Body composition

60% water in men, 50% in women, body fat ranges from under 10% in well trained athletes to nearly 50% in obese patients, protein comprises 15% of body composition and mineral comprises 5.3% of normal body composition.

Body composition is the ratio of fat to nonfat (bone, muscle, and other tissue) in the body.

With aging the water content declines with 10-15% loss of total body water so that by age 80 years total body water constitutes only 45% of total body weight.

Two thirds of total body water is intracellular, i.e., in tissue cells.

One third of total body water is extracellular and is divided into the intravascular plasma compartment and the interstitial compartment.

Approximately 25% of extracellular fluid is composed of plasma volume and the remainder is interstitial fluid.

Interstitial fluid that is part of the extracellular fluid bathes cells.

About one third of total body water is extracellular and the remaining two thirds is intracellular water.

The average young adult male is made up of 18% body weight of protein, 7%, mineral. 15% fat, and 60% water.

Intracellular fluid accounts for 40% of body weight, while the extracellular component is about 20% of body weight.

Plasma makes up about 5% of body weight.

Total blood volume is about 8% of body weight.

Approximately 25% of the extracellular fluid compartment is in the vascular space and 75% is interstitial space.

Plasma volume equal to 5% of body weight, about 3500 cc for a 70 kg man.

Extracellular fluid volume difficult to measure as there is no specific definition for the space and other substances distribute in this space and include glandular secretions and gastrointestinal contents.

Transcellular fluids include the CSF and eye fluids.

Extracellular fluid volume is 20% of body weight or about 14 L in a 70 kg man (3.5 L for plasma and 10.5 L for interstitial fluid.

Small amounts of extracellular fluid, transcellular fluid, are in pleural, synovial, cerebrospinal, intraocular and peritoneal spaces.

Loss of body fluid with aging is limited to the intracellular compartment as the extracellular fluid volume is well maintained.

Interstitial fluid volume space cannot be measured directly and the volume can be calculated by subtracting the plasma volume of the extracellular volume.

The extracellular fluid volume/intracellular fluid volume ratio larger in infants and children than in adults, and accounts for more rapid and severe development of dehydration in children.

Intracellular volume cannot be measured directly but is calculated by subtracting extracellular fluid volume from total body water.

Water content of lean body is constant 71-72 mL/100 gm of tissue.

Fat is relatively free of water so that the ratio of TBW to body weight varies with the amount of fat present: total body water is lower in women than men, and decreases with age.

Patients with cancer experience body composition changes, including loss of skeletal muscle to the point of sarcopenia.
Body composition changes are estimated to play a role in approximately 20% of all cancer deaths.

Body composition changes occur as a result of factors which include deficits in intracellular anabolic signaling pathways involved in muscle and whole-body protein synthesis.
Weight loss correlates with worse cancer survival.

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