Role of muscle morphology in the development of insulin resistance and metabolic syndrome.

Type IIB muscle fibres are among the most insulin-insensitive muscle fibres and are not adapted to oxidation of fat during muscle work. The first characteristic of this type of muscle fibre most probably reflects or contributes to further development of insulin resistance contribute to further perpetuation of obesity and to the channeling of excess free fatty acids to the liver followed by secondary deterioration of its function. The impaired functioning of the liver is epitomized, among other changes, by impairment of insulin extraction. The increasing hyperinsulinaemia is followed by inhibition of synthesis of specific proteins such as carrier proteins for transporting testosterone (sex hormone binding globulin, SHBG). This results in an increased free testosterone concentration which induces androgenization in women and may further increase insulin insensitivity in abdominal obesity in women. The poor capillarization and changed muscle morphology in spite of great interindividual variety is observed in several pathological conditions characterised by insulin sensitivity (stroke, PCO, hypertension, diabetes, obesity). It is suggested that, in addition to the previous concept of the main role of intraabdominal adipose tissue, even muscles and liver are also important organs contributing to the pathogenesis and development of the metabolic syndrome.