Section
5
Assessing Adiposity
Methods Not Feasible in Population-level Research
The following methods are important in furthering our understanding of body composition but will not be discussed in detail in this review as they lack feasibility outside of specialized research centers, which make them generally unavailable for use in community-based studies or in large population-based studies.
Magnetic resonance imaging (MRI) is considered a reference method for adipose tissue, including adipose tissue distribution, skeletal muscle mass, and other internal tissues and organs. Its primary application has been in quantifying the distribution of adipose tissue into visceral, subcutaneous, and intermuscular depots. The approach uses complex software to convert instrument-acquired signals into images of the interior of the body and to interpret those images in terms of FM and FFM tissues. Post-processing imaging analysis software quantifies the image area, and thus volume, of a specific tissue. The method is available only in specialized radiology facilities and hospitals. This limited availability, combined with high cost, makes this method feasible only for specialized research studies.
Magnetic resonance spectroscopy (MRS) provides information on the chemical composition or physical nature of a specific structural region within a tissue imaged by MRI. MRS methods have been developed to distinguish the fat within the muscle cell (intramyocellular lipid) from that outside of the muscle cell (extramyocellular lipid) and the fat within the liver (intrahepatic lipid) in-vivo. This method has been used extensively by many groups to report associations between intramyocellular lipid and/or intrahepatic lipid and insulin resistance as well as the effects of exercise on the intramyocellular lipid of muscle. Post-processing imaging analysis software quantifies the image area, and thus volume, of a specific tissue. The method is available only in specialized radiology facilities and hospitals. This limited availability, combined with high cost, makes this method feasible only for specialized research studies.
Quantitative magnetic resonance (QMR) is a relatively new non-imaging technique that uses an electromagnetic field to detect the hydrogen atoms in three groups: fat tissue, fat-free tissue, and free water. This technique does not pose any health or safety concerns and can be repeated many times within or across days, allowing for the assessment of short-term changes in body composition. Systems are available for infants, children, and adults. The advantages of the QMR device for studies involving humans include rapid data collection, no special participation requirements on the part of the participant, no sedation, no ionizing radiation, and high precision for FM and TBW measurements. Equipment is available from a single manufacturer only and is very expensive, making this method feasible only for studies in specialized research studies.
Deuterium dilution uses the stable isotope dilution tracer technique of deuterium oxide (D2O) and oxygen-18 labeled water (H218O) to calculate the volume of TBW. The tracer sodium bromide (NaBr) can be used to measure extracellular water space (ECW). Plasma or saliva samples are obtained at baseline and 3 hours after oral administration of the tracers in adults and 2 hours after in young children. FFM is calculated from TBW/hydration factor, and FM is calculated as measured body weight minus FFM. Administration of these tracers and collection of samples are easy in older children but challenging in newborns. The equipment and labor required for sample analyses are significant; sample analyses must be sent to specialized research centers. The isotopes themselves may be costly depending on amount needed and vendor. Thus, these methods are often impractical for large-scale studies.