Skeletal muscle insulin resistance in morbid obesity: the role of interleukin-6 and leptin.

BACKGROUND: Although insulin resistance in obesity is established, the link between excess body fat and skeletal muscle insulin resistance is obscure. The aim of this study was to investigate whether cytokines secreted from the subcutaneous adipose tissue are related to the sensitivity of glucose metabolism to insulin in skeletal muscle. METHODS: A meal was given to 14 obese and 10 non-obese women. Plasma samples were taken for 360 min from a forearm vein and from the radial artery for glucose and insulin measurements. Interleukin-6, leptin, TNFα, resistin and adiponectin were measured preprandially from the radial artery and from the superficial epigastric vein. Forearm blood flow was measured with plethysmography. RESULTS: (1) In obese vs non-obese: (a) Glucose uptake by skeletal muscle was decreased (AUC (0-360)369 ± 55 vs. 877 ± 146 μmol/100 g tissue, p=0.001) (b) arterial interleukin-6 (2.5 ± 0.5 vs. 1 ± 0.1 pg/ml, p=0.013) and subcutaneous venous interleukin-6 (5 ± 0.5 vs. 3.4 ± 0.5 pg/ml, p=0.027) were increased (c) arterial leptin (63 ± 7 vs. 5 ± 0.6 ng/ml, p<0.0001) and subcutaneous venous leptin 80 ± 8 vs. 6.5 ± 0.7 ng/ml, p<0.0001) were increased. (2) Arterial interleukin-6 (p=0.002) and subcutaneous venous interleukin-6 (p=0.014) were negatively associated with forearm glucose uptake in obese. (3) No association was found between leptin and forearm glucose uptake, after correcting with fat mass. CONCLUSIONS: In morbid obesity: (1) Subcutaneous adipose tissue releases interleukin-6 which could then mediate insulin resistance in skeletal muscle. (2) Although there is increased secretion of leptin by the subcutaneous adipose tissue, leptin levels are not correlated to the sensitivity of glucose metabolism to insulin in muscle.