🍽️Nutrition Assessment

Unit 4 Overview: Body Composition Analysis
4.1 Bioelectrical Impedance Analysis (BIA)
4.2 Dual-Energy X-ray Absorptiometry (DXA)
4.3 Hydrostatic Weighing and Air Displacement Plethysmography
4.4 Comparing Body Composition Methods

🍽️nutrition assessment review

4.2 Dual-Energy X-ray Absorptiometry (DXA)

6 min read•Last Updated on July 30, 2024

Dual-Energy X-ray Absorptiometry (DXA) is a powerful tool for assessing body composition. It uses low-dose X-rays to measure bone density, fat mass, and lean mass, providing detailed insights into overall health and fitness.

DXA offers high precision and regional analysis, making it valuable for tracking changes over time. While it has limitations like hydration assumptions and cost, its accuracy and comprehensive data make it a go-to method for body composition assessment.

DXA for Body Composition

Principles and Technology

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  • DXA uses two low-dose X-ray beams with different energy levels to measure bone mineral density (BMD) and soft tissue composition, including fat mass and lean mass
  • The two X-ray beams are attenuated differently by bone, fat, and lean tissue, allowing the system to differentiate between these components
    • Lower energy beam is more attenuated by soft tissue, while higher energy beam is more attenuated by bone
  • DXA scanners typically use a fan-beam or pencil-beam technology to scan the entire body or specific regions of interest (arms, legs, trunk)
  • The attenuation of the X-ray beams is measured by detectors, and the data is processed using complex algorithms to generate body composition estimates
    • Algorithms account for differences in attenuation coefficients of fat, lean tissue, and bone
  • DXA assumes a constant hydration of lean tissue (73%) and a fixed attenuation ratio for fat and lean tissue (R-value), which may introduce errors in certain populations or conditions (dehydration, obesity)

Body Composition Estimates

  • DXA provides estimates of total body and regional fat mass, lean mass, and bone mineral content (BMC)
    • Fat mass represents the amount of adipose tissue in the body
    • Lean mass includes muscle, organs, and other non-bone, non-fat tissues
    • BMC represents the amount of mineral content within the bones
  • Body fat percentage is calculated by dividing fat mass by total body mass
    • Example: If a person has 20 kg of fat mass and a total body mass of 80 kg, their body fat percentage would be 25% (20 kg / 80 kg * 100)
  • Appendicular lean mass index (ALMI) is calculated by dividing the lean mass of the arms and legs by height squared, and is used to assess sarcopenia
    • Sarcopenia is the age-related loss of muscle mass and function
    • Example: If a person has a combined arm and leg lean mass of 20 kg and a height of 1.6 m, their ALMI would be 7.8 kg/m² (20 kg / 1.6 m²)

DXA Scan Procedure

Patient Preparation

  • Before the scan, the patient's height and weight are measured, and they are asked to remove any metal objects or clothing containing metal (jewelry, zippers, underwire bras)
    • Metal objects can interfere with the X-ray beams and affect the accuracy of the results
  • The patient lies supine on the DXA table, with their arms by their sides and their legs slightly apart
    • Proper positioning is crucial for accurate and reproducible results
  • The DXA operator positions the patient correctly within the scan area and ensures that the patient remains still throughout the scan
    • Movement during the scan can cause artifacts and errors in the results

Scan Process and Duration

  • The scan typically takes 5-20 minutes, depending on the scanner model and the regions of interest being assessed
    • Whole-body scans generally take longer than regional scans (spine, hip)
  • The DXA scanner moves over the patient's body, emitting low-dose X-ray beams at two different energy levels
    • The detectors measure the attenuation of the X-ray beams as they pass through the patient's body
  • After the scan, the DXA software processes the data and generates a report with body composition estimates and images of the scanned regions
    • The report includes values for total and regional fat mass, lean mass, BMC, and body fat percentage
    • The images show the distribution of fat, lean tissue, and bone in the scanned regions

Interpreting DXA Results

Body Composition Estimates and Indices

  • DXA reports provide estimates of total body and regional (arms, legs, trunk) fat mass, lean mass, and bone mineral content (BMC)
    • These values can be used to assess overall body composition and distribution of fat and lean tissue
  • Body fat percentage is calculated by dividing fat mass by total body mass
    • This value represents the proportion of total body mass that is composed of fat
    • Healthy body fat percentage ranges vary by age, sex, and ethnicity
  • Appendicular lean mass index (ALMI) is calculated by dividing the lean mass of the arms and legs by height squared, and is used to assess sarcopenia
    • Low ALMI values may indicate the presence of sarcopenia, which is associated with increased risk of falls, fractures, and disability in older adults
    • ALMI cutoff values for sarcopenia vary by population and reference data used

Potential Sources of Error

  • Interpreting DXA results requires consideration of the patient's age, sex, ethnicity, and clinical status, as well as the use of appropriate reference data
    • Reference data should be specific to the population being assessed and the DXA scanner model used
  • Potential sources of error in DXA results include:
    • Incorrect patient positioning or movement during the scan
      • Can cause artifacts and errors in the results
    • Presence of metal implants, prostheses, or contrast agents
      • Can interfere with the X-ray beams and affect the accuracy of the results
    • Changes in hydration status or fluid balance
      • Can affect the assumed constant hydration of lean tissue and introduce errors in the estimates
    • Extreme obesity or very low body fat levels
      • Can exceed the limits of the DXA scanner's calibration and affect the accuracy of the results
    • Differences in calibration and software between DXA scanners
      • Can lead to variability in results between different scanners and manufacturers

DXA Advantages vs Limitations

Advantages

  • DXA has several advantages compared to other body composition assessment methods:
    • High precision and accuracy compared to other body composition methods (skinfold thickness, bioelectrical impedance analysis)
    • Ability to assess regional body composition and bone mineral density
      • Allows for evaluation of fat and lean tissue distribution and identification of regional changes
    • Low radiation exposure compared to other X-ray based methods (computed tomography)
      • Effective dose from a whole-body DXA scan is typically less than 1 μSv
    • Quick and non-invasive procedure
      • Does not require any special preparation or fasting
    • Widely available in clinical and research settings
      • Many hospitals, clinics, and research centers have DXA scanners

Limitations

  • Despite its advantages, DXA also has some limitations:
    • Assumption of constant hydration and fixed attenuation ratios may introduce errors
      • Changes in hydration status or fluid balance can affect the accuracy of the results
    • Limited ability to distinguish between visceral and subcutaneous fat
      • DXA cannot directly measure visceral fat, which is associated with increased metabolic risk
    • Influenced by changes in fluid balance and hydration status
      • Dehydration or edema can affect the estimates of lean mass and body fat percentage
    • May underestimate fat mass in very obese individuals
      • The limited thickness of the DXA table and the high attenuation of the X-ray beams by fat can lead to underestimation of fat mass in individuals with a BMI > 35 kg/m²
    • Requires specialized equipment and trained operators
      • DXA scanners are expensive and require regular maintenance and calibration
    • More expensive than some other body composition methods, such as bioelectrical impedance analysis (BIA)
      • The cost of a DXA scan may limit its accessibility and use in some settings