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

d

Y

X

Y = mX + c Y = % Body FatX = 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

SIRI EQUATION

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

BROZEK EQUATION

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 100

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 Fatdue to variation in density of fat free mass

Example: predicted value = 15% Body Fat95% confidence in true value = 15 ± 1.96 x S.E.E.

= 15 ± (1.96 x 2.77) = 9.57% - 20.43%

ID # Body Density % Fat via Siri’s equation

Sum of 10 Skinfolds

22 1.100 0 63 16 1.101 -0.4 74 24 1.102 -0.8 57 2 1.103 -1.2 55 5 1.103 -1.2 97 9 1.105 -2.0 69 26 1.105 -2.0 87 28 1.129 -11.6 64 25 1.130 -12.0 88

Body fat predictions for 9 professional football players (Adams et al., 1982).

Obvious ErrorsIn 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

0

10

20

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70

0 10 20 30 40 50 60 70

Hydro-Densitometry %Fat

DXA

%Fa

t

n = 91 subjectsr = 0.92SD = 3.7%SEE = 0.045

Beware of the illusion of Validity

BODPOD vs U W Weighing – Influence of clothingFields et al. 2000

RESULTS: In 67 females UWW Db (1.030±0.020 g/cm3) was higher (P<0.01) than BOD POD Db (1.028±0.020 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. 2002

Review of BODPOD vs U W WeighingFields et al. 2002

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 ± 4.5%BF

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 4.1%BF– 95% confidence of ± 8%BF !!!!!!

“Which is better UW Weighing or Skinfold predictions?”

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 SEE’s 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 TRUE!!!

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.