III.2.A. Adipose tissue distribution

The main components of the adipose tissue are adipocytes. The most important functions of adipose tissue:

• energy storage
• protection of the body against physical trauma, such as mechanical effects or cold
• a covering for various organs
• storage of various molecules (e.g. vitamin E)

The two main types are the white adipose tissue and the brown adipose tissue.

1. White adipose tissue
   1. Its main function is energy storage and release (release of TG).
   2. The percentage of body fat in men is  ̴ 20% and in women is  ̴ 30%.
   3. Sympathetic innervation:
      1. the sympathetic activation increases during eating;
      2. the neurotransmitters E and N increase the release of TG via the β-adrenergic receptor;
      3. E inhibits insulin-stimulated Glu uptake in the muscles, and increases Glu uptake in the lack of insulin.
2. Subcutaneous adipose tissue
   1. The role of long-term energy storage.
   2. The main source of the hormone leptin.
   3. It can be estimated through measurement of the skinfold thickness.
3. Visceral (central) adipose tissue
   1. It has a role of transitional storage.
   2. It is measured by abdominal computed tomography (CT).
   3. Hormones and metabolites produced by the visceral adipose tissue directly affect the liver and the pancreas, because they are released through the hepatic portal vein.    
   4. Types:
      1. omental adipose tissue, which is innervated by β-adrenergic nerves;
      2. visceral adipose tissue, the deeper depots.
4. Protective storage
   1. Surrounding the joints, in the soles, palms and orbit.  
5. Brown adipose tissue
   1. Its main role is heat generation.
   2. It surrounds the abdominal aorta, kidneys, trachea and esophagus.
   3. It is characterized by sympathetic innervation and dense vascularization.

The body fat content varies widely throughout life: it is  ̴12% at birth, and rises to 30% at 6 months of age. It falls to  ̴18% at 10 years of age. There is a difference between women and men in the normal range of body fat content in adulthood.  The normal range in men is 15–18% and in women is 20–25%. There is a large difference in visceral fat content between men and women: 20% in men and 6% in women.

Two adipogenous factors are responsible for the differences in fat: the nuclear peroxisome-proliferator-activated receptor gamma (PPAR-γ) and the cytosine-cytosine-adenine-adenine-thymine enhancer binding protein-α (C/EBP-α).

PPAR-γ

Sources: liver, heart, skeletal muscle and kidney.

Roles:  regulation of the β-oxidation of fatty acids;

            decrease of TG,

            increase of HDL cholesterol level;

            its level is increased by fasting.

C/EBP-α.

The distribution of fat may be changed by external factors such as physical exercise, or the administration of drugs or hormones.