Regional Differences in Fat Distribution and Physiology
The visceral fat depot is contained within the body cavity surrounding the internal organs and is composed of mesenteric and omental depots. Visceral fat represents about 20 and 6% of total body fat in men and women, respectively.
The subcutaneous fat depot is located under the skin, particularly in the abdominal region. In the lower body, all adipose depots are subcutaneous and the largest sites of storage are the gluteal and femoral regions [20]. Visceral and subcutaneous fat express and secrete various amounts of different cytokines. Leptin is mainly secreted by peripheral fat, while adi- ponectin and Interleukin-6 are secreted in higher quantities by visceral fat. For TNF-α, the results are less clear. In addition, visceral fat is more sensitive to catecholamines, and therefore to lipolysis, and more resistant to insulin than subcutaneous fat [21].It has long been known that glucocorticoids act on adipose tissue and can inhibit proliferation but enhance lipid storage leading to fat hypertrophy. Visceral adipocytes, which contain higher levels of glucocorticoid receptors, are more sensitive to the effect of this hormone. Accordingly, in Cushing’s syndrome with hypercorticism, visceral fat is particularly increased. More recently, profound differences have been demonstrated in the level of the enzyme that can convert inactive cortisone to active cortisol, 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1), with very high activity in visceral fat and barely detectable activity in subcutaneous fat [20, 22]. Therefore, visceral fat is able to produce cortisol locally, which could act inside adipocytes and increase lipid accumulation. Interestingly, this enzyme is activated by TNF-α (Fig. 4).
Fig. 4 Pathophysiology of visceral fat hypertrophy: hypothetical mechanisms
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