skip to content »

Children's Nutrition Research Center

Houston, Texas

Children's Nutrition Research Center - CNRC
USDA/ARS Children's Nutrition Research Center
not shown on screen

Lab - Body Composition

Body composition studies play a key role in understanding childhood growth and development. Questions such as whether body weight or lean body mass is the best indicator of when premature babies are healthy enough to go home, or whether very chubby babies are more prone to develop cardiovascular problems as adults, can only be answered with research using precise body composition measurements and appropriate standard reference values.

The CNRC Body Composition Laboratory is the only laboratory of its type in the nation that can provide a complete complement of body composition measurements in all populations ranging from low-birth-weight infants to adults. These high-precision measurements are associated with body water, mineral, protein and fat content. These measurements are used in a wide variety of CNRC research studies and in collaborative projects with other organizations. The laboratory also develops and validates new methods for assessing body composition in children. Data from the body composition laboratory research is also being used to develop age, gender and ethnic-specific body composition standard references, such those for bone mineral content and bone mineral density.

The Body Composition Laboratory uses prompt gamma activation analysis and total body potassium counting to measure whole body muscle mass and lean tissue quality and quantity. Additional methods include stable isotope dilution for measuring the body's water compartments; dual-energy x-ray absorptiometry (DXA) for measuring bone, fat and lean-body mass; Air displacement plethysmography (BodPod) for determining fat-free mass and body fat; Bioelectrical impedance analysis (BIA) for water and fat ratios; Computed tomography (CT) for measuring fat distribution and bone density; portable Lipometer for measuring subcutaneous fat; and portable Ultrasound for localized bone density measurements.

Prompt Gamma Activation Analysis

Prompt Gamma Activation AnalysisPrompt Gamma Activation Analysis is the gold-standard method for measuring the amount of nitrogen in the body, which is a direct indicator of total body protein (TBP).

Substantial losses of total body protein can occur in chronic diseases and in aging. Such losses impact negatively on immunity and quality of life,and on growth rates in children. Direct measurements of total body nitrogen (TBN) monitor the integrated changes in TBP over time and allow comparison with normal subjects.

When this measurement is combined with measurements from the Total Body Potassium Counter (40K), scientists can determine total organ and muscle mass.

Learn more about Prompt Gamma Activation Analysis.

Total Body Potassium Counting

Total Body Potassium CounterThis extremely sensitive device measures the gamma rays emitted from an isotope of potassium known as 40K, which exists naturally in the human body at a known natural abundance (0.012%). This knowledge, plus the fact that potassium is only found inside body cells and is not present in stored triglycerides, makes 40K data an accurate index of the body's total cell mass (the active growing tissues in the body), which in turn can be used to estimate fat-free mass.*

*Although the terms lean body mass (LBM) and fat free mass (FFM) are often used interchangeably, it is more accurate to think of body weight (mass) as the sum of LBM (muscles, organs and other non-fat tissues such as bone) plus adipose (fat) tissue, or alternatively, as the sum of extractable fat (i.e., the pure fat component of adipose and other tissues) plus fat-free mass (FFM), with FFM including the water, protein and other non-fat components of adipose and other fat-containing tissues.

Learn more about Total Body Potassium Counting.

Dual Energy X-Ray Absorptiometry (DXA)

Dual Energy X-Ray Absorptiometry (DXA)DXA was originally developed to determine bone mineral density and to aid in the treatment of osteoporosis. More recently, the technique has been expanded to include the analysis of fat mass and lean body mass in addition to bone mass.

The basic principle of DXA data acquisition is based on the differences between bone and soft tissue attenuation at high and low x-ray levels. As an x-ray beam passes through the subject, detectors register the varying levels of x-rays that are absorbed by the anatomical structures of the subject. The raw scan data, which includes values of tissue and bone, are captured and sent to a computer. The computer generates an image of the body in pinpoint pixels, which can be 'counted' to assess bone status and fat distribution. The radiation exposure during DXA scanning is very low.

Learn more about how DXA is used in nutrition research.

Air Displacement Plethysmograph (BodPod)

Air Displacement Plethysmograph (BodPod)The BodPod uses a technique called Air Displacement Plethysmography (ADP) to measure body volume , which in turn can be used to estimate percent body fat.

This technique relies on the physics of Boyle's Law, which states that pressure and volume vary inversely with one another. In other words, as pressure goes up, volume goes down, and vice versa . Monitoring pressure changes in a closed chamber allows one to calculate volume.

Learn more about how the BodPod works!

Bioelectrical impedance analysis (BIA)

Bioelectrical impedance analysis (BIA) The resistance to an applied electric current flowing through the body is related to the volumes of conductive tissues that the current passes through. This measurement can estimate water and fat ratios.

Bioelectrical impedance analysis is based on the conductive and non-conductive properties of various biological tissues. Most of the body's fat-free mass is composed of conductive tissues such as muscle, while fat is part of the non-conductive tissue mass. The volume of these tissues can be estimated from the measurement of the resistance to an applied electric current flowing through the body.

Water is a constant fraction of fat-free mass, usually about 73 percent. The water measurement can therefore be used to estimate levels of fat-free mass. Water ratios, however, can change with the onset of certain illnesses (mostly in the extra-cellular compartment). Accurate body water measurements using BIA can be important in studying disease.

Learn more about how BIA works!

Computed Tomography (CT) Scan

Computed tomography produce cross-sectional scans of the body, which is particularly useful in determining ratios of visceral (intra-abdominal) fat versus subcutaneous fat. CT is also used to obtain true three-dimensional bone mineral density (BMD in g/cm 3 ), usually of the lumbar spine. The BMD measurement looks specifically at trabecular bone - the softer, spongy bone within a vertebra that is most closely associated with bone loss in osteoporosis and other bone diseases.

Lipometer

This small, portable, optical device enables researchers to measure the thickness of subcutaneous adipose tissue at any location in the body in a rapid, safe and non-invasive manner.

Ultrasound

Ultrasound measurements operate by emitting high frequency sound waves that penetrate the skin surface and pass through the fat layer, bounce off the muscle or bone, and return to the ultrasound unit. The time for the transmission of the ultrasound wave is recorded and converted to a body composition measurement based on comparative data. This portable technology can be used to estimate the bone density of individuals using localized measurements, such as of the tibia and radius, as well as to estimate subcutaneous fat.

E-mail this page to a friend