Fat Tissue Growth and Development in Humans.

Lipid storage and release from fat cells in adipose tissue are key factors in the regulation of the energy balance. During infancy and adolescence, adipose tissue is growing by a combination of increase in fat cell size (to a lesser extent) and (above all) the number of these cells. In adults, fat cell number is constant over time in spite of a large turnover (about 10% of the fat cells per year) when body weight is stable. A decrease in body weight only changes fat cell size (becoming smaller), whereas an increase in body weight causes elevation of both fat cell size and number in adults. An important source of renewal of fat cells during the entire life span is the bone marrow. This is most apparent in obesity when ∼20% of all fat cells are derived from the bone marrow. Fat cell turnover is also important for the size of fat cells. Low turnover may cause large fat cells which, in turn, is linked to cardiovascular disease and type 2 diabetes. There is also a rapid turnover of fat cell lipids, which constitute a single active pool and are renewed about 6 times during the life span of individual fat cells. Overweight and obesity are associated with decreased lipid turnover due to high input in combination with low output of lipids from the fat cells. Low fat cell lipid turnover is associated with insulin resistance and dyslipidemia. Thus, changes in the turnover of fat cells and their lipid content are important for the development of adipose tissue mass and its cellularity (fat cell size and number) and, in turn, for metabolic disturbances.