Visceral Adipose Tissue (VAT)

Visceral adipose tissue is the fat inside the abdominal cavity, the omental, mesenteric and retroperitoneal depots sitting among the organs, as distinct from subcutaneous fat beneath the skin. It differs functionally too: visceral adipocytes are more lipolytically active, less responsive to insulin's suppression of lipolysis, more densely infiltrated by macrophages, and secrete more interleukin-6. The omental and mesenteric portions drain into the portal vein, delivering fatty acids and cytokines straight to the liver.

Quantification requires cross-sectional imaging. Computed tomography or magnetic resonance at a single lumbar slice is the standard, with absorptiometry android estimates and waist circumference as cruder proxies. Visceral fat correlates with insulin resistance, liver fat and cardiometabolic risk independently of body mass index, the observation underlying both metabolically healthy obesity and metabolically unhealthy normal weight. Ectopic fat in liver, muscle, pericardium and pancreas behaves similarly and tends to accumulate with it.

This is the strongest argument for measuring fat distribution rather than weight alone. Two people at the same BMI with different visceral burdens carry different metabolic risk, and visceral fat tends to fall early in weight loss, part of why glycaemic and hepatic markers improve before much weight has gone.

Whether visceral fat causes the risk or marks it remains unsettled: the portal free fatty acid hypothesis has serious critics, and surgical omentectomy has not produced the metabolic benefit a causal role would predict. The commercial misuse is blunter. No drug, food or exercise selectively empties one depot, so anything advertised as targeting belly fat claims a mechanism that does not exist, and impedance scales displaying a visceral fat rating are reporting an equation output, not a measurement.