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 Equations to 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

SIRI EQUATION Assumptions: Density of FAT MASS0.9 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 MASS0.9 gm/ml Density of LEAN BODY MASS 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% %

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 Units of measurement are density not % Body Fat Residual Plot Residual is difference between

S.E.E - 1 SEE + 1 SEE - 2 SEE + 2 SEE Density 1.04 gm/ml gm/ml % Fat 26.0 % Fat 2.29 % Fat % Confidence 95% Confidence Beware of the illusion of Validity

BODPOD vs U W Weighing – Influence of clothing Fields et al RESULTS: In 67 females UWW Db (1.030±0.020 g/cm 3 ) was higher (P<0.01) than BOD POD Db (1.028±0.020 g/cm 3 ). This is a difference of 1.0% fat. The R 2 was 0.94, SEE was g/cm 3 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 Weighing Fields 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 DEXA Fields et al Note the SEE values (2.4 – 4.1 % Body Fat)

BODPOD vs DEXA Fields et al “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/cm 3 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.Bioelectrical Impedance

The Limb skinfold thicknesses, particularly Triceps and Medial Calf Sites, are well related to breast size. However the lower trunk site NIRs have the highest relationships with breast NIR and size measurements. In conclusion breast size appears to relate best to limb skinfold thickness and breast composition relates best to lower trunk site fat/water ratio as indicated by the NIR measurements. NIRTricepsForearmFront ThighMedial CalfIliac CrestSupraspinaleAbdominal Right Breast Above Breast Size (cm)* Skinfolds (mm)TricepsForearmFront ThighMedial CalfIliac CrestSupraspinaleAbdominal Right Breast Above Breast Size (cm)* Breast Size = Maximum Chest Girth (cm) – Chest Girth Below Breasts (cm) Correlation Coefficients in shaded celss significant (P < 0.05) SIZE vs COMPOSITION