Adipose Tissue Mass

views updated

Adipose Tissue Mass

Adipose tissue is a specialized form of connective tissue that acts to support, connect, or protect organs. It consists of triglycerides, which is are lipids that comprises a glycerol backbone with three associated fatty acids.

White adipose tissue that surrounds organs helps provide cushioning to the kidneys and the lymph glands. Adipose tissue also serves an important function as an insulator against cold. The insulation property is due to subcutaneous adipose tissue, which is located in the subcutaneous layer of the skin. The adipose tissue does not conduct heat nearly as readily as do other tissues.

The degree of insulation depends on the amount of subcutaneous adipose tissue that is present. In athletic endeavors, when insulation is not as critical due to the generation of heat during exercise, adipose tissue mass in athletes can be less than in non-athletes who rely on the tissue's insulation property. For example, excess adipose tissue translates to more body weight and increased exercise effort for a marathon runner or cross-country skier. In contrast, mountain climbers benefit from the increased insulation afforded by an increased supply of adipose tissue.

Adipose tissue also plays an important role as an energy source. Energy stored as adipose tissue represents a reservoir that can be drawn on when other, more easily utilizable sources of energy (carbohydrate and protein) have been exhausted. Little water is present in adipose tissue; as a result, the energy available per gram of fat is more than double the energy available per gram of either carbohydrate or protein.

This energy store is not, however, as easily accessible other energy sources such as dietary glucose. The human body is not efficient in the conversion of lipid to carbohydrate. As well, the brain normally uses carbohydrates as the energy source. Finally, foods provide a ready supply of carbohydrate and protein, which will be utilized first. Excess dietary carbohydrate and protein that is not immediately utilized can be converted to fat and ultimately stored as adipose tissue. In a healthy person, this backup energy source is advantageous. For example, a marathon runner will draw on adipose tissue as the more readily accessible energy sources are exhausted during the course of the run. However, over time, the continued intake of excess energy can lead to the creation of the plentiful adipose tissue that is a hallmark of obesity.

The reason that white adipose tissue is an excellent energy reservoir is explainable by its structure. Up to 85% of the weight of a cell is occupied by lipid. The lipid occupies almost all the volume of a cell, with the nucleus and mitochondria typically being relegated to the periphery.

The process whereby fat is liberated from adipose tissue is termed lipolysis. In lipolysis, the trigly-cerides in the adipose cells are enzymatically broken down to free the fatty acids from glycerol. The fatty acids are then available for use in muscle regeneration or as a source of energy.

The distribution of adipose tissue in the body varies from person to person. In general, adipose tissue tends to be located more on the upper body in men than in women. This pattern is variously called an android, central, male, upper-body segment, or "apple" distribution. The latter term denotes the body shape. Correspondingly, women, who generally have more adipose tissue on the lower body, are described as having producing a "pear" distribution of adipose tissue (other terms include gynoid, female, and lower-body segment).

Aside from gender, ethnicity, and diet, the distribution of adipose tissue and even the amount that accumulates over time can be dictated in part by an individual's genetic makeup. In females, the physiological changes that accompany menopause influence adipose tissue distribution, with a post-menopausal shift to an upper body distribution. Exercise is an important means of reducing the accumulation of adipose tissue.

Shedding adipose tissue as a consequence of athletic activity can bring an unhealthy risk. Since approximately four percent of body fat is vital to maintain organ function, having too little fat can be a health hazard. The standard measure of body fat is termed the body mass index (BMI), which is calculated by multiplying body weight in pounds by 700 and then dividing the results by the square of the height in inches. A BMI lower than 18 is considered a risk factor for premature death. Elite athletes will monitor their BMI during the normal course of training and alter their diet to compensate for a decreasing reserve of fat.

see also Body fat.