Body fat turned out to be a hormone-making organ, not inert padding
For a long time, fat was thought of as a passive store, a cushion that also kept the cold out. It is now recognised as a major endocrine organ, pumping out leptin, oestrogen, resistin and inflammatory signals. Where it sits in the body changes what it does and how much it matters.
Adipose tissue is loose connective tissue made mostly of fat cells, the adipocytes, mixed with precursor cells, fibroblasts, blood vessel linings and immune cells. Its chief job is storing energy as lipids, which the body can burn when needed, while also shielding organs and insulating skin. Human fat tissue is 61 to 94 percent lipid by content. There are two basic kinds: white fat stores energy, and brown fat produces heat. The Swiss naturalist Conrad Gessner first described adipose tissue, specifically the brown variety, in 1551.
Fat collects in distinct depots: under the skin, around internal organs, inside bone marrow, between muscles and in breast tissue. Location makes a big difference. Visceral fat packed between the stomach, liver and intestines is semi-fluid and metabolically distinct, and excess amounts are linked with type 2 diabetes, insulin resistance and heart disease. Subcutaneous fat in the hips and thighs is not tied to those classic problems, and some evidence hints it may even be protective. A special visceral pad around the heart, epicardial fat, releases molecules that may influence the heart's function.
Hormones steer where fat goes. Oestrogen directs storage to the hips, thighs and buttocks, which is why the typical female pattern differs; after menopause, as ovarian oestrogen falls, fat shifts toward the waist. Men tend to store more in the abdomen. Insulin controls traffic in and out: when it is high, fatty acids move into fat cells, and only when it drops can they leave. Fat is also a leading source of aromatase, the enzyme that makes the hormone estradiol, in both sexes.
Some depots are odd. Marrow fat grows during calorie restriction, a trait that sets it apart from other depots. Ectopic fat accumulates in organs meant to hold little, such as the liver and pancreas, and interferes with how they work. And fat has become a surprisingly useful source of adult stem cells, which can be reprogrammed without the ethical questions attached to embryonic cells.
Source: Adipose tissue
