Adipose tissue is the main triglyceride deposit in mammals, including humans. It consists of a set of numerous cells, called adipocytes, which are responsible for the synthesis of triglycerides and their release in the form of glycerol plus fatty acids. The orientation of the adipocyte towards one or another metabolic path depends above all on the nutritional state of the organism.
In an individual of 70 kg the amount of fat accumulated in adipose tissue is 11-15 kg, a sufficient caloric reserve for about two months of life.
Below the skin adipocytes clump together to form a more or less thick layer, commonly called subcutaneous adipose tissue or hypodermis.
The thickness and distribution of these panniculus adiposus depends on various factors, some of which are very obvious. For example, in some areas of the body the panniculus adiposus is reduced (hands, feet, forearm), especially when compared with those present in other regions (abdomen, hips, buttocks).
The thickness and distribution of subcutaneous adipose tissue is different in the two sexes. In women, for example, the adipose mass is generally concentrated in the hips, buttocks, thighs and abdomen below the navel (gynoid obesity). In fact, nature has wisely wished that the lipid stocks were distributed in areas essential for completing the pregnancy, even in conditions of famine.
In men an android type distribution prevails (fat mass concentrated in the face, neck, shoulders and above all in the abdomen above the navel). This last condition is more dangerous than the previous one, because an excess of androgenic adipose tissue is associated with higher levels of glycaemia, triglycerides and blood pressure.
The extent of fat masses also depends on the state of nutrition: it increases in case of excessive caloric intake (obesity) and decreases in the presence of chronic nutritional deficits. Finally, in welfare societies, adipose tissue is generally higher in the elderly than in adolescents and young adults.
Many people believe that once fat is accumulated in certain areas of the body it is impossible to remove it. In reality the triglycerides of adipose tissue are not stationary, but are constantly mobilized and redeposited, renewing themselves every 10-15 days.
Classic functions of adipose tissue
Energy reserve
Impact protection and mechanical support for various organs
Modeling of the body shape
Thermal insulation (white adipose tissue); increased body temperature and disposal of excess food in the form of heat (brown adipose tissue).
To learn more: fat functions; adipokines; white and brown adipose tissue (adipose tissue is no longer simply considered an inert deposit, but a real endocrine organ).
Increased adipose tissue
The increase in fat masses can occur in two ways:
for hyperplasia: increase in the number of adipocytes
for hypertrophy: increase in the lipid content of each adipocyte
Until a few years ago, adipocyte hyperplasia was thought to occur only in infancy. Today we know that this phenomenon can also occur in adults, especially when we pass from moderate to overweight to a condition of obesity. The confirmation of this phenomenon, in part intuitive, has been given by the discovery of preadipocytes, undifferentiated cells immersed in the adipose tissue of adults. These cells retain the ability to divide and, if stimulated and activated, to originate new adipocytes. Once formed, these new fat cells will remain so until the death of the individual, they may then increase or decrease in volume but not in number.
According to the lipostatic theory, which sees the reduction of the lipid content of adipocytes as the main factor stimulating appetite, a large number of empty fat cells would be responsible for uncontrolled hunger attacks, which very often frustrate the slimming diets proposed to obese people.
It is therefore important to prevent an excessive increase in adipose tissue and in the number of adipocytes, especially in children, where this phenomenon would condemn them, with high probability, to remain obese for the rest of their lives.
In a normal-weight subject the number of fat cells is around 25-30 billion, in obese subjects this value increases on average between 40 and 100 billion.
CONTINUE: white and brown adipose tissue »
