Methods of Body Fat Measurement
Hydrostatic (underwater) weighing: Archimedes’ principle is the theory on which this technique for measuring body composition is based. It states that when a body is immersed in water, it is buoyed by a counterforce equal to the weight of the water displaced. The density of body fat is less than that of water, therefore contributing to the buoyancy of the body, as does air in the lungs. Muscle and bone tissue have a greater density than that of water and can cause a body to sink, therefore an individual with greater fat free mass but the same overall body mass, will weigh more in water due to the higher body density and lower percentage body fat. Although hydrostatic weighing is considered the “standard” when measuring body composition, there are numerous sources of error associated with the procedure. The formulas used to convert body density to percent fat assume a density of 0.900 g/ml for the fat mass and 1.100 g/ml for the density of the fat free mass. However the density of the fat free mass varies with age, gender, and ethnicity, use of a single general formula to convert body density to percentage body fat for all populations would therefore result in systematic errors. Even with flawless measurement technique, hydrostatic weighing results in an error estimate of ±2% body fat due to the individual differences in the density of fat free mass.
Plethysmography (air displacement): This method is based on a two compartment model of body composition, fat mass and fat free mass, based on Boyle’s law it uses the inverse relationship between pressure and volume to derive body volume for a subject. If both volume and bodyweight are known body density can be calculated by using the following equation:
Body Density = mass/volume, where mass = body weight (kg) and volume = liters (l)
The change in pressure and volume between the empty chamber and the chamber with the subject present can be used to calculate the volume of the subject, since one side of the equation is already known.
Boyles Law: P¹ x V¹ = P² x V² Where P = pressure and V = volume
Body Mass Index (BMI): This method is used to assess weight relative to height, it is calculated by dividing bodyweight in kilograms by height in meters squared (kg/m²) A BMI ranging between 25.0 to 29.9kg/m² is considered overweight whereas a BMI of 30 or greater is considered obese. However due to the large margin of error associated with estimating percentage body fat from BMI (+/- 5% fat) it is not considered a useful tool for evaluating an individual’s body fat.
Skinfold measurement: A skinfold thickness is two layers of skin and two layers of fat, to measure the skinfold thickness, a calipers is used, this should be calibrated and have a constant pressure of 10 g/mm². Body fat percentage determined form skinfold measurements correlates well with that determined through hydrostatic weighing (r = 0.79 – 0.90). This technique is based on the principle that the amount of subcutaneous fat is proportional to the total amount of body fat. However, age gender and ethnicity all have an effect on the proportion of subcutaneous fat to total fat, which means that these variables must be considered when using the regression equations to convert sum of skinfolds to percentage body fat. Assuming that the correct technique ahs been used and that the correct equations have been applied, the prediction of percentage body fat has an accuracy of ±3.5%
Bioelectrical Impedance Analysis
A relatively easy to administer method of measuring body fat percentage. Bioelectrical impedance analyses works by determining the electrical impedance of the body tissues, which provides an estimate of total body water, which can be used to estimate fat free mass and percentage body fat. It involves the passing of a small electrical current through the body and measuring the impedance to current flow. Fat free tissue is a good conductor of electrical currents, therefore body fat impedes current flow, and the impedance to current flow is inversely related to the fat free mass and total body water, both of which can be estimated by using this technique. Generally bioelectrical impedance analyses yields similar results to skinfold measurements with regards to percentage body fat estimates, but the protocol that MUST be followed by the subject make it a little difficult.
Prior to assessment the subject must do the following to achieve maximum accuracy in results:
- 4 hours fasting prior
- No exercise within 12 hours
- No alcohol within 48 hours
- Each subject must void completely before assessment
- Abstain from diuretic agents including caffeine, unless medically prescribed