Body weight is the sum of bone, muscle, organs, body fluids, and adipose tissue. All of these components are subject to normal change as a reflection of growth, reproductive status, variation in exercise levels, and effects of aging. Maintaining a constant body weight is a complex system of neural, hormonal and chemical mechanisms that keeps the balance between energy intake and energy expenditure within fairly precise limits. Abnormalities of these mechanisms result is weight fluctuations and the most common are overweight and obesity. The inability to gain weight can also be a problem. In the elderly or in children unintentional weight loss can be especially detrimental and should be assessed and treated early to prevent malnutrition.
The body contains of fat mass – the fat from all body sources including the brain, skeleton, and adipose tissue, and the fat free mass. Fat free mass consists of water, protein and mineral components.
The range of total body fat expressed as a percentage of total body weight that is associated with optimum health is 8% to 24% in males and 21% to 35% in females. Professional and elite athletes have body fats much lower than those of the average person. Fat in the body is categorized as either essential or storage. Essential fat, which is necessary for normal physiologic functioning, is stored in small amounts in the bone marrow, heart, lung, liver, spleen, kidneys, muscles, and lipid-rich tissues in the nervous system. In women essential fat is higher, because it also includes sex-specific body fat in the breasts, pelvic regions and thighs.
Storage fat is accumulates under the skin and around the internal organs to protect them from trauma. It is the primary energy reserve of the body and stored as triglycerides in depots made up of adipose tissue. Adipose tissue is located primarily under the skin. There are two types of adipose tissue: white and brown. White adipose tissue stores energy as a repository for triglycerides, serve as a cushion to protect abdominal organs, and insulates the body to preserve the heat. Brown adipose tissue seen in infants and in very small amounts in adults, regulates energy expenditure. Most storage fat is expendable. The totality of fat stores in adipocytes is capable of extensive variation, thus allowing for changing requirements of growth, reproduction, and aging as well as fluctuations in environmental and physiologic circumstances such as the availability of food and the demands of physical exercises.
The greatest level of fatness in normal growth occurs at the age of 6 months. In lean children, fat cell size then decreases, but this decrease doesn’t occur in obese children. Fat cell number increases in both lean and obese children throughout childhood into adolescent, but the number increases faster in obese children. After adolescence, increases in body fat occur primarily by an increase in fat cell size. The number of fat cells can increase throughout life, however, cell numbers do not increase until maximum cell size is reached. The number of cells doesn’t decrease with weight loss. Prevention is the key, because once fat is gained and maintained over time, it is more difficult to lose.
Most depot fat comes directly from dietary triglycerides. Excess dietary carbohydrate and protein are also converted to fatty acids in the liver.
Under normal feeding conditions little dietary carbohydrate is used to produce adipose tissue, and it requires about three times as much energy to convert excess energy from carbohydrate to fat storage as it does to convert excess energy from dietary fat to fat storage. However, carbohydrate energy makes individuals fatter by suppressing fat oxidation.
Total calories remain critical as the variable for weight management.