Adiposity

Adiposity is how much fat a body carries and where it sits. Adipocytes store energy as a single triglyceride droplet and expand by hypertrophy first, then by recruiting new cells. The tissue is not inert padding: it secretes leptin in rough proportion to fat mass, adiponectin in inverse proportion, and, as depots enlarge and attract macrophages, inflammatory mediators including tumour necrosis factor alpha and interleukin-6.

Fat mass is measured rather than inferred. Dual-energy X-ray absorptiometry partitions the body into fat, lean soft tissue and bone mineral, while bioelectrical impedance estimates it from conductivity and is far less reliable. Healthy adult men sit roughly in the ten to twenty percent range and women around ten points higher, the difference being essential fat rather than excess. In the body composition substudy of the sixty-eight-week STEP 1 semaglutide trial, the fat proportion of body mass fell and the lean proportion rose, although absolute lean mass declined as well.

Distribution matters more than the total. Two people with identical fat mass can differ sharply in insulin sensitivity, liver fat and cardiovascular risk depending on how much of it is visceral rather than subcutaneous. The same point reframes weight change: losing six kilograms is a different event if it is mostly fat than if a third is skeletal muscle.

The recurring error is precision without accuracy. Consumer impedance scales report body fat to a tenth of a percent while shifting several percentage points with hydration, recent meals or skin temperature, so a week-to-week change on one is usually water. The opposite error is treating any weight loss as fat loss; a trial reporting only kilograms cannot distinguish them, and compounds marketed as fat-specific rarely rest on a composition endpoint at all.