Body Composition Overview

1
Body Composition Overview

• What do you want to find out?
• Bone mineral density?
• Lean body mass?
• body fat?
• Regional body composition?

2
Total Body Density

• If you know the density, you can
  determine/estimate composition
• Difficulty is volume measurement
• Hydrostatic weighing (measure displaced water)
  gold standard
• Density mass (weight)/volume

3
Total Body Density Determine fat

• 1 / density fat mass/density fat fat free
  mass/ density of fat free tissue
• Assume constants density of fat, density fat
  free mass and amount of fat free mass (taken from
  reference body
• Problem The density of fat and fat free tissue
  are not constant and can vary widely between
  individuals. Also varies with fluid shifts and
  other disease states.
• Problem The person varies from reference body
  only in the amount of fat

4
Total Body Density

• Underwater weighing requires measurement of air
  in the lungs at the time of weighing
• Theoretical error 2
• Practical error upwards of 10
• No regional/specific information on individual
  populations

5
Total Body Water

• Water in two compartments body cells and
  extracellular fluids
• Dilution of known tracer typically stable
  isotopes of water (tritium, deuterium, oxygen-18)
• Grams of tracer administered will be diluted and
  volume calculated

6
Total Body Water

• Body water w f Cdose / C body water
• W moles of water in original sample
• F fractionation compared to water
• Cdose enrichment or concentration of dose
• Cbody water enrichment or conc in body sample

7
Total Body Water

• Assumptions
• Tracer only in body water the labeled atoms can
  undergo exchange with other organics. 2-5
  error for all isotopes
• Equal distribution of tracer to all compartments
  not usually a problem
• Fast rate of equilibrium IV takes 2-3 hours.
  Oral up to 6 or more
• No tracer metabolism constant excretion and
  dilution. Plateau method with several
  determinations after ingestion

8
Total Body Water

• Fat will not take up water
• Use a hydration constant to determine fat free
  mass
• Problems encountered must assume the hydration
  of fat free mass equal in different individuals
  and disease states, measurement error,
  instrumentation required

9
Total Body Water fat free mass

• Fat will not take up water
• Use a hydration constant (.73) to determine fat
  free mass
• Problems encountered must assume the hydration
  of fat free mass equal in different individuals
  and disease states, measurement error,
  instrumentation required

10
Bod Pod

• Naturally occurring radioactive potassium isotope
  in the body 40K
• Count gamma emissions
• Potssium will be proportional to metabolically
  active tissue - FFM
• Total body potassium correction 40K counts
• (correction is calibration factor for each
  machine)

11
Bod Pod

• Fat free mass conversion factor total body
  potassium
• Precision is excellent, accuracy dependent upon
  size of patient, state of nourishment, hydration
• Good over the short term within individuals

12
DEXA (dual energy x-ray absorptiometry)

• Underlying principle xrays are attenuated by
  body tissue, each to a different degree depending
  on frequency (energy). DEXA uses two beams at
  different energies. Ratio can accurately measure
  attenuation of each component.
• Attenuation constant between individuals. Thus,
  each pixel contains information on fat and
  lean tissue.

13
DEXA

• Advantages
• Scan the entire body for total fat, lean
  mass, and bone density
• Area determinations that tell both the fat and
  lean mass in a single region of interest
• Non-invasive, relatively inexpensive, fast
• Good for absolute measurement and following
  changes in an individual

14
DEXA

• Bone mineral density
• Attenuation calibrated to phantoms of known
  calcium content
• Very precise appr. 1
• Good accuracy 2

15
DEXA BMD Caveats

• Assumed that there is no change in attenuation
  with thickness. This is not true over about 20
  cm
• DEXA MEASURES PERCENT COMPOSITION NOT ABSOLUTE
  VALUES
• Despite labels, DEXA does not measure true bone
  density. It is the attenuation in a particular
  surface, there is no depth to the measurement

16
Electrical Impedance

• Water with electrolytes conducts water much
  better than fat, bone or air.
• Volume of a tissue can be calculated by combined
  resistance
• Dependent upon changes in volume, geometry,
  electrolytes, body temp

17
Electrical Impedance
18
Electrical Impedance

• Basic one frequency model
• Body water stature / resistance
• Stature is calculated based on body weight
• Simple equations with R gt 0.9, CV 3
• One frequency model only fair in reporting total
  body water after weight loss

19
Electrical Impedance
20
Electrical Impedance

• Best used for epidemiologic studies
• Choice of predictive equations important
• Influenced by fluid shifts
• Biggest drawback is the need for appropriately
  calibrated, cross-validated predictive equations
  (sme age, sex, ethnicity, and health)

21
Electrical Impedance

• Newer instruments use multiple frequencies with
  increased accuracy and wider capabilities.

22
CT

• Area of interest outlined on the slice (r 0.94
  with planimetry on cadavers)
• Specific density for each tissue of interest
  derived from standards
• Allows true volume measurement and thus true
  density.

23
CT

• Excellent correlation between body fat mass and
  cross-sectional abdominal adipose tissue area
• Men r0.92
• Women r0.97
• Nine scans required
• Difficulty is radiation exposure and cost

24
CT vs fat in males
25
CT vs fat in females
26
MRI

• Same principal as CT
• May need as few as 1 to 4 slices at the L4-L5
  level (questionable benefit)
• Major drawback is cost.
• Considered by some to be reference techniques for
  body composition

27
MRI
28
Summary

• Most important decide what you need to know for
  your research project.
• Be aware of errors in each method and assumptions
• Balance cost, safety and availability