The regulation of adipose tissue distribution in humans.

The regulation of adipose tissue distribution is an important problem in view of the close epidemiological and metabolic associations between centralized fat accumulation and disease. With visceral fat accumulation multiple endocrine perturbations are found, including elevated cortisol and androgens in women, as well as low growth hormone (GH) and, in men, testosterone (T) secretion. These abnormalities probably derive from a hypersensitive hypothalamo-pituitary-adrenal axis, with hyperinsulinemia related to a marked insulin resistance as a consequence. These hormonal changes exert profound effects on adipose tissue metabolism and distribution. At the adipocyte level cortisol and insulin promote lipid accumulation by expressing lipoprotein lipase activity, while T, GH and probably estrogens exert opposite effects. The consequences will most likely be more expressed in visceral than subcutaneous adipose tissues because of a higher cellularity, innervation and blood flow. Furthermore, the density of cortisol and androgen receptors seems to be higher in this than other adipose tissue regions. The endocrine perturbations found in visceral obesity with an abundance of the lipid accumulating hormones cortisol and insulin, and a relatively low secretion of the lipid mobilizing sex steroid hormones and GH would therefore be expected to be followed by visceral fat accumulation. The potential significance of local synthesis of steroid hormones in adipose tissue requires more attention. Although studies in vitro are informative when elucidating detailed mechanisms of hormonal interactions, they might not give a true picture of the regional integrated regulation of adipose tissue lipid storage and mobilization. Such information can be obtained by regional measurements of lipid mobilization by free fatty acid turnover or by microdialysis techniques, both showing lower rates of mobilization in leg than in upper body adipose tissues. More detailed information can be obtained by physiological oral administration of triglycerides, labelled with a small amount of oleic acid, followed by measurements of the regional uptake and turn-over of adipose tissue triglycerides. Such studies show lipid uptake in the order omental = retroperitoneal > subcutaneous abdominal > subcutaneous femoral adipose tissues in men, with a similar rank order for half-life of the triglyceride, indicating also a turn-over of triglycerides in that order. T amplifies these differences in men. In premenopausal women subcutaneous abdominal has a higher turnover than femoral adipose tissue. Results of studies in vitro indicate that this difference is diminished at the menopause, and restored by estrogen substitution, suggesting that the functional effects of estrogens in women are similar to those of T in men. The mechanisms are, however, probably indirect because of the apparent absence of specific estrogen and progesterone receptors in human adipose tissue. This interpretation from the studies referred to above fits well with physiological, and clinical conditions with increased visceral fat mass, where the balance between the lipid accumulating hormone couple (cortisol and insulin) and the hormones which prevent lipid accumulation and instead activate lipid mobilization pathways (sex steroid hormones and GH) is shifted to the advantage of the former. Such conditions include Cushing's syndrome, the polycystic ovary syndrome, menopause, aging, GH-deficiency, depression, smoking and excess alcohol intake. With appropriate interventions against hypercortisolemia and substitution of deficient sex steroids and GH, visceral fat mass is decreasing. Based on this evidence from physiological, clinical, interventional observations and detailed studies of mechanisms at cellular and molecular levels it is suggested that the combined endocrine abnormalities in the syndrome of visceral obesity direct storage fat to visceral adipose depots. Therefore, measurements of visceral fat accumulat