Mass and Volume The One Compartment Model
Weight (mass) and Body Volume
The concept behind 'BMI'
Trivially easy to calculate from weight and height but far, far less relevant and useful and accurate than the 4-compartment model.
Body Mass Index
The two-compartment model is a simplistic method for assessing body composition, dividing the body into two compartments: fat mass (FM) and fat-free mass (FFM). In this model, body composition is determined based on body density. The relationship between body density and the two-compartment model is based on the assumption that fat mass and fat-free mass have different densities. Fat mass has a lower density (around 0.9 g/cm³) than fat-free mass (around 1.1 g/cm³).
To estimate body density using the two-compartment model, one method involves estimating body volume and then using this volume along with body mass to calculate density. One way to (poorly) estimate body volume is by using height as a proxy. For example, the body mass index (BMI) uses height and mass to provide a crude estimate of body composition. However, using height as a proxy for body volume has several limitations, leading to inaccurate estimations of body density, fat mass, and fat-free mass.
Some of the pitfalls of this technique include:
Individual variability: People have different body shapes and sizes, which can significantly affect body volume. Relying solely on height as a proxy for body volume does not account for these individual differences, leading to inaccurate estimations of body density and composition.
Age and sex differences: Body composition varies with age and sex, which can influence the distribution of fat mass and fat-free mass. This method does not account for these differences, potentially resulting in inaccurate estimations of body composition.
Inaccurate assumptions: The two-compartment model is based on the assumption that fat mass and fat-free mass have constant densities. However, this is not true, as the density of both compartments can vary among individuals and under different physiological conditions.
Inability to account for hydration status: Hydration status can significantly affect body density, as water is a major component of fat-free mass. The two-compartment model does not account for fluctuations in hydration, leading to inaccurate estimations of body composition.
Inability to differentiate between fat-free mass components: The two-compartment model does not distinguish between different components of fat-free mass, such as muscle, bone, and water. This can result in inaccurate estimations of body composition, particularly when assessing changes over time or in response to interventions.
Given these limitations, the two-compartment model and the use of height as a proxy for body volume are not the most accurate methods for estimating body composition. More advanced techniques, such as dual-energy X-ray absorptiometry (DXA), bioelectrical impedance analysis (BIA), or air displacement plethysmography (ADP), provide more accurate and reliable assessments of body composition, taking into account individual differences and the various components of fat mass and fat-free mass.