Majority of the fat in the body is deposited in either of the two forms: visceral fat or subcutaneous fat. Although, both comprise of fat containing adipocytes, still they have different significance on the future health events of a person.
Subcutaneous fat occurs below the skin and in between the skeletal muscles (intermuscular fat). Fat occurring between organs and inside the organs is referred to as visceral fat. Visceral fat is also referred to as intra-abdominal fat because most of it occurs inside the abdomen. However, some visceral fat also occurs in the pelvic region. Therefore, subcutaneous fat is mainly seen in limbs where as the visceral fat is mainly seen as abdominal obesity or central obesity ("pot belly" or "beer belly"). The body habitus for them are "pear shaped" (relative excess of subcutaneous fat in hips, buttocks and limbs) and "apple shaped" (relative excess of abdominal fat) respectively.
Abdominal obesity or increased visceral fat occurs due to imbalance between caloric intake and expenditure. The excess calories are deposited as fat in the body. The exact cause of this imbalance and subsequent obesity is not yet known completely. Excess visceral fat may also be deposited secondary to some disease (e.g. Cushing's Syndrome, Polycystic Ovary Syndrome). Toxicity due to prolonged therapy with corticosteroids (for example in asthma, allergies, etc.) results in Cushing's Syndrome and "apple shaped" body habitus with central obesity is one of most common presenting features of corticosteroid toxicity.
The two most popular methods to measure abdominal obesity and gauge the amount of visceral fat are absolute waist circumference and waist-hip ratio. Absolute waist circumference is simply the circumference of waist and acceptable values are <102 cm for males and <88 for females. Waist-hip ratio is ratio of circumference of waist to that of hip and acceptable values are <0.9 for men and <0.85 for women.
Compared to subcutaneous fat, excess visceral fat has a much stronger correlation with adverse metabolic and cardiovascular disease outcomes. It is associated with increased risk of Type 2 Diabetes Mellitus and abnormalities in blood lipid profile (dyslipidemias). It is also associated with increased risk of hypertension, coronary artery disease and heart attack. Increased visceral fat is also associated with an increased risk of Alzheimer's disease and asthma. In males, excess visceral fat is associated with relatively lower testosterone levels in the blood.
All treatments and preventive measures are based on correcting the imbalance between caloric intake and expenditure. For prevention, the both should be balanced and for treatment, the caloric expenditure should be more than intake till the desired weight is achieved.