© 2003 by the American Diabetes Association, Inc.
Correlation Between Midthigh Low- Density Muscle and Insulin Resistance in Obese Nondiabetic Patients in KoreaFrom the Division of Endocrinology and Metabolism, Department of Internal Medicine, Yonsei University, College of Medicine, Seoul, Korea
OBJECTIVEWe investigated the link between lipid-rich skeletal muscle, namely low-density muscle, and insulin resistance in Korea. RESEARCH DESIGN AND METHODSAbdominal adipose tissue areas and midthigh skeletal muscle areas of 75 obese nondiabetic subjects (23 men, 52 women; mean age ± SD, 41.9 ± 14.1 years) were measured by computed tomography (CT). The midthigh skeletal muscle areas were subdivided into low-density muscle (0 to +30 Hounsfield units) and normal-density muscle (+31 to +100 Hounsfield units). The homeostasis model assessment (HOMA) score was calculated to assess whole-body insulin sensitivity. RESULTSThe abdominal visceral fat area and the midthigh low-density muscle area were found to be well correlated with the HOMA score (r = 0.471, P < 0.01 and r = 0.513, P < 0.01, respectively). The correlation between low-density muscle area and insulin resistance persisted after adjusting for BMI or total body fat mass (r = 0.451, P < 0.01 and r = 0.522, P < 0.01, respectively) and even after adjusting for abdominal visceral fat area (r = 0.399, P < 0.01). CONCLUSIONSThe midthigh low-density muscle area seems to be a reliable determinant of insulin resistance in Korean obese nondiabetic patients.
Abbreviations: CT, computed tomography HOMA, homeostasis model assessment HU, Hounsfield units OGTT, oral glucose tolerance test
The close relationship between abdominal adiposity and insulin resistance has been described in previous studies (13). Moreover, visceral adipose tissue is well recognized to be significantly related to insulin resistance of obese type 2 diabetic patients and even patients with normal weight (4,5). Recently, the role of intramuscular lipid components in insulin resistance became the subject of attention (68). Low-density muscle represents lipid-rich skeletal muscle, which includes fat components between and inside the muscle fibers. Many other studies have already shown that low-density muscle is significantly related to insulin resistance in obese type 2 diabetic patients. However, this relation has not been investigated in Korea, where the prevalence of both obesity and diabetes is relatively low. Therefore, the current study was undertaken to investigate the potential link between low-density muscle and insulin resistance in the Korean population.
Subjects A total of 75 subjects (23 men, 52 women; mean age ± SD, 41.9 ± 14.1 years) with sedentary lifestyle were enrolled in this cross-sectional study. Of these, 69 patients were obese (BMI >25 kg/m2) and the remainder were overweight (BMI 2325 kg/m2), according to the revised definition of adult obesity in the Asian-Pacific race proposed at the Hong Kong meeting (9). A total of 33 premenopausal women and 19 women with natural menopause (mean age ± SD, 31.2 ± 7.8 and 54.8 ± 7.9 years, respectively) were included. Subjects were divided into a normal glucose tolerance group (n = 46) and an impaired glucose tolerance group (n = 29), according to the results of an oral glucose tolerance test (OGTT). Individuals with a history of or evidence of hypertension, any type of diabetes, or cardiovascular disease were excluded. Individuals with hyperlipidemia (concentration of plasma total cholesterol >350 mmol/l or concentration of triacylglycerol >300 mmol/l) were excluded. Those taking any kind of oral or parenteral medications were excluded, and none of our subjects engaged in any regular exercise. The clinical and biochemical characteristics of the subjects are described in detail in Table 1. The study protocol was approved by the Yonsei University College of Medicine ethical committee, and informed consent was obtained from each subject.
Anthropometric parameters Body weight and height were measured in the morning, without clothing and shoes. BMI was calculated as body weight in kilograms divided by height in meters squared (kg/m2).
Biochemical profiles
OGTT
Insulin sensitivity
Body composition
Regional fat distribution
Statistical analysis
The mean values of abdominal visceral and subcutaneous adipose tissues were 120.9 ± 65.4 and 250.6 ± 97.4 cm2, respectively, and the mean midthigh muscle areas were 106.1 ± 28.6 cm2 for normal density muscle and 17.3 ± 6.8 cm2 for low density muscle, respectively (Table 2).
Fasting serum triglyceride levels and free fatty acid levels were found to be correlated with the HOMA scores (r = 0.354, P < 0.01 and r = 0.553, P < 0.01; Fig. 1). Fasting serum insulin levels were significantly related to BMI (r = 0.250, P < 0.05), the abdominal visceral fat area (r = 0.405, P < 0.01), and the midthigh low-density muscle area (r = 0.532, P < 0.01). Fasting serum free fatty acid levels were correlated with the midthigh low-density muscle area (r = 0.272, P < 0.05).
The abdominal subcutaneous fat area did not correlate with the HOMA score, but the abdominal visceral fat area was well correlated with the HOMA score (r = 0.471, P < 0.01; Fig. 2A). The ratio of visceral to subcutaneous abdominal fat area also correlated with the HOMA score (r = 0.434, P < 0.01; Fig. 2B). The midthigh low-density muscle area was found to be linearly correlated with the HOMA score (r = 0.513, P < 0.01; Fig. 2C). After adjusting for BMI and total body fat, the correlation between the low-density muscle area and the HOMA score remained valid (r = 0.451, P < 0.01 and r = 0.522, P < 0.01, respectively). Even after controlling for abdominal visceral fat area, the low-density muscle area correlated with the HOMA score (r = 0.399, P < 0.01).
Total body weight, BMI, and total fat mass were shown to decline with age (r = -0.352, P < 0.01; r = -0.362, P < 0.01; and r = -0.402, P < 0.01; respectively). The abdominal subcutaneous fat area decreased with age (r = -0.438, P < 0.01), but the ratio of visceral to subcutaneous abdominal fat increased significantly with age (r = 0.369, P < 0.01), without a corresponding statistically significant increase in the abdominal visceral fat area. Midthigh subcutaneous fat areas decreased with age (r = -0.572, P < 0.01), but neither the low-density muscle area nor the normal-density muscle area was significantly correlated with age. Although the BMI was identical in both sexes, the percentage of total body fat was higher in women (39.3 ± 6.0 vs. 28.5 ± 8.2%, P < 0.01) and the percentage of fat-free mass was higher in men (P < 0.01). Fasting serum insulin levels and the HOMA score in both sexes were identical. The abdominal visceral fat area was greater in men than in women (172.6 ± 83.5 vs. 98.0 ± 38.3 cm2, P < 0.01), but the abdominal and midthigh subcutaneous fat area was greater in women (P < 0.01). The midthigh low-density muscle area was similar in both sexes (18.5 ± 8.1 vs. 16.7 ± 6.2 cm2, NS), but the normal density muscle area was greater in men than in women (P < 0.01). In men, the abdominal visceral fat area was related most closely to the HOMA score (r = 0.652, P < 0.01), and this correlation persisted after adjustment for BMI and total fat mass (r = 0.53, P < 0.05 and r = 0.67, P < 0.01, respectively). In women, only the midthigh low-density muscle area was correlated with the HOMA score (r = 0.59, P < 0.01) independent of BMI, total fat mass, or abdominal visceral fat area. Postmenopausal women had a greater abdominal visceral fat area than premenopausal women (113.0 ± 35.0 vs. 89.5 ± 38.0 cm2, P < 0.01), but the abdominal and midthigh subcutaneous fat areas were greater in premenopausal women than in postmenopausal women (P < 0.05). The midthigh low-density muscle area was identical in both groups, but the normal-density muscle area was greater in the premenopausal women than in the postmenopausal women (104.9 ± 14.7 vs. 86.0 ± 23.6 cm2, P < 0.01). Patients with impaired glucose tolerance had higher HOMA scores (3.3 ± 2.4 vs. 1.7 ± 1.2, P < 0.01) and greater abdominal visceral fat and midthigh low-density muscle areas (143.0 ± 80.6 vs. 107.0 ± 49.8 cm2, P < 0.05 and 19.1 ± 6.9 vs. 16.1 ± 6.5 cm2, P = 0.06, respectively) than those with normal glucose tolerance. Men and postmenopausal women showed a greater glycemic response to a 75-g oral glucose load than premenopausal women (7.9 ± 1.1 and 7.8 ± 0.9 vs. 7.2 ± 1.0 mmol/l, respectively, P < 0.05).
Recent evidence suggests that low-density muscle is closely linked to insulin resistance in obese Caucasian patients (6,13,14). The present study confirms that low-density muscle is strongly associated with insulin resistance in nondiabetic obese subjects in Korea. To evaluate regional fat distribution, we used single-slice images from the midabdomen at L4L5 and from the midthigh, which have been frequently used to measure the quantity and distribution of adipose tissue in the thigh in many other studies (6,12,15). Goodpaster et al. (16) showed that skeletal muscle attenuation by single-slice CT scans well demonstrate muscle fiber lipid content in percutaneous biopsy specimens. Therefore, skeletal muscle attenuation in vivo as determined by CT may provide valuable information about the association between muscle composition and muscle function. The abdominal visceral fat area increased with age in both sexes; a more marked fat redistribution was noted in women (r = 0.382, P < 0.01). Both hormonal and body compositional changes occur with aging, primarily due to a decrease in lipolytic activity and the consequent prevalence of liposynthesis, resulting in visceral fat accumulation (17). Diminished leptin action or leptin resistance was proposed to explain the metabolic decrease associated with aging. Previous investigators have asserted that age is an independent predictor of low-density muscle, which is associated with insulin resistance in obesity irrespective of sex or age (18,19). A recent study based on muscle biopsies of obese children demonstrated that skeletal muscle triglyceride stores are not a consequence of aging but occur early in the natural course of obesity (20). Many differences in anthropometric and biochemical parameters, body composition, and regional fat distribution were observed in both sexes in our study, suggesting hormonal effects. Because circulating leptin levels are known to be strongly related to the percentage of body fat and because leptin values in women are twice those in men, higher estrogen levels in women might be responsible for the sexual dimorphism of leptin concentrations, but this hypothesis has not been confirmed. Our results suggest that the menopause transition is associated with an accumulation of visceral adipose tissue. The menopause transition is associated with a reduction in resting metabolic rate, physical activity, and fat-free mass and an increase in fat mass and abdominal adipose tissue accumulation. Abdominal visceral adipose tissue was found to be a more important predictor of insulin resistance than low-density muscle in postmenopausal women in our study, which is consistent with the findings of previous studies (21,22), whereas low-density muscle seems to play a key role in premenopausal women. Few studies have been conducted in the Asian populations with regard to racial differences in body composition, regional fat distribution, and insulin resistance (2328). Our results show that abdominal visceral adiposity and low-density muscle are related to insulin resistance in mildly obese Korean subjects (under grade II). The prevalence and severity of obesity and related complications are reported to be relatively low in Korea compared with westernized countries. The prevalence of abdominal or visceral obesity may be much higher, although only preliminary data of a small population are currently available (29,30). It is clear that the global issues of obesity per se and abdominal obesity are relevant in Korea and should not be overlooked. Moreover, the significance of insulin resistance and its determinants should be emphasized in the Korean population, because Koreans are more susceptible to glucose intolerance and diabetes. The maximal secretory function of ß-cells is relatively low, and consequently, even slight stresses on ß-cells, including mild obesity, can substantially disrupt the metabolic balance and prematurely induce the overt failure of ß-cells to meet the metabolic demands of insulin in obese patients (31,32). This hypothesis provides presumptive evidence that may explain recent data concerning the rapidly increasing prevalence of metabolic syndrome and type 2 diabetes in Korea. Further global epidemiologic studies are needed if we are to fully understand metabolic disparities with respect to the different ethnicities. Previously, Greco et al. (33) emphasized the role of abnormal fat deposition within skeletal muscle on obesity-related insulin resistance. They found that lipid deprivation selectively depletes intramyocellular lipid stores and induces a normal metabolic state. Potential mechanisms for this association include apparent defects in fatty acid metabolism at the mitochondrial level in obese individuals with type 2 diabetes. Substantial evidence indicates that perturbations in fatty acid oxidation are involved in the accumulation of skeletal muscle triglyceride and the pathogenesis of insulin resistance. Moreover, recently acquired knowledge of insulin receptor signaling indicates that the accumulation of lipid products within skeletal muscle can interfere with insulin signaling and finally produce insulin resistance (6,34). Although low-density muscle accounts for a relatively small portion of the total skeletal muscle, it seems to be a valuable marker of insulin resistance in the Korean population. From our data and previous epidemiologic data, we speculate that the significance of low-density muscle as well as visceral adipose tissues deserves much consideration with regard to genetically determined low ß-cell capacity. The mechanisms whereby triglyceride contents within the skeletal muscle provoke insulin resistance should be investigated further.
This study was supported by the BK 21 Project for Medical Science, Yonsei University. We thank Professor J. M. Nam, PhD, for statistical advice and Dr. John Roberts, of Yonsei Medical College, for helpful discussion and critical reading of the manuscript in this study.
Address correspondence and reprint requests to Prof. Dr. Chul-Woo Ahn, Division of Endocrinology and Metabolism, Department for Internal Medicine, Yong-dong Severance Hospital, Yonsei University, College of Medicine, 146-92, Dogok-dong, Kangnam-ku, P.O. Box 135-720, Seoul, Korea. E-mail: acw{at}yumc.yonsei.ac.kr. Received for publication 21 November 2002 and accepted in revised form 23 February 2003. A table elsewhere in this issue shows conventional and Système International (SI) units and conversion factors for many substances.
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