Validation of Methods of Estimating % Body Fat

1. Validation of Methods of Estimating % Body Fat

2. 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 How do you validate these techniques?

3. Regression Equationsto Predict % Body Fat Y = mX + c Y = % Body Fat X = Anthropometric measure (Skinfolds etc) Correlation Coefficient (r) Standard Error of Estimate (SEE)

4. 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

5. 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

6. 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

7. Siri Equation: % Fat = (4.95/Density)-4.5) x 100

8. 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%

9. Obvious Errors In 9 of 29 measured, the density of FFM was clearly not 1.1 gm/ml

10. 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

11. DEXA vs. Hydro-Densitometry

12. Beware of the illusion of Validity

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

14. Review of BODPOD vs U W WeighingFields et al. 2002

15. 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 U W WeighingFields et al. 2002

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

17. “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 !!!!!! BODPOD vs DEXAFields et al. 2002

18. “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

19. Typical SEE’s for Doubly Indirect Methods

20. 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!!!