What methods are included to calculate total body fat?

Methods to Calculate Total Body Fat

Total body fat is calculated using multiple methods, with dual-energy X-ray absorptiometry (DXA) serving as the gold standard reference method, while practical clinical approaches include skinfold thickness measurements, bioelectrical impedance analysis (BIA), and anthropometric measurements including weight, height, circumferences, and bone breadths. 1

Gold Standard Method

• DXA is the reference standard for measuring body composition in clinical settings, providing direct measurement of three compartments: fat mass, lean mass, and bone mass 1
• DXA allows segmental body composition analysis including visceral fat mass and appendicular skeletal muscle mass 1
• The method is fast, noninvasive, and exposes patients to acceptable radiation doses (2-5 mSv) 1
• Important caveat: DXA measurements are influenced by hydration status, making timing critical—in dialysis patients, assessments should be obtained after treatment when fluid compartments are balanced 1

Practical Clinical Methods

Skinfold Thickness Measurements

Skinfold measurements are the most simple, long-established, and inexpensive method for assessing body fat, showing excellent correlation with DXA (r = 0.94). 1

Standard 4-Site Method (Durnin and Womersley):

• Measure four skinfolds: biceps, triceps, subscapular, and suprailiac 1
• Sum all four measurements in millimeters 1
• Compute the logarithm of the sum 1
• Apply age- and sex-adjusted equations to compute body density (D, g/mL) 1
• Calculate fat mass: Fat mass (kg) = body weight (kg) - [(4.95/D) - 4.5] 1
• Calculate fat-free mass: FFM (kg) = body weight (kg) - fat mass (kg) 1

Alternative 3-Site Method:

• Measures chest, abdomen, and thigh skinfolds, showing high correlation (r = 0.98) with the 7-skinfold model 1
• This simplified approach enhances feasibility while maintaining accuracy 1

Performance Characteristics:

• Skinfold thickness showed excellent agreement with DXA (mean difference ± SD: 0.47 ± 2.8 kg; 95% limits of agreement: -5.0 to 6.0 kg) 1
• Skinfold measurements are preferable over BIA because they show less sex-specific variability and are less affected by fluid status changes 1
• Standard errors are 2.6 kg for fat-free mass and 3.5% for percent body fat 1

Technical Requirements:

• Requires considerable technical skill and meticulous attention to site location 1
• Measurements should be within 4 mm of each other; if not, take four measurements and record the mean 1
• Limitation: Difficult to obtain reliable readings in older patients with loose connective tissue or obese individuals with large folds 1

Bioelectrical Impedance Analysis (BIA)

• BIA uses hand-to-foot surface electrodes providing raw electrical data: reactance, resistance, and impedance 1
• Critical understanding: BIA only directly measures fluid-related compartments (fat-free mass); fat mass is obtained by subtraction from body weight 1
• BIA showed good correlation with DXA (r = 0.91) but demonstrated greater mean prediction error, particularly showing sex-specific variability 1
• Major limitation: Fluid status significantly affects BIA readings, making it inadequate for critically ill patients 1
• Sun's equation is the most appropriate BIA equation for population-level studies 1

Anthropometric Measurements

Combining skinfold thicknesses with circumferences and bone breadth measures provides more precise prediction of body fat than skinfolds alone. 2

For Men:

• Best equation: BFM (kg) = -40.750 + {(0.397 × waist circumference) + [6.568 × (log triceps SF + log subscapular SF + log abdominal SF)]} 2
• Waist circumference is a particularly strong predictor in men 1

For Women:

• Best equation: BFM (kg) = -75.231 + {(0.512 × hip circumference) + [8.889 × (log chin SF + log triceps SF + log subscapular SF)] + (1.905 × knee breadth)} 2

Simple Anthropometric Predictors:

• BMI has the strongest association with total and visceral fat mass (r = 0.89 for total fat mass, r = 0.77 for visceral fat mass) 3
• Waist circumference and abdominal sagittal diameter are excellent predictors of total abdominal fat (r = 0.87 to 0.93) and visceral fat (r = 0.84 to 0.93) 4
• Waist-to-hip ratio is the least predictive measure and should not be relied upon 4

Additional Methods

• Near-infrared interactance: Showed correlation with skinfolds (r = 0.78) but is less commonly used 1
• Computed tomography (CT): Provides criterion measure of total abdominal and visceral fat but is expensive, involves radiation exposure, and shows wide variation when compared to other methods 4, 5
• Creatinine kinetics: Correlates with muscle mass but shows significant differences from anthropometric measures for lean body mass 1

Method Comparison and Selection

• DXA identifies higher fat mass than BIA, with larger variations at both extremes of BMI 1
• Results from DXA and BIA should not be clinically compared interchangeably 1
• For longitudinal monitoring, the same method should be used consistently, as results are confounded by hydration status and inter-method variability 1
• In overweight and obese patients, adding fat-free mass measurements to weight-based equations provides no additional advantage 1
• Predicted fat mass and percent fat have significantly stronger associations with obesity-related biomarkers than BMI alone 6

Common Pitfalls

• Hydration status significantly affects both DXA and BIA measurements; ensure consistent timing and hydration state 1
• Skinfold measurements underestimate percent body fat, especially at extremes of body fatness, if linear rather than quadratic equations are used 1
• Age must be included in generalized equations as it is independently related to body composition 1
• Race-specific skinfold equations should be used when available to account for racial differences in body composition 1
• BMI-based calculations do not account for differences in body composition and may misclassify individuals with high muscle mass 7