BOD POD Study Notes

BOD POD Overview

  • BOD POD is a method for assessing body composition (e.g., body fat percentage).
  • It operates similarly to hydrostatic weighing but utilizes air displacement instead of water displacement.

Key Concepts of BOD POD

Methodology

  • Hydrostatic Weighing vs BOD POD:
    • Hydrostatic weighing measures the volume of water displaced by the body to determine body density.
    • BOD POD measures the volume of air displaced by the body to determine body density.
  • After measuring air volume, equations (such as Siri or Brozak) are used to convert density into body fat.

Mechanism of Action

  • BOD POD consists of two closed cylinders separated by a diaphragm.
  • The diaphragm moves, temporarily altering the volume of the chamber and measuring the resulting pressure changes.
    • Diaphragm Movement:
    • The diaphragm functions like a bass speaker, moving air in and out of the test chamber.
    • Changes in air volume cause corresponding changes in pressure, which are used to calculate the subject's volume.
  • Pressure-volume relationships dictate that as the diaphragm displaces air, the pressure changes correspondently based on the volume occupied by the subject.

Pressure Measurement

  • Normal Pressure Values:
    • The BOD POD measures base pressure with no subject in the chamber.
    • A calibration cylinder (of known volume) can also be used for comparison in pressure changes to ensure accurate readings.
  • Volume Occupied by Subject:
    • Larger subjects will displace more air, resulting in a larger pressure increase compared to smaller subjects.
    • Measurement process involves:
    1. Measure pressure with no one in the BOD POD.
    2. Measure pressure with a known volume inside to calibrate.
    3. Measure pressure with the subject and analyze volume based on pressure changes.

Types of Air in BOD POD

  • There are two types of air to consider:
    1. Adiabatic Air: Air not influenced by heat sources that retains different compressibility characteristics compared to isothermal air.
    2. Isothermal Air: Air in contact with the body or in the lungs, affected by body heat, leading to different compressibility.
  • Impact of Air Type on Measurements:
    • Adiabatic air will yield different pressure readings than isothermal air.
    • If an individual is measured (versus a mannequin without body heat), the isothermal air leads to underestimation of volume.

Factors Affecting Measurement

Body Hair and Surface Area
  • Body hair traps isothermal air, impacting the compression and thus the measured air volume inside the chamber.
  • Precautions:
    • All subjects are required to wear swim caps to standardize this effect.
Lungs and Thoracic Gas Volume

  • Thoracic Gas Volume: Average volume of air in the lungs during normal breathing, needing consideration as lung air is warm (isothermal).

  • To account for thoracic gas volume:

    1. Use Tidal Volume (TV) - the volume of air inhaled or exhaled during normal respiration.
    2. Include Functional Residual Capacity (FRC) - total air remaining post-exhalation, combining residual volume and expiratory reserve volume.

  • Calculation for Thoracic Gas Volume:

    Thoracic Gas Volume=TV2+FRC\text{Thoracic Gas Volume} = \frac{TV}{2} + FRC

  • Measurements should ideally represent normal breathing for accuracy.

Clothing and External Factors
  • Clothing significantly influences the volume of isothermal air reflected in measurements.
  • Recommended Attire:
    • Men: Tight-fitting shorts (e.g., compression shorts); must avoid padded clothing which traps air.
    • Women: One-piece swimsuits are optimal, or alternatively, snug yoga shorts with a sports bra (minimizing the trapping of air).
  • Consistency in clothing is essential for serial measurements.
  • Recommendation: Subject garments must be dry to avoid moisture affecting air compression.
GI Tract and Eating Conditions
  • BOD POD doesn’t account for air in the closed GI tract; however, food density changes can complicate density calculations.
  • For accuracy, subjects should ideally come in a fasted state, although this is less critical compared to hydrostatic weighing.

Procedure for Using the BOD POD

Pre-Test Instructions

  • Void before Testing: Subjects should use the bathroom to minimize excess volume from food or liquid.
  • Clothing Check: Verify that subjects wear the recommended clothing to prevent air trapping effects.
  • Calibration:
    • BOD POD must be calibrated multiple times with and without the known volume cylinder prior to starting.
  • Weight Measurement: While subjects stand on the scale, any removable jewelry should be taken off before measurement.

Testing Process

  • Entering the Chamber: Subjects must sit comfortably, hands by their sides, and minimize movement during getting measured.
  • It is vital for subjects to breathe normally without disruptions (e.g., coughing, sneezing, laughing) as this affects pressure readings.
  • Each measurement lasts approximately thirty seconds, with breaks to let fresh air in and acclimatize chamber conditions.

Results Collection

  • Minimum of two complete measurements are necessary and recorded; if differences are significant, a third test may be required.
  • Results showing consistent pressure readings (within 150 mL) are finalized; otherwise, retesting may be needed.
  • Lean individuals might have difficulty getting results if their readings fall below the threshold interpreted by BOD POD algorithms (usually <1-2% body fat).

Lung Volume Calculation Options

  1. Measured: Involves spirometry to directly assess thoracic gas volume, typically less utilized due to complexity.
  2. Predicted (Most Common): Computation using age, sex, and height to estimate thoracic gas volume.