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Validation of Methods of Estimating % Body Fat


Validation of Methods of Estimating % Body Fat – PowerPoint PPT presentation

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Title: Validation of Methods of Estimating % Body Fat

Validation of Methods of Estimating Body Fat
How do you validate these techniques?
  • There can be no direct validation
  • Measure subjects with technique to get fat then
    kill them, blend them and dissolve out lipid
  • Validation of Indirect techniques is by
    comparison to other Indirect techniques
  • Which analysis indicates validity
  • Correlation
  • Test of Difference of means between tests
  • Linear regression slope of unity
  • Standard Error of Estimate

Regression Equationsto Predict Body Fat
Y mX c Y Body Fat X Anthropometric
measure (Skinfolds etc) Correlation Coefficient
(r) Standard Error of Estimate (SEE)
Predicting Fat from Density
ASSUMPTIONS Body can be divided into two
components Fat Non-Fat (Fat Free) Masses Each
has different, known and constant densities
Assumptions Density of FAT MASS 0.9
gm/ml Density of NON-FAT MASS 1.1
gm/ml Equation Fat (4.95/Density)-4.5) x 100
Assumptions Density of FAT MASS 0.9
gm/ml Density of LEAN BODY MASS 1.095
gm/ml (some essential lipids in Lean Body
Mass) Equation Fat (4.57/Density)-4.142) x
Siri Equation Fat (4.95/Density)-4.5) x 100
Error in Prediction of Fat
Standard Error of Estimate for Fat from
Densitometry S.E.E. 2.77 Body Fat due to
variation in density of fat free mass Example
predicted value 15 Body Fat 95 confidence
in true value 15 1.96 x S.E.E. 15 (1.96
x 2.77) 9.57 - 20.43
Obvious Errors In 9 of 29 measured, the density
of FFM was clearly not 1.1 gm/ml
Variability of Constants
  • The existence of this table infers that we should
    know the precise density of FFM.
  • However, using arbitrary cut-offs between age
    groups merely highlights the problem

DEXA vs. Hydro-Densitometry
Beware of the illusion of Validity
BODPOD vs U W Weighing Influence of
clothingFields et al. 2000
  • RESULTS In 67 females UWW Db (1.0300.020 g/cm3)
    was higher (Plt0.01) than BOD POD Db (1.0280.020
    g/cm3). This is a difference of 1.0 fat.
  • The R2 was 0.94, SEE was 0.005 g/cm3 and the
    regression between Db by UWW and BOD POB did not
    significantly deviate from the line of identity.
  • CONCLUSION This study supports the use of the
    BOD POD as a substitute for UWW. However, caution
    should be made in using the BOD POD if subjects
    are clothed in anything other than a tight
    fitting swimsuit.

Review of BODPOD vs U W WeighingFields et al.
Review of BODPOD vs U W WeighingFields et al.
  • the SEEs reported in 4 of the 12 studies ranged
    from 1.8 to 2.3 BF. These SEEs are in the
    excellent to ideal range (2.5 BF) according to
    Lohman (1992).
  • SEE 2.3 BF gives
  • 95 confidence of 1.96 x 2.3 BF
  • 95 confidence of 4.5BF

Review of BODPOD vs DEXAFields et al. 2002
  • Note the SEE values (2.4 4.1 Body Fat)

BODPOD vs DEXAFields et al. 2002
  • SEEs ranged from 2.4 to 3.5 BF?
  • which were distributed among the good, very
    good, and excellent categories, as subjectively
    assessed by Lohman (1992)
  • SEE 4.1 BF gives
  • 95 confidence of 1.96 x 4.1BF
  • 95 confidence of 8BF !!!!!!

Which is better UW Weighing or Skinfold
  • Based upon densitometry

fat from skinfolds is predicted using equations
developed from UW Weighing of subjects. UW
Weighing S.E.E. 2.77 Fat Skinfolds
S.E.E. 3.7 Fat
Typical SEEs for Doubly Indirect Methods
The New York Obesity Research Center
  • The assumed density of 1.1 g/cm3 is based on
    observations made in a limited number of human
    cadavers suggesting relatively stable proportions
    of water, protein, glycogen and minerals. To the
    extent that these proportions change in any
    individual subject will introduce corresponding
    errors in the assumed density of fat-free mass.
  • A number of studies suggest that the density of
    fat-free mass is relatively stable across age and
    sex groups, although some variation is recognized
    at the extremes of age and in patients who have
    underlying medical and surgical conditions. NOT
  • Additionally, there may exist race differences in
    the density of fat-free mass as well as variation
    among special groups such as body builders or
    other types of athletic participants. Thus, while
    underwater weighing and the two-compartment model
    served as a reference technique for several
    decades, newer approaches without these various
    assumptions are now replacing hydrodensitometry
    as the clinical reference method. MISLEADING!!!

Beware of Garbage
  • BIA (Bioelectrical Impedance) - The only method
    that is based on measuring something, not
    estimating anything, is Bio-Impedance
    measurement. Bio-Impedance is a means of
    measuring electrical signals as they pass through
    the fat, lean mass, and water in the body.
    Through laboratory research we know the actual
    impedance or conductivity of various tissues in
    the body, and we know that by measuring current
    between two electrodes and applying this
    information to complex proven scientific formulas
    accurate body composition can be determined. The
    fact that the measurement is based on a reading
    of lean mass and not an estimate of fat mass,
    lends to a much more comprehensive testing method
    and results.
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