Assumptions inherent in prediction of % Fat from Skinfolds

Based upon densitometry

“Which is better UW Weighing or Skinfold predictions?”

%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

Assumptions inherent in prediction of % Fat from Skinfolds

Constant Skinfold Patterning Constant Skinfold Compressibility Constant Tissue Densities Constant Ratio of external/internal adipose

tissue Constant Fat (lipid) content of adipose tissue

YUHASZ

Male: % Fat = 0.1051(Sum 6 SF) + 2.585

Female: % Fat = 0.1548(Sum 6 SF) + 3.580

Canadian University Students

Can never give a negative answer.

What if weight alone changes or is different?

Durnin & Womersley

Density = a (log10Sum 4 SF) + c Overpredicts by 3 - 5% Fat British (left side) Age and gender specific equations Upper body sites Electronic Skinfold Caliper

Ultrasound

High Frequency Sound (6 MHz)

Some sound reflected at tissue interfaces

Time taken for return of sound used to estimate distance based upon assumed speed of sound in that tissue

% Fat prediction from Ultrasound

Regression equations predicting densitometrically determined % Fat

S.E.E.’s comparable to skinfold predictions

Beware of “predict anything from anything” once it is in a computer

RADIOGRAPHY

Measurements from radiographs– uncompressed tissue thicknesses

Regression equations predicting densitometrically determined % Fat

Not used any more due to possible negative health consequences

BIOELECTRICAL IMPEDANCE ANALYSIS (BIA)

BIA measured by passing a microcurrent through the body

% Fat predicted from sex, age, height, weight & activity level + BIA

Influenced by hydration level Claims that you can guess %

fat more accurately

Typical BIA Equations

Males– FFM = -10.68 + 0.65H2/R + 0.26W + 0.02R

Females– FFM = -9.53 + 0.69H2/R + 0.17W + 0.02R

Where – FFM = fat free mass (kg)– H = height (cm)– W = body weight (kg)– R – resistance (ohms)

% BF = 100 x (BW-FFM)/BW

Major types of BIA analyzers

Client Friendly

Site Specific?

BIA Protocol

Very sensitive to changes in body water– normal hydration

caffeine, dehydration, exercise, edema, fed/fasted

Sensitive to body temperature– Avoid exercise

Sensitive to placement of electrodes– conductor length vs. height

Near Infra-Red Spectrophotometry (NIR)FUTREX

Near Infra-Red light emitted from probe

Reflected light monitored Changes due to differing

optical densities Influenced by hydration Relative fat may be useful

Dual-Energy X-ray Absorptiometry

DEXA, DXADual Energy X-ray Absorptiometry

Two different energy level X-rays Lean, fat, and bone mass each reduce

(attenuate) the X-ray signal in unique ways Whole body Regional Osteoporosis

BMI = 12.6%Fat = 3.2%

BMI = 23.7%Fat = 48.1%

BMI = 18.1%Fat = 23.1%

What DEXA Measures

Fat and fat-free mass (based upon the standards)

Bone Mineral Mass Regional results for the above

DEXA Cannot Measure...

Protein Mass 3-D Fat Distribution Hydration Status Tissue inside bone (brain, marrow,

blood)

Next generation of Body Composition Models

Two compartment plus– Water– Bone mineral– Protein

3 or 4 compartment models now regarded as the reference standard rather than underwater weighing

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 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

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 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%

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

30

40

50

60

70

0 10 20 30 40 50 60 70

Hydro-Densitometry %Fat

DX

A %

Fat

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

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 !!!!!!

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.