12: Body Composition- Laboratory Methods (Chapter 14)
- Page ID
- 116933
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Several indirect in vivo laboratory methods are available to assess body composition. Their selection depends on the study objective, the precision and accuracy required, the cost, convenience to the subject and their health, and equipment and technical expertise available. This chapter describes both non-scanning and scanning in vivo laboratory techniques. Non-scanning techniques include total body potassium, total body water (via isotope dilution or bioelectrical impedance analysis (BIA)), neutron activation analysis, densitometry (via under-water weighing or air-displacement plethysmography), and total body electrical conductivity. Four scanning techniques are included: computerized tomography, magnetic resonance imaging, dual energy X‑ray absorptiometry (DXA), and ultrasound. Each technique generates body composition data in a different way, so the results are not interchangeable. For each technique, the characteristics, including both the advantages, limitations and assumptions applied to generate the body composition data, are outlined. In addition, their potential applications are summarized, with emphasis on the alterations in the relative proportions of the body components that may occur in certain life-stage groups or disease states. Such alterations may invalidate the determination of fat and fat-free mass in a 2‑component model of body composition. In a 2‑component model, constants for the hydration and density of fat-free mass which ignore the inter-individual variability in these properties, are often assumed.
Many factors have the potential to alter values for these assumed constants, especially growth and maturation in children, aging, pregnancy, and obesity. Consequently, researchers have developed specific constants for the hydration and density of fat-free mass specific for these circumstances. Their use will improve the estimates of fat-free mass when applying the simple 2‑component model based on measurements of total body potassium, total body water, or whole-body density. From a comparison of nine in vivo body composition methods conducted by Field and co-workers (2015), measurement of densitometry via air displacement plethysmography was selected as the method with the highest degree of accuracy and reliability and with the least degree of technical error to track and monitor whole-body composition across the lifespan, provided any alterations in the relative proportions of the body components, are taken into account.
In clinical patients with certain disease states, over- or underhydration and abnormalities in mineral mass may occur, resulting in substantial variability in both the hydration and density of fat-free mass. In these circumstances, application of multi-component models that include measurements of protein and/or bone minerals (via neutron activation or DXA) as well as total body water and density will minimize assumptions related to the structure, hydration, and density of fat-free mass. The 4-component model is considered the criterion method whereby whole-body composition can be most accurately assessed. A simplified version based on measurements from both DXA and BIA holds promise for monitoring conditions in certain disease states.
The four scanning techniques are widely used in clinical settings to quantify in vivo body composition, especially at the tissue-organ level, when investigations of bone density, skeletal muscle mass, and the deposition of visceral ectopic fat with their corresponding relationship with osteoporosis, sarcopenia, or cardiometabolic risk, respectively, are needed.
- 12.1: An introduction to techniques used to measure body composition (14.0)
- This page discusses the importance of accurate body composition measurement methods for health studies and treatment monitoring, emphasizing the superiority of multi-component models, particularly the 4-component model, over simpler methods for clinical populations. It highlights various scanning techniques like CT, MRI, and DXA, each with distinct advantages and limitations.
- 12.2: Chemical analysis of cadavers (14.1)
- This page discusses limited studies on body composition based on direct chemical analysis of human cadavers, mainly from individuals who died from illness between 1945 and 1968, which may not reflect healthy adults. Findings from six cadavers show fat-free tissues consist of about 72% water and 20% protein, with stable potassium levels at around 69mmol/kg. However, fat content varied widely, ranging from 4.3% to 27.9% of body weight.
- 12.3: Total body potassium (TBK) (14.2)
- This page discusses the measurement of the radioactive isotope 40K in the body using whole-body gamma-spectrometry, highlighting the need for shielding and extended counting times. It emphasizes the importance of calibrating for individual body composition for accuracy, due to variations in potassium concentration based on demographics and conditions.
- 12.4: Total body water from isotope dilution (14.3)
- This page outlines the significance of measuring hydration status and total body water, highlighting deuterium as an effective measurement tool, especially through FTIR methods. It emphasizes the impact of hydration variations across demographics, such as age, sex, and health conditions, on body composition analysis.
- 12.5: Multiple dilution methods (14.4)
- This page discusses multiple dilution methods for estimating body fluid compartments, including extracellular mass (ECM) and body-cell mass (BCM). It highlights how total body water and extracellular water are measured, inferring intracellular water. The method is useful in evaluating BCM in malnourished patients, noting a correlation between reduced BCM and increased ECM. It mentions a precision rate of 2.
- 12.6: In vivo activation analysis (14.5)
- This page discusses in vivo neutron activation analysis (NAA), a method for estimating chemical elements in the human body and body composition, including key elements like nitrogen and calcium. It effectively calculates the carbon-to-oxygen ratio for assessing fat mass. However, radiation exposure from NAA limits its use, making dual X-ray absorptiometry (DXA) a preferred alternative for calcium assessment, despite NAA's significance in tracking protein loss in patients.
- 12.7: Densitometry (14.6)
- This page outlines the evolution of body density assessment from a 2-component to a 4-component model, incorporating hydration and bone mineral content. It highlights the shift from hydrostatic weighing to air-displacement plethysmography for improved measurement comfort and efficiency. Methods for estimating thoracic gas volume and the impact of demographic factors, particularly in children, are discussed.
- 12.8: Total body electrical conductivity (14.7)
- This page discusses Total Body Electrical Conductivity (TOBEC) as a non-invasive method for measuring body composition based on electrical conductivity changes in an electromagnetic field. It effectively distinguishes fat-free mass, which conducts electricity well, from fat. Although fast and suitable for those unable to undergo underwater weighing, TOBEC is expensive and relies on calibration equations.
- 12.9: Bioelectrical impedance (14.8)
- This page provides an overview of bioelectrical impedance analysis (BIA) for body composition assessment, detailing methods like SF-BIA, MF-BIA, BIS, and BIVA. It discusses their limitations, especially in obese and clinical populations, and the necessity of using raw BIA data for bedside assessments. Challenges in establishing cut-points for malnutrition and the use of phase angle and impedance ratios for diagnosing conditions like sarcopenia are noted.
- 12.10: Computerized tomography (14.9)
- This page discusses computerized tomography (CT) as a crucial imaging technique for evaluating body composition and tissue conditions. While it offers high-resolution images, its high radiation exposure and cost are limitations. Innovations like peripheral quantitative CT (pQCT) seek to minimize radiation while assessing bone health. CT is becoming more important in studying sarcopenia, but the absence of clear diagnostic standards hampers its clinical use.
- 12.11: Magnetic resonance imaging (14.10)
- This page discusses MRI as a non-ionizing imaging method ideal for infants and long-term monitoring, focusing on body fat and muscle mass assessment. It highlights the technology's ability to detect minor body composition changes and the reduction in scan times from 10 minutes to under 2 minutes. While effective in distinguishing adipose from lean tissue through hydrogen nuclei behavior, it requires technical expertise and can be expensive.
- 12.12: Dual energy X‑ray absorptiometry (14.11)
- This page discusses dual-energy X-ray absorptiometry (DXA) as a cost-effective and quick method for body composition assessment across all ages, excluding pregnant women. It involves low radiation exposure and is suitable for individuals with extreme obesity. DXA measures X-ray absorption to determine fat and lean mass but can be influenced by factors like hydration.
- 12.13: Ultrasound (14.12)
- This page discusses diagnostic ultrasound, a safe and cost-effective imaging technique that uses sound waves to assess tissue thickness without radiation. It is valuable for prenatal imaging and monitoring body composition but is affected by operator skill and variability in technology. There are calls for further research to establish standardized methods and reference data to enhance clinical effectiveness.
- 12.14: Summary - Method comparisons (14.13)
- This page discusses the growing use of advanced in vivo methods for assessing body composition, highlighting the limitations of BMI as an obesity indicator and its role in diseases like diabetes and cardiovascular conditions. It notes that no single method is suitable across all ages but identifies air displacement plethysmography as the most accurate and reliable. The evaluation of methods takes into account factors such as cost, patient cooperation, feasibility, and age-related assumptions.