Body composition analysis can be used several ways. There are several different components to the body and body composition analysis can detect several of these components. In the health and fitness industry as well as the health and disease research, body composition is utilized to determine total body fat, distribution of body fat and internal vs. external body fat. The determination of body fat also helps prescribe target weights based on body composition instead of height-weight tables.

Great web site for body composition!

There are several compartment models for the human body. They range from a simple two compartment model to a four compartment model. The goal in body composition is to measure one of these compartments and assume that the relationship between the compartments is constant to estimate another compartment. In the clinical setting the fat compartment is most commonly measured because excess body fat is associated with modern chronic disease.


Even though it appears easy, the fat -free components can be

  • FFM - fat free mass
  • LFM - lipid free mass

Because lipids are essential components of every cell, lipid can included in fat free mass. If the total lipid component of the body were separated 10% of the lipid (Le) would be included in the FFM; 90% would be nonessential or storage of fuel (Ln).

As illustrated in the figure to the right, the LFM can be divided into mineral, protein and water.

In the classic cadaver analysis, the components are

  • FM - fat mass
  • TBW - total body water
  • Bone mineral
  • Non-bone mineral
  • Protein

These five components have been split into the two, three and four component models for body composition. The assumption is that these components are relative. With these models the measurement of one element of the component the whole body composition can be determined. However, each component can be divided into smaller components.

For example, this figure illustrates how the components can be divided based on atomic, molecular, cellular and functional aspects.





ECS - Extracellular solids
ECF - extracellular fluid

Total body water can be divided into:

  • Intracelllular - 55%
  • Extracellular - 45%

Extracellular body water:

  • Plasma - 7.5%
  • Interstitial - 20%
  • Bone - 7.5%
  • Dense Connective Tissue - 4.5%
  • Transcellular - 1.5%


Anthropometry is the measurement of man. In body composition, anthropometry includes

  • Height
  • Weight
  • Skinfolds
  • Girths or Circumferences
  • Breadths or Diameters
  • Lengths

Each of these measurement assess segments of the body which may be related to body composition. The specific sites for an anthropometric protocol are determined by taking several sites and correlating combinations of these sites to percent body fat determined by hydrostatic weighing. The largest error in anthropometry is in the measurement of skinfolds. This error varies from 3-5%. When using the skinfold measurements to estimate percent body fat the error of hydrostatic weighing should be multiplied by the error of skinfold measurement.

The error of skinfold measurement has been reported to be:

      • 3% for young adults
      • 4.4% in older adults
      • 3.9% in obese

The measurement of anthropometry is a skill. Locating the correct site and then measuring that site takes practice. In some labs, students must practice on 100 people and match the primary investigator before being allowed to take samples in research projects. Click on skinfolds, girths or diameters to see the various sites and the techniques to measure each site.

In general:

  • The more sites used in the estimation of body composition, the more accurate the protocol.
  • The single best site is the triceps to estimate body fat
  • Anthropometric protocols are specific to populations.
  • Anthropometric protocols cannot measure the extremes of the populations
  • Anthropometric protocols assume a constant ratio of internal to external body fat
  • The most common error is measurement technique
Measurement of biilliac diameter.

Landmarks found on the skeleton

Abdominal circumference


Measuring the distribution of body fat is important in modern chronic disease. Upper body fat distribution or android obesity is associated with the following diseases

  • Coronary Heart Disease
  • Type 2 Diabetes
  • Hypertension
  • Gallbladder Disease
  • Cancer
  • Hyperlipidemia
  • Menstrual Irregularities
  • Reproductive Hormone Dysfunction

Different methods used to categorize body fat distribution include:

  • Waist to Hip Ratio
    Abdominal 1/Gluteal Circumferences
  • Waist to Thigh Ratio
    Abdominal 1/Thigh Circumferences
  • Trunk to Extremity Ratio
    (sum biceps + triceps + calf skinfolds) / (sum of suprailiac + subscapular + abdominal skinfolds)
  • Central to Peripheral Ratio
    [(sum of suprailiac + subscapular skinfolds) / 2 ]/ [(sum of biceps + triceps + thigh + calf skinfolds) / 4 ]
  • Upper to Lower Body Ratio
    (sum of biceps + triceps + subscapular skinfolds) / (sum of supraililac + thigh + calf skinfolds)

 Because the risk of disease is associated with visceral fat and not subcutaneous fat deposits on the abdomen or trunk, the use of skinfolds, alone, may not adequately categorize body fat distribution.  Girths of the anatomical areas or girths in combination with subcutaneous skinfold methods may prove to be the better methods.  However, none of these have been investigated as much as the waist to hip ratio.   The most common method is the waist to hip ratio.  

 We reviewed the literature and found 29 different ways to measure waist to hip ratio. The waist measurements were anywhere between the 10th rib and the iliac crest, whereas the hip measurements were anywhere between the iliiac crest and greater trochantor.

We had three of these sites (waist, iliiac crest, and hip) measured on 119 obese women and 81 obese men. We compared the percentage of the sample who would be classified as upper and lower body fat distribution using two combinations of these sites.

The results showed significantly different results when the different methods were used. For example, about 58% of the men were classified as upper body distribution when using the waist:hip ratio vs. 72% would be using the iliac crest:hip ratio.
Similar, but more dramatic discrepancies were found for women. About 25% of the women would be considered upper body fat distribution with the waist:hip ratio whereas 70% would be classified upper body fat with the iliac crest: hip ratio.

 Research activities needed to standardize procedures to categorize body fat distribution include:

  1. Determine the amount of visceral fat that is detrimental to health
  2. Determine the girth measurements that correlate best with the visceral fat deposits
  3. Standardize the girth measurements
  4. Determine the criterion values for body fat distribution categories and the associated health risk
  • Wallace, J.P., P.G. Bogle, K.T. Murray, and W.C. Miller. Variation in the anthropometric dimensions for estimating upper and lower body obesity. American Journal of Human Biology 6:699-709, 1994.

In reality, the CT scans below are the most accurate on visceral fat measures.


Hydrometry is the measurement of the total body water. Most hydrometry techniques utilize a tracer which is diluted in one of the water components. Once this water component is determined, total body water and the remaining components are estimated,

Assumptions for hydrometry include:

  1. Tracer distributed only in body water compartments
  2. Tracer is equally distributed
  3. Equilibrium is rapid
  4. Tracer is not metabolized

Tracers used in hydrometry include:

  • Tritium - TBW
  • Deuterium - TBW
  • Oxygen 18 - TBW
  • Double Labeled Water - TBW
  • Bromide Dilution - ECW
  • Radioactive potassium (42K) - ICW

Double labeled water is probably the most promising method because it can also be used to estimate energy expenditure.

One of the assumptions is the TBW is 73% of the FFM or 60% of body weight. However, these ratios do change across the life-span and with diseases.

From TBW, fat free mass is calculated; then fat mass; then percent fat.

Error: 10% for TBW, 2% for %fat, and 0.5% for FFM


In measuring muscle metabolites, the muscle mass is measured, From the muscle mass, the lean body mass is estimated. From the total body weight the fat weight is estimated.

Two metabolites specific to muscle are

  • 3- Methyl histidine
  • Creatinine


3- Methyl histidine (3-MH) is an amino acid, unique to muscle, which reflects muscle breakdown. 3-MH is only found in muscle at a concentration of 3.31 + 0.05 umole/g. It is not affected by age. Therefore the measurement of 3 MH can reflect the whole muscle mass, which in tern, can be used to estimate lean tissue and eventually body fat..


Assumptions include

  • the muscle mass is stable
  • no other sources of 3MH such as meat ingestion

98% of the body's Creatinine is found in the muscle. Therefore, measuring the amount of creatinine in the body can reflect the whole muscle mass similar to 3-MH techniques. Assumptions for creatinine are the same as for 3-HM.

Error for both is approximately 4%.


Densitometry is a measurement of the density of the whole body. Density = Mass/Volume. Mass is easily measured with a scale. Volume is measured by either water or air displacement. Hydrostatic weighing measures volume by water displacement and the Bod Pod measures volume by air displacement.

Hydrostatic weighing can be done two ways; the actual volume of water displaced or based on Archimedes principle of a body immersed in a fluid is acted on by a buoyant force which is equal to the weight of the displaced fluid

The volume of the body is simply how much water was displaced when submerged. This volume can be measured by

  1. measuring the volume of the water displaced
  2. the weight of the individual underwater.

The subject's loss of weight in the water is equal to the weight of the volume of water displaced.

The assumptions are the density of body fat is 0.9007 and the density of lean tissue is 1.1.

In both situations, the amount of gas trapped in the body have to be accounted for; GI and residual volume. The residual volume is often measured out of the water even though the pressure of the water could reduce the residual volume. The GI gasses are always assumed to be a constant volume for a fasting individual.

Hydrostatic weighing is often considered the "gold standard" of body fat assessment. It utilizes the simple two component model where the intracellular lipid is considered in the total body fat.

Error = 2%


Instead of water, an air tight chamber is utilized for displacement.

The Bod Pod is the most common method for air displacement.

This technique utilizes two air chambers. The subject sits in the first and the second serves as a reference volume. As the subject sits in the first chamber and the door is sealed, the air pressure oscillates between the two chambers. The concept of pressure vs. volume at a constant temperature is used to determine the volume of the body.

Subject breathes normally…through circuitry connected to outside, but uses inside air during the measurements.

Volume of the body is calculated correcting for Body surface area and Air trapped in thorax

Body Volume = body volumeraw – Surface Area +40% VTG (thoracic gas)

Error = 1% for percent fat

  • Demster and Aitkens, A new air displacement method for the determination of human body composition, Medicine and Science in Sport and Exercise 27: 1629-1697, 1995.


X-ray and Ultra sound work on the same principle in body composition. That is, an image is taken of a limb. The thickness of the subcutaneous fat layer is determined and compared to the thickness of the organ (muscle) to determine percent fat.

Assumptions are:

  • Ratio of internal to external body fat is constant
  • Camera angle does not alter the thickness
  • Training of technicians

Error is 8% for both


A fiber optic probe is connected to a digital analyzer that indirectly measures the tissue composition (fat and water) at various sites on the body. This method is based on studies that show optical densities are linearly related to subcutaneous and total body fat.

The biceps is the most often used single site for estimating body fat using the NIR method. The NIR light penetrates the tissues and is reflected off the bone back to the detector. The NIR data is entered into a prediction equation with the person's height, weight, frame size, and level of activity to estimate the percent body fat.

Error: 5%


Biological Tissue has electrical properties. An electrical current is applied across the body; from one limb to another. This electrical current follows the path of least resistance (water).  The combined resistance can estimate total body water.


1. Conductive properties of biological tissue
2.  Water tissue is highly conductive

  • cerebraspinal fluid
  • blood
  • muscle

3.  Other tissue is not

  • lung
  • bone
  • fat

4.  Normal hydration
5.  Electrolyte normalcy
6.  Populations are different

Error = 3.5 - 5.0%


Whole body counters measure one element of the body which reflects whole compartments. These methods include:



Potassium is an element found in every cell. The percentages of different types of potassium are

  • 39K = 93.1%
  • 40K = 0.0118%
  • 41K = 6.9%

40K is radioactive and can be detected by a gamma counter. The whole body is subjected to the counter. The whole lean mass is then calculated from the 40K counts. Percent fat is then calculated from lean mass and total body weight.

Error - 4% in adults; 6% in children


Neutron activation is a method of determining concentrations of elements in samples. In the case of body composition, neutron activation is used to assay the

  • Calcium
  • Chlorine
  • Phosphorus
  • Nitrogen
  • Sodium 
  • Hydrogen
  • Oxygen
  • Carbon

In neutron activation the body is irradiated by neutrons and the gamma rays given off by specific elements are measured. More than one element can be measured at the same time.

Neutron activation is more frequently used to assess the nutritional status in the clinical setting.


Error - 5%; 2-9% for chlorine


When an X-ray beam is presented through a body, it is absorbed in the tissue, based on the density of the tissue. X-ray beams of different intensities are absorbed at different energies. Thus, the DEXA utilizes both low (~40 keV) and high energy (70-100 keV) x-ray beams.

The x-ray beams are presented in the moving arm and the absorbance is measured in the bed below the subject.

DEXA was first used to measure bone density for osteoporosis evaluation. More recently it has been used for body composition.

Mass of the Bone Tissue and Mass of Soft tissue are calculated using the low and high energy absorbance.

Using the 2-component model, percent fat can be calculated.

The bone tissue with the overlying soft tissue is reconstructed by the computer to provide an image showing either the bone or both.


DEXA has not been validated with cadaver analysis. The soft tissue calculations are in need of more standardization.

Error - 4%


Body images from DEXA. The amount of subcutaneous fat can be see in these images. Visceral fat, however, cannot.


Computed tomography is a radiologic technique that measures internal and external body fat.

A CT scanner looks like a big, square doughnut. The patient aperture (opening) is 60 cm to 70 cm (24" to 28") in diameter. Inside the covers of the CT scanner is a rotating frame which has an x-ray tube mounted on one side and the banana shaped detector mounted on the opposite side.


X-ray tubes and detectors are located at the opposite poles of a large ring which rotates around the subject. The computer generates a three dimensional image of organs, including adipose tissue.

A fan beam of x-ray is created as the rotating frame spins the x-ray tube and detector around the patient (see figure to the right). Each time the x-ray tube and detector make a 360° rotation, an image or "slice" has been acquired. This "slice" is collimated (focused) to a thickness between 1 mm and 10 mm using lead shutters in front of the x-ray tube and x-ray detector.

The advantage of the CT scan is the measurement of internal, specifically intraabdominal body fat.

Error = 0.6% (based on 2 cadavers)


Magnetic resonance imaging (MRI) measures the Nucleus of Hydrogen Atoms which are abundant in all biological material by using a strong magnetic field to re-align the cell nuclei against the field. Other elements that will respond to the magnetic fields include

  • Hydrogen - 1H
  • Carbon - 13C
  • Fluoride - 19F
  • Sodium - 23Na
  • Phosphorus - 31P
  • Potassium - 39K


The subject is placed in a magnetic field (10,000 x earth). Protons align with the magnetic field. Protons flip or absorb energy when subjected to the pulsed radio frequency. The time to release energy is also measured because it takes longer for adipose tissue to release energy than lean tissue. The signal is caught in the image. Different tissues give off different energies and appear as different densities in the scan.

Both subcutaneous and visceral fat can be detected with MRI. Anthropometric measurements are used to calculate these values.



The error is 2-10%



TOBEC is based on the principle of body fat and fat free tissue differ in electrical properties. This technique has been used in the animal industry to measure body composition of small animals or limbs of larger animals.

The subject is placed in a cylinder that generates a very weak electromagnetic field. Conductivity of the body is measured estimating lean tissue.

Based on the two component model, percent fat is then calculated.



Cadaver analysis the most accurate method of body composition because the body can be divided into its parts and analyzed completely.

The cadaver is also used to validate the other measures.


In a different lab class, students performed percent body fat using six different anthropometric protocols, a bioimpediance scale, bioelectrical impedance, hydrostatic weighing and visualization.

The variation of the percent body fat measurements are illustrated in the figure to the left. These measurements were performed on about 100 subjects. The range of body fat was 18 to 30 percent on the same population, on the same day.
The estimates of percent body fat appeared more constant for women than for men.
When observing the standard deviations, the variations in circumferences were the least; whereas the variation in skinfold measurements were the most variant.

As illustrated in the figure to the left, the greater the skinfold thickness, the greater the variation in that measurement.

Thus, there is more error in the measurement of skinfolds when the individual has more subcutaneous fat.


Different target weights are used in the clinical setting than in athletic performance. Target weights should be based on lowering the risk of disease than on optimal performance.

Minimal Weight
Below Average
Above Average
At Risk
< 8% or < 14%
8-14% to 23%
23 to 33%
> 33%
< 5%
5 to 15%
15 to 25%
> 25%
Because >33% for women and >25% for men are considered at risk, targeting weight to be under these values should reduce risk.


1. Determine fat weight (FW) from total weight (TW) and percent fat (%F)


Fat Wt = Total Wt x %Fat


2. Determine lean weight (LW) from total weight (TW) and fat weight (FW)


Lean Wt = Total Wt - Fat Wt


3. Choose the target percent fat.

a. for a short term goal, choose a 3-5% fat weight loss

b. for a long term goal, choose a range of percent fat that is a range of 5% and below the at risk percentage

Women = 25-30%
Men = 18 - 23%




4. Convert the target percent fat to target percent lean (based on a two component model)

25% fat = 75% lean
23% fat = 77% lean




5. Calculate a target weight (TarWt) from the target lean (TarLe) and Lean weight (LW). Calculate the low end of the target weight first.


Target Wt = Lean Wt/(Target Lean x 100)


6. Calculate the target weight at the higher end of the range.



Provide target weights in range of weights. ALWAYS give target weights as whole numbers.


This page was last updated 24-Nov-2010
Webmaster: Janet P. Wallace, PhD, FACSM
Copyright 1998, The Trustees of Indiana University