blood pressure <140/90 mmHg, and a fasting glucose level <110 mg/dl.
Interestingly, our offspring subjects not only had higher HOMA-IR, but also had higher serum triglycerides and RLP-C levels compared with normal control subjects. However, there was no significant difference in age, BMI, total cholesterol, HDL cholesterol, and LDL cholesterol levels between the two groups. These findings, therefore, suggest that triglyceride or RLP-C, but not BMI, is associated with insulin resistance in nonobese nonhypertensive glucose-tolerant relatives of type 2 diabetic patients.
The mechanisms by which serum triglyceride or RLP-C level is associated with insulin resistance in nonobese nonhypertensive glucose-tolerant relatives of type 2 diabetic patients are presently not known. Although insulin resistance has been suggested to be an underlying defect leading to the development of endogenous hypertriglyceridemia (21), there are some data suggesting that elevated triglyceride levels are preceding factors for the development of insulin resistance (22,23). In families with multiple cases of hypertriglyceridemia, increased serum triglycerides levels serve as a risk marker for subsequent development of type 2 diabetes (22). Mingrone et al. (23) reported on two sisters with extreme hypertriglyceridemia and diabetes in whom the normalization of serum triglycerides by operation improved insulin resistance and glucose tolerance. We recently disclosed that Japanese type 2 diabetic patients with insulin resistance had significantly higher triglyceride levels than those with normal insulin sensitivity (9,24,25). We later demonstrated that physical exercise lowers triglyceride, fasting glucose, and HOMA-IR levels in Japanese type 2 diabetic patients without affecting BMI (26). Furthermore, we showed that bezafibrate not only reduces serum triglyceride levels but also improves insulin sensitivity and glycemic control in type 2 diabetic patients (27). Paolisso et al. (28) recently demonstrated that simvastatin and atorvastatin, a lipid-lowering drug, reduce serum triglycerides, insulin resistance, and HbA1c in type 2 diabetic patients. An in vitro study (29) showed that incubating IM-9 lymphocytes with VLDL downregulates the cell’s insulin receptor. Insulin binding to erythrocytes in the blood of patients with hypertriglyceridemia is reported to be low (30).
Regardless of the mechanism, our present study showing that both high RLP-C and insulin resistance exist in glucose-tolerant relatives of type 2 diabetic patients might suggest that the events preceding the onset of diabetes contribute to the evolution of CHD. Alternatively, our present study might support the view that the duration of diabetes and the level of glycemia, the risk factors for microvascular disease, are not risk factors for CHD in type 2 diabetic patients (5,6).
In summary, nonobese nonhypertensive Japanese glucose-tolerant relatives of type 2 diabetic patients are characterized by impairments in insulin sensitivity, high serum triglyceride levels, and high RLP-C levels. Further study should be undertaken to clarify whether diabetic offspring in association with high RLP-C and insulin resistance is a risk factor for coronary atherosclerosis.
We thank the Department of Biochemistry, Kansai-Denryoku Hospital, Shionogi Biomedical Laboratory, and Ootsuka Pharmaceutical Company, Japan, for their help in our research.
Address correspondence to Ataru Taniguchi, M.D., First Department of Internal Medicine, Kansai-Denryoku Hospital, 2-1-7 Fukushima, Fukushima-ku, Osaka-city, Osaka 553-0003 Japan. E-mail: email@example.com.