The number of fat cells
Fat cells are where the body stores excess energy. In a typical fat cell, the nucleus and other organelles occupy a small space to one side while the cell's main volume is given over to fat storage — and the cell volume can grow or shrink with weight.
The number of fat cells is fixed at puberty. When adults gain weight, the cell count does not increase; the cell diameter does. An important finding in obesity is that obese children end puberty with more fat cells than normal-weight children. Even if these children lose weight in adult life, the cell count stays the same — having extra storage capacity is one of the factors that makes obese children likely to remain obese.
How fat is distributed in the body
Fat distribution differs between women and men. In women, fat collects more under the skin; in men, it gathers around the organs inside the abdomen. As a result, the belly swells in men but there is little subcutaneous accumulation — which is why liposuction can make little difference in men, since it is a technique that removes subcutaneous fat.
Women have genetically determined body types, named after the regions where subcutaneous fat collects. Barely noticeable at normal weight, these differences emerge with weight gain. In the android (apple) type the waist widens; in the gynoid (pear) type fat collects on the hips, and the breasts may also be large. In the thyroidal type the outer thighs and hips gain fat with breasts usually not very large, while in the lymphatic type only the legs widen, including the ankles.
How liposuction affects fat distribution
It used to be believed that removing fat along with its cells during liposuction reduced the total cell count. But recent studies have shown that stem cells within the fat tissue restore the fat-cell number towards the fixed level set at puberty. However, this replacement happens across the body as a whole, while the regionally reduced fat stays low. So people who gain weight after liposuction may find the fat appears in different areas rather than the treated one — and the fat cells remaining in the treated area can expand by increasing their cell diameter.
