Multicomponent models of body composition are a class of inference models that use more than one source of measurement data to achieve a more valid estimate of body composition.
The human body can be described in terms of its different components (or “compartments”), each of which contains different types of atomic, molecular, cellular or tissue material. These components can be assessed and combined to describe the composition of the human body; as described in Table 1, models typically use between 2 and 4 components, but 5 and 6component models also exist The components which comprise different multicomponent models are summarised in Figure 1.
Figure 1 Components measured to generate different multicomponent models.
Source: MRC Epidemiology Unit.
Important: Figure is not to scale and represents the assumptions of the models, not the exact relationships between different components invivo.
*Includes water, protein, glycogen, bone mineral content, and non osseous mineral content.
**Includes protein, glycogen, bone mineral content, and nonosseous mineral content.
***There is a difference between mineral content and mineral mass. Measures of mineral content are typically converted to mineral mass to reflect the ashing process.
2component models
Assessment of body composition requires quantification of at least two components; we refer to this as a 2component (2C) model, the most common of which divides total body mass into fatfree mass (FFM) and fat mass (FM).This model assumes that fatfree mass and fat mass have densities of 1.1 kg/L and 0.9 kg/L respectively and this approach typically uses the following equations to estimate % body fat:
Measurements that allow inferences to be made on these components include:
However, the 2C model is subject to error due to interindividual variation in the composition of the fatfree mass compartment. Fatfree mass consists of minerals, proteins and water. The 2C model assumes constant values for water content (hydration fraction), bone mineral content (BMC), and density of fatfree mass, but these do vary between ethnicities, age groups, pubertal status, pregnancy, weight loss status, and in patients with deranged hydration (e.g. chronic renal failure, cirrhosis). The 2C model is therefore not ideal for assessing fatfree mass under these conditions and circumstances.
3 and 4component models
Models that assess 3 or 4 components expand upon 2component models by measuring fatfree mass constituents with greater certainty. The 3component (3C) model divides body mass into fat, water, and fatfree dry mass (proteins and minerals). This model avoids the assumption that water content of fatfree mass is constant between individuals, however the ratio between the proteins and minerals of fatfree dry mass is assumed to be constant. This model requires the following data:
The 4component (4C) model further divides fatfree dry mass into protein and bone mineral content (BMC). This model requires the same information applied in the 3C approach, with the addition of the measurement of bone mineral content by dualenergy Xray absorptiometry) Estimating bone mineral content and protein mass avoids the assumption that the proteintomineral ratio in fatfree mass is constant; the 4C model still assumes that the ratio between bone mineral and nonosseous mineral content is constant. However, this model is robust to major differences in this ratio.
Table 1 Summary of differences between 2, 3, and 4component models.
Model  Components  Measurements needed  Assumptions relating to composition 
2component model 
1. Fat mass
2. Fatfree mass 
Densitometry*
or Hydrometry or Bioelectrical impedance analysis or Whole body counting of total body potassium 
Constant density of fatfree mass
Constant water content of fatfree mass Constant bone mineral to muscle ratio in fatfree mass 
3component model 
1. Fat mass
2. Water 3. Fatfree dry mass 
Densitometry*
and Hydrometry 
Constant proteintomineral ratio of fatfree dry mass 
4component model 
1. Fat mass
2. Water 3. Protein 4. Bone mineral content 
Densitometry*
and Hydrometry and Dualenergy Xray absorptiometry (DEXA) 
Constant bone mineral content to nonosseous mineral content ratio 
*Densitometry methods include: hydrostatic underwater weighing and air displacement plethysmography.
5 and 6component models
Other models have been developed, which assess 56 components. The five component model (5C) divides total body mass into water, fat mass, protein mass, bone mineral mass and nonosseous mineral mass (notably soft tissue minerals). While, the six component model developed by Wang et al. (2015), includes estimates for residual mass components soft tissue mineral and glycogen.
However, these models are used less frequently than the 4C model, which is considered the reference method for the invivo assessment of overall body composition (but not for fat distribution). It is robust to interindividual variability in the composition of FFM as more measurements are performed on this compartment, thus the inference requires fewer assumptions.
Several measurements are required to construct 3C and 4C models, as shown in Table 1. Information about these methods is available on separate pages.
Body weight and body volume are measured using densitometry (e.g. hydrostatic underwater weighing or air displacement plethysmography) and total body water is measured using hydrometry (isotope dilution) or bioelectric impedance analysis if following the 3C model.
For the 4C model, the same data applied in the 3C approach are used, with the addition of the measurement of bone mineral content (BMC) using dualenergy Xray absorptiometry (DEXA).
Taking into account the various assumptions underlying the densities and constant ratios (proteintomineral ratio in the 3C model; and the bone mineral content to nonosseous mineral ratio in the 4C model), fat mass can be estimated from the combined measurements of:
The above measures are combined using the formulae in Table 2. There are different equations available for the same input variables but their output results for fat mass are very similar as they are all based on similar assumptions. They share assumed constant densities for fat 0.9007 g/cm^{3}, water (0.99371 g/cm^{3} and for bone mineral (2.982 g/cm^{3}), but two different approaches are taken in developing the equations:
Table 2 shows a selection of multicomponent models to estimate overall body fat. In these models, four quantities are measured: body volume, total body water, bone mineral and body mass.
Table 2 Selection of multicomponent models to estimate overall body fat mass.
Models  Equations 

3component model 

1 Siri (1961)  FM = 2.057*BV0.786*TBW1.286*BM 
2 Lohman (1986)  FM = 6.386*BV+3.961*M6.09*BM 
3 Silva (2004)  FM = 2.122*BV0.779*TBW1.356*BM 
4 to 6component models 

4 Selinger (1977)  FM = 2.747*BV0.714*TBW+1.129*Mo2.037*BM 
5 Lohman (1992)  FM = 2.747*BV0.714*TBW+1.146*Mo2.053*BM 
6 Heymsfield et al. (1990)  FM = 2.748*BV0.6744*TBW+1.4746*TBBA2.051*BM 
7 Baumgartner et al. (1991)

FM = 2.747*BV0.7175*TBW+1.148*Mo2.058BM 
8 Fuller et al. (1992)  FM = 2.747*BV0.710*TBW+1.460*TBBA2.05*BM 
9 Withers et al. (1992)  FM = 2.513*BV0.739*TBW+0.947*Mo1.790*BM 
10 Friedl et al. (1992)  FM = 2.559*BV0.734*TBW+0.983*Mo1.841*BM 
11 Siconolfi et al. (1995)  FM = 2.7474*BV0.7145*TBW+1.1457*Mo2.0503*BM 
12 Heymsfield et al. (1996)  FM = 2.513*BV0.739*TBW+0.947*Mo1.79*BM 
13 Forslund et al. (1996)  FM = 2.559*BV0.734*TBW+0.983*Mo1.841*BM 
14 Wang et al. (2002)^{a}  FM = 2.748*BV 0.699*TBW+1.129*Mo2.051*BM 
15 Wang et al. (2014)^{b}  FM = 2.720*BV 0.715*TBW+1.108*Mo2.020*BM 
Adapted from Wang (2005) and Heymsfield et al. (2015).
BM = Body mass (kg); BV = body volume (L); FM = fat mass (kg); M= total mineral mass (kg); Mo = bone mineral mass (kg); TBBA = total body bone ash (kg); TBW = total body water (L).
Total mineral mass (M) includes bone mineral mass (Mo) and nonosseous mineral mass.
Bone mineral mass (Mo) includes total body bone ash (TBBA) and nonosseous mineral mass.
^{a}5component equation.
^{b}6component equation.
Some equations in Table 2 require that the bone mineral content from DEXA is converted to bone mineral mass (Mo) or total body mineral mass (Mo + nonosseous) in Table 2.
Body mineral content or total body bone ash (TBBA) is typically converted to bone mineral mass (Mo) by multiplying TBBA*1.0436. This is to reflect the ashing process. Most DEXA systems have adjusted for this process (see Heymsfield 2015).
Different conversion factors also exist for the derivation of total body water mass from labelled water dilution volumes. Each equation takes a different strategy to derive soft nonosseous mineral mass. Nonosseous minerals, glycogen and other residual mass components are taken into account. Refer to each equation for the various strategies/assumptions and conversation factors (see reference list).
An overview of multicomponent models is outlined in Table 3.
Strengths
Limitations
Table 3 Characteristics of multicomponent models.
Consideration  Comment 

Number of participants  Small 
Relative cost  High 
Participant burden  High as several techniques are required 
Researcher burden of data collection  High as several techniques are required 
Researcher burden of coding and data analysis  High 
Risk of reactivity bias  No 
Risk of recall bias  No 
Risk of social desirability bias  No 
Risk of observer bias  No 
Space required  High 
Availability  Medium 
Suitability for field use  Low 
Participant literacy required  No 
Cognitively demanding  No 
Considerations relating to the use multicomponent models in specific populations are described in Table 4.
Table 3 Use of multicomponent models in different populations.
Population  Comment 

Pregnancy  3C models are typically used as DEXA (required for 4C) is not feasible (ionizing radiation )*. 
Infancy and lactation  Suitable. 
Toddlers and young children  Suitable (3C approach might be used more often as the use of DEXA in this population can be challenging). 
Adolescents  Suitable. 
Adults  Suitable. 
Older Adults  Suitable. 
Ethnic groups  Suitable. 
Other (obesity)  Suitable. 
*Forsum et al. (2014).
Refer to section: practical considerations for objective anthropometry