11 Methods of Studying Body Composition

Ms. Shumayla and Dr. Meenal Dhall

Contents of this unit

1. Introduction

2. Assessment of Body Composition

2.1 Direct Assessment

2.2 Indirect Assessment

2.2.1 Densitometry

2.2.1.1 Hydrodensitometry or Under Water Weighing

2.2.1.2 Air Displacement Plethysmography

2.2.2 Hydrometry 

2.2.2.1. Total Body potassium

2.2.2.2. Neutron activation analysis

2.2.3. Dual Energy X-Ray Absorptiometry Method (DEXA)

2.2.3.1 Triple energy X-ray Absorptiometry

2.2.4. Bioelectrical Impedence Analysis (BIA)

2.2.5. Imaging Methods

2.2.5.1. Ultrasound

2.2.5.2. Computed Tomography

2.2.5.3. Magnetic Resonance Imaging (MRI)

2.2.6. Anthropometry

2.2.6.1 Weight/ Height indices

2.2.6.2. Skinfold Thickness

2.2.6.3 Circumferences and Breadths

Learning Objectives:

After the unit student will be able to

• To Understand the need of assessment of Body Composition
• To understand the two basic assessment method of body composition.
• The basic principal, methodology and application of these methods.

1. Introduction

Body composition and growth are the key components of health. The current prevalence of obesity in children and adults has highlighted the importance of body fat. However, other components of body composition also influence health outcomes, and its measurement is increasingly considered valuable in clinical practice. There are growing evidences which links body composition with health risk together.Cureton (1947) indicated that, although individuals of a given height could have similar weights, they could still differ in proportions of bone, muscle, and fat. Limitations of this nature relative to height– weight measures were demonstrated by Behnke et al. They reported that excessive weight for height and age could be regarded erroneously as obesity. By determining the specific gravity of football players who ranged in weight from 170 to 260 pounds, they found that 11 of 17 overweight players were classified within low-fat categories. Keys and Brozek corroborated these results; they found subjects to be misclassified by insurance scales when measured through body composition methods of evaluation. New research suggests that longevity of human life can be increased by losing fat and not by losing weight. Addition to this fact the body composition is important in athletic performance. The body composition techniques are now not only laboratory procedure but it is being used in ordinary medical practices, health clubs, or even at home. A person’s health status can be more accurately assessed by measuring body composition; also the effects of dietary and physical activity programs can be better directed. Hence, it becomes important to study the body composition.

2.The following Two procedures are basically used to evaluate body composition:

1. Direct measurement by chemical analysis of the animal body or human body
2. Indirect estimation by hydrostatic weighing, simple anthropometric measurements, and other clinical and laboratory procedures

2.1 Direct Assessment:

Two approaches directly assess body composition. One technique dissolves the body in a chemical solution to determine its mixture of fat and fat-free components. The other physically dissects (Cadaver technique) fat, fat-free adipose tissue, muscle, and bone. Considerable research has chemically assessed body composition in various animal species, but few studies have directly determined human fat content. These labor-intensive and tedious analyses require specialized laboratory equipment and involve ethical questions and legal hurdles in obtaining cadavers for research purposes. Direct body composition assessment suggests that while considerable individual differences exist in total body fatness, the compositions of skeletal mass and the fat-free and fat tissues remain relatively stable. Researchers have developed mathematical equations to indirectly predict the body’s fat percentage on the basis of the assumed constancy of these tissues.

2.2 Indirect Assessment:

Diverse indirect procedures assess body composition. One involves Archimedes’ principle applied to hydrostatic weighing (also referred to as hydrodensitometry, or underwater weighing).

This method computes percentage body fat from body density (ratio of body mass to body volume). Other procedures predict body fat from skin fold thickness and girth measurements, X-ray, total body electrical conductivity or bioimpedance (including segmental impedance), near -infrared interactance, ultrasound, computed tomography, airplethysmography, and magnetic resonance imaging.

Archimedes’ principal: The principal is named in honor of Archimedes, who supposedly discovered it while attempting to determine whether a king’s gold crown counterfeit. This principal states that upward buoyant force exerted on a immersed body in fluid is equal to weight of the fluid the body has displaced.

Source: http://physics.weber.edu/carroll/archimedes/images/buoyancy.gif

2.2.2.1Densitometry:

The body density (Db) is equivalent to the ratio of its mass (MA) and volume (V)

Db= MA/V

It is inversely related to the body fat content- greater the proportion of fat, the lower the body density. Thus, the body density permits an estimate of percentage of body weight that is fat. The term densitometry refers to the procedure of estimating body composition from body density (Malina et al. 2004). It involves the following methods of measuring body composition:

2.2.1.1 Hydrodensitometry or under water Weighing:

This method is basically used to measure the body volume. In this method subject sits in a specially constructed tank on a suspended chair or frame. This method requires a high degree of water confidence as subject has to exhale all the air before submerging in the water (Brodie 1998). After that Archimedes’ principal is applied to compare the mass of the subject in air and in water. It computes body volume as the difference between body mass measured in air (Ma) and body weight measured during water submersion (Ww ; the correct term because the body mass remain unchanged under water). Body volumes equals loss of weight in water with the appropriate temperature correction for water‘s density. The following formula computes body density (Db) from underwater weighing variables:

Db         = mass/volume

= Ma / [(Ma *Ww) / Dw] – RLV

For ease in composition, the following formula can be used to compute body density:

Db = Ma *Ww / (Ma – Ww – RLV * Dw)

Calculating Body Fat Percentage. The equation used to estimates the body’s fat percentage incorporates whole-body density. The simplified equation derived by UC Berkeley scientist William Siri (1919–1998) substitute’s 0.90 g cm-3for the density of fat and 1.10 g cm-3for the density of the fat-free tissues. The final derivation, referred to as the Siri equation, computes percentage body fat as:

Percentage body fat – (495 / body density) – 450

The above equation assumes the two -component model of body composition in which the body comprises of FFM and FM; the density of fat extracted from adipose tissue equals 0.90 g cm-3and 1.10 g cm-3 for fat-free tissue at 37°C.( McArdle. et. Al 2010)

2.2.1.2 Air Displacement:

This is an alternate method to the hydrodensitometry which has potential clinical value. This method is simply based on classic gas laws and does not require water submission. Air Displacement Plethysmgraph (ADP) is a BOD POD that also uses the principal of densitometry to determine body composition. The BOD POD measures the body mass (weight) using a very precise scale, and volume by sitting inside the BOD POD.

The BOD POD differs from underwater weighing, as BOD POD uses air as an alternative of water to measure body volume, based on the physical relationship between pressure and volume.

The BOD POD consists of two chambers. The molded seat inside the BOD POD divides it into two chambers namely front (test) and rear (reference) chambers and provides a common wall between these two chambers. A diaphragm is mounted on the common wall for oscillation during testing by computer control.

During the body volume measurement, the BOD POD door is closed by a series of electromagnets and a seal, and the diaphragm effectively moves back and forth between the two chambers. When the volume of one chamber increased, the same amount of the volume decreased in other chamber, and vice versa. The change in volume effect the pressure in both the chambers leads to the magnitude of pressure to change. During the body volume measurement, the subject was advised to breathe normally. Thus, the relevant measurement of lung volume for the BOD POD is not residual volume, but the average lung volume during normal tidal breathing (average thoracic gas volume). This is a much easier measurement to obtain and no difficult maneuvers are required. To achieve optimal accuracy, the volume of air in the lungs must be determined. This may be done either by directly measuring the average thoracic lung volume or by using an estimated value based on standard prediction equations.

2.2.2 Hydrometry:

Water is the largest compositional component of the body. It varies from 55% to 65% of body weight in normally hydrated young adult males, with lower values for females. Most of the water in the body is in lean tissues, so the measurements of TBW provide a means for estimating FFM. A reduction in Body water can lead to dehydration, that causes remarkable changes in body and can be life threatening and hence TBW analysis should be an integral part of Body composition.

Dilution is a method to determine total body water content (TBW) by administration of a tracer and measuring the tracer’s concentration a short time later. After collection of a body fluid (blood, urine, or saliva) to measure pre-dose background levels, a tracer of labeled water (tritium, deuterium, or oxygen-18) is administered. And after a short period of time another sample from the body is obtained. The total Body Water calculated on the basis of measured dilution of tracer element and the assumed exchange rate of the tracer with non-aqueous compound. The limitation of this method includes a high isotope and analysis cost, and time necessary for the preparation and administration of the samples. (Heymsfeild, 1997).

2.2.2.1. Total Body Counting (Total Body potassium)

In human body, naturally distributed radioactive potassium present I three forms of isotopes 39K, 40K and 41 k. the Total body counting or Total body potassium, measures the amount of potassium 40 (40K) in the body. Potassium is found almost entirely within cell bodies, measuring the amount of potassium in the body can provide an estimate of body cell mass. Once total body potassium is known, fat-free mass can be calculated assuming a constant concentration of potassium in FFM. To measure 40K in human body there are three general criteria

• A gamma ray detector should be used and should be placed close enough to the subject.
• The gamma detector used should have proper shielding to reduce the other natural cosmic and radioactive rays.
• Also, to identify gamma rays among 40K, a computerized analyzer should be present in the system.

“Shadow shield” counter is a scanning design used for whole body count/ total body counting. This is a small and portable counter. In this subject lies straight on the bed in supine position and scanned slowly beneath the shielded detector. Also shield can be built as a part of the room while using a multidetector counter as can be seen in figure

2.2.2.2. Neutron Activation Analysis:

The first demonstration that Human Body composition can be analyzed by using neutron activation was reported 40 years back (Anderson et.al 1964). These techniques are highly accurate for tissue-specific body composition, with a typical body scan occupying up to 1 hour. Asthe subject has expose to a neutron field, output of gamma rays can be measured as the cell nucleus relaxes and again go back to its pre- exposed state. Gamma output can be measured immediately after activation (the technique is known as prompt gamma neutron activation) or at a somewhat delayed period (the technique known as delayed gamma neutron activation”). Using these techniques, many elements in the body can be measured like carbon, nitrogen, sodium, and calcium. A significant concern with these techniques is that these techniques involves high levels of neutron radiation exposure and therefore has not been used in large-scale population research.

2.2.3. DEXA (Dual Energy X-Ray Absorptiometry) :

DEXA is used to measure bone mineral and soft tissue composition of the body. It provides estimates of the composition of the total body and of specific regions in the form of bone mineral, fat-free soft tissues and fat.

This method requires a low radiation exposure in the form of two photon beams, one of low energy and other of high energy which passes through the body. Radiation exposure with DEXA is low (0.05 to 1.5 mrem), depending on the machine and how quickly the total body scan is done (Lohman 1992). DEXA scans take about 20 minutes, but some of the updated machines can complete a whole body scan in 5 minutes.

The DEXA unit measures the attenuation of the low-dose of X-ray beam as it passes through different tissues of the body. The amount of photon beam absorbed by the atoms in bone mineral and soft tissues of the body is recorded during the scan and convertedto estimate bone mineral and soft tissues (Goran, 1997) . The DEXAinstrument must be linked with appropriate computer algorithms to measure bone mineral, fat tissue content and fat free soft tissue content of the total body. The algorithms also permit divisionof the body into anatomical segments- arms, legs, trunk and head- to permit estimation of regional body composition.

The derivation of fat and fat-free soft tissue from DEXA scans is based on the ratio of soft tissue attenuation of the low-energy and high- energy photon beams as they pass through the body. The attenuation of the low energy soft tissues is known based on scans of pure fat and fat-free soft tissues and theoretical calculations and is assumed to be constant. The attenuation for fat is lower than that for fat-free soft tissues. Using these constants and the scans from the DEXA unit, the amount of fat and fat-free soft tissue is calculated.

The derivation of bone mineral requires adjustment for the soft tissue overlying bone. DEXA technology provides an estimate of total body bone mineral content (g) and total bone area (cm3). The ratio of total- body bone mineral to total bone area is used to estimate bone mineral density (g/cm3). DEXA basically measures the cross-sectional area of a scan (total bone area) and not bone volume; hence, expressing bone mineral relative to bone area is only an approximation of bone mineral density.

Many researchers are working for its development. In theory, it may be possible to obtain estimates not only for bone and fat but also for body water and protein mass but only if the energies are from the Compton scattering, photoelectric and pair production regions.

2.2.4. BIA (Bioelectrical Impedance Analysis):

Bio-electrical Impedance Analysis or Bioimpedance Analysis (BIA) is a method of assessing body composition, the measurement of body fat in relation to lean body mass. The Human body has the tendency to conduct an electric current. The aqueous tissues in the body have dissolved electrolytes, because of that they are the considered as the conductors of an electrical current, whereas body fat and bone have relatively lower electrolytic content and hence poor conducting properties. This difference in electrolyte content permits an estimate of FFM from the magnitude of the body’s electrical conductivity or from the body’s impedance to an electrical current as it flows from the source(usually the ankle ) and the sink (usually the wrist) electrodes. The Fat Free Mass has high conductivity and low impedance, whereas Fat Mass has low water and electrolyte content, and has low conductivity and high impedance.

Principal behind BIA: Impedance is the frequency dependent opposition of a conductor to the flow of electric current. Impedance is a vector (two-dimensional) quantity which consists of two independent scalar (one-dimensional) phenomena: resistance(R) and reactance (Zc)(Scully,1993). Impedance can be calculated by following equation   Z2 = R2 + Zc2

The general theoretical model of BIA treats the human body as a single cylinder, with measurements made between electrodes placed on the wrist and ankle. Adjustment of the bioelectrical data for height allows the estimation of total body water (TBW). In practice, empirical derivation of regression equations relating height2/impedance to TBW is required and these equations are then applied subsequently to predict TBW, which is converted to FFM.

2.2.5. Imaging methods:

Imaging methods are considered to be among the most accurate approaches for the in-vivo quantification of body composition. The widely available methods are ultrasound imaging; X-ray computed axial tomography (CAT) and magnetic resonance imaging (MRI)2.2.5.1. Ultrasound:

Ultrasound is most of the familiar and oldest among all the three imaging methods. In this method ultrasound rays reflect from the boundaries of tissues which are interpretable by a trained technician. The image quality in this method may be poor but it is the widely used in the evaluation for the measurement of adipose tissues deposited. Ultrasound imaging is free of hazards and that’s why it is useful in the assessment of human body composition. Detecting differences in adipose tissue distribution can be more explored by using this technique. This is low in cost and the equipment used in this technique is movable which are in its favor.

2.2.5.2. Computed Tomography:

This technique uses collimated X-rays to provide a fan-shaped beam which pass through the body, an array of detectors to detect the transmitted radiation is placed on the opposite side of the subject. The X-ray source and detector assembly rotated 360o around the subject. The transmitted intensity is recorded at each degree of rotation to provide information about the internal structures along that beam path. The basic anatomical image of CT is similar to that of MRI, except it contains additional information for the tissue’s true density at each pixel. This information combined with the anatomical location of the pixel within the image can be used to identify it as adipose, muscle, skin, viscera, or bone tissue.(Heymsfeild, 1997)

The examination of body composition by CT is a rapid procedure, and is easyto perform and the equipment is widely spread. The radiation dose during CT can limit its usefulness especially in healthy and young individuals. This technique is considered as costly and the analyses of the images can be time consuming. However characteristic CT numbers automatically separate the different tissues and further atomization of the determinations of depots would facilitate the post processing. (Kullberg, 2014)

2.2.5.3. Magnetic Resonance Imaging (MRI):

The atoms and molecules in human body are in random orientation as the magnetic field of earth is very weak but when the human body placed in a strong magnetic field some of the nuclei will attempt to align with or against magnetic field. Hydrogen protons in particular have a high affinity for alignment with the magnetic field. After the hydrogen protons are aligned in a known direction, a pulsed radio-frequency is turned off, the protons gradually return to the original state, in the process releasing energy that is absorbed in the form ofa radio frequency signal. The MRI system uses this signal to generate the cross sectional images. During MRI imaging, the subject must hold his or her breath to reduce the effect of respiratory motion on image quality. A series of images for whole body analysis can now be acquired in less than 30 minutes. These advances have made MRI a much more useful instrument for body composition research.

A major advantage of technologies like MRI and CT is the lack of ionizing radiation. Radiation exposure is still a concern in long term longitudinal studies with repeated measurements over time and study of children and women in child bearing years.

While Comparing CT with MRI, the MRI method is slightly overestimate subcutaneous adipose tissue volume and underestimate visceral adipose tissue volumes. But MRI can be considered sufficiently accurate for measurements of adipose tissue volumes in assessment of body composition.

2.2.6. Anthropometry:

Decades ago Anthropometry was the only technique present for quantifying body size and proportion. Anthropometric measurements like skin fold thickness, circumferences, and body length and body width were used in developing equations for predicting body fat. Anthropometry provides simple approach to body composition. The anthropometric equipment are portable and relatively inexpensive, also procedures are noninvasive and there is a substantial literature available (Cameron 1986, Gibson 1990, Weiner and Lourie 1981) . Though anthropometry require adequate training by an experienced professional and quality control, including analysis of inter and intra-observer reliability and calibration of equipment during a study or during the course of work.

2.2.6.1. Height-weight indices:

A number of height/weight indices have been proposed in literature. BMI (Body Mass Index) is the most widely used index and has been recommended by World Health Organization (WHO) as a crude indicator of nutritional status (Gibney at al., 2009). The adults are categorized as undernourished, normal and overweight/obese, according to BMI cut-offs <18.5 kg/m2, 18.5-24-9 kg/m2 and >25 kg/m2 respectively. The children are screened as per the weight-for-height and BMI for age reference charts due to discrepant rate of growth. It is used as adiposity marker in identifying individuals at risk of metabolic disorders. However, its utilization has been debated as a consequence of inability to distinguish fat mass from fat free mass.

2.2.6.2. Skinfold Thickness

Skinfold thickness:

Skinfold thickness is widely accepted as a prognosticator of body fatness because of following two reasons:

• –About 40-60% of the total body fat is in subcutaneous region of the body
• –Skinfold thickness can be measured directly by using a well-calibrated caliper.

The most often measured skin folds for the assessment of total body fat are biceps, triceps, sub scapular, suprailiac and abdomen. Despite relatively strong correlation of skin fold thickness at a single site with body fat percentage, no single site is accurate predictor of percentage of body fat reflecting inter-individual variation in distribution of subcutaneous adipose tissue and proportion of total adipose tissue under skin. The triceps skin fold thickness is highly sensitive to the nutritional status and hence used directly as an indicator. According to the literature, four different calipers have been used: adipometer, harpenden, Holtain and Lange.

2.2.6.3. Circumferences and Breadth:

In past several decades, more than 17 sites for circumference measurements have been used in equation s for predicting body fatness. Midarm, midthigh, waist and hip circumferences are the most common and more frequently used circumferences, because they indicate difference among people in major regions of the body. Circumferences are considered more reliable than skin folds and can be measured irrespective of body size and fatness. A simple steel tape can be used to measure the circumference at almost all the sites.

Bone Breadths: Skeletal breadth along with the length is distinct determinant of fat free mass. Measurements of bone breadths at waist, elbow, ankle and shoulder using the suitable caliper are used to measure growth in children. While in adults they provide information on individual frame size. Bone breadth is not as popular as skin fold thickness and circumferences are because the caliper used is relatively expensive.

Summary:

• Body composition analysis of the fat and lean components of the body can be determined through direct and indirect methods of research.
• Direct methods employ cadaver studies, whereas indirect measurement methods of investigation incorporate: densitometry, dilution, total body nitrogen, total body potassium content, x-ray, computed tomography, magnetic resonance, ultrasound, electrical conductivity evaluation, photon absorptiometry, air displacement plethysmography, and anthropmetric measures.