The metabolic syndrome--a neuroendocrine disorder?

Central obesity is a powerful predictor for disease. By utilizing salivary cortisol measurements throughout the day, it has now been possible to show on a population basis that perceived stress-related cortisol secretion frequently is elevated in this condition. This is followed by insulin resistance, central accumulation of body fat, dyslipidaemia and hypertension (the metabolic syndrome). Socio-economic and psychosocial handicaps are probably central inducers of hyperactivity of the hypothalamic-pituitary adrenal (HPA) axis. Alcohol, smoking and traits of psychiatric disease are also involved. In a minor part of the population a dysregulated, depressed function of the HPA axis is present, associated with low secretion of sex steroid and growth hormones, and increased activity of the sympathetic nervous system. This condition is followed by consistent abnormalities indicating the metabolic syndrome. Such 'burned-out' function of the HPA axis has previously been seen in subjects exposed to environmental stress of long duration. The feedback control of the HPA axis by central glucocorticoid receptors (GR) seems inefficient, associated with a polymorphism in the 5' end of the GR gene locus. Homozygotes constitute about 14% of Swedish men (women to be examined). Such men have a poorly controlled cortisol secretion, abdominal obesity, insulin resistance and hypertension. Furthermore, polymorphisms have been identified in the regulatory domain of the GR gene that are associated with elevated cortisol secretion; polymorphisms in dopamine and leptin receptor genes are associated with sympathetic nervous system activity, with elevated and low blood pressure, respectively. These results suggest a complex neuroendocrine background to the metabolic syndrome, where the kinetics of the regulation of the HPA axis play a central role.