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 and number (the latter is most important). 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. A major source of renewal of fat cells throughout life is bone marrow. This is most apparent in obesity when ~ 20% of all fat cells are derived from bone marrow. Fat cell turnover is also instrumental for the size of fat cells.
Low turnover may cause large fat cells which, in turn, are linked to cardiovascular disease and type 2 diabetes. There is also a rapid turnover of fat cell lipids, which are renewed about 6 times during the life span of 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 different forms of adipose tissue (few large or many small fat cells) and also for metabolic disturbances.