Why measure body composition?
Everyday experience shows us that human beings come in many different shapes and sizes. These differences are a result of different genetic background and realisation of this genetic potential, modified by nutritional and environmental influences, over the individual’s life course. Differences in gross body habitus are, in turn, a reflection of different individual body compositions; the masses of the individual tissues and organs that comprise the body. The term, body composition, is not, however, limited to describing the relative sizes of anatomically identifiable body components. It is also used more commonly to describe the body in terms of conceptual models that represent body constituents in terms of compartments that represent biologically functional entities such as lean mass or total fat mass (Fig. 1).
The composition of the body, at any point in time, captures the outcome of all prior influences upon human physiology that have led to the accumulation of all these different constituents up to that point . Other than the underlying genetic milieu, principal among these influences are nutrition and illness. Body composition represents an historic record of life’s balance of energy and nutrient intake. When nutritional intake is mismatched to requirement change in body composition occurs. For example, it has long been recognised that inadequate nutrition may lead to altered body composition as seen in wasting and stunting , while, conversely, over-nutrition leads to obesity particularly in children in populations undergoing nutritional transition . Similarly, body composition may be adversely affected by ill-health, e.g., surgical trauma may lead to short-term loss of lean mass while long-term infection with tuberculosis is associated with wasting . The composition of the body, as well as being a reflection of nutritional experience, is also a gauge of body function. Optimal bodily functioning and good health demand a body composition that provides the ideal masses of tissues and organs that support the physiological and biochemical processes that underpin a healthy life. These two factors are not independent; nutrition impacts upon body composition which in turn impacts on bodily function. Knowledge of body composition provides insights into both nutritional status and functional capacity of the human body.
Yesterday: what do we already know about human body composition?
How do we measure body composition?
The analysis of body composition has a long and rich history (those interested in a detailed account of the history of body composition analysis are referred to Wang et al. ). The first quantitative studies of the human body date to early cadaver analyses in the 1840s. The obvious drawback of cadaver analysis is that it can only be undertaken at time of death and can only provide retrospective information. Since the early part of the 20th century, few cadaver studies have been undertaken, with the notable exception of the work of Clarys and colleagues , and the focus has been on the development of technologies for the quantitative assessment of body composition in vivo (Table 1). One of the first measurements, by Shaffer and Coleman in 1909 , was the estimation of muscle mass from the measurement of the excretion rate of creatinine, but the real advance came with the availability of isotopes of atoms of nutritional interest.
The availability of the stable isotope of hydrogen, deuterium, in the form of deuterated water in the mid-1930s allowed the measurement of total body water (TBW) by von Hevesey and Hofer . TBW is under tight physiological control and in healthy humans the hydration of the non-fat compartment of the body, the fat-free mass (FFM), is relatively constant at 0.732 mL/g FFM. If FFM is known, by difference with body weight (BW-FFM), body fat may be determined. This simple two-compartment (2C) model of the human body has become the mainstay of body composition assessment in clinical nutrition. A little later, in 1940s and 1950s, the 2C model was approached from the other direction, first estimation of fat, the development of methods based on Archimedes principle and the different densities of fat and FFM, allowed quantification of body fat mass. While remaining a research technique in a few centres, densitometric methods based on under-water weighing largely languished in the 1970s through 1990s but were revitalised with the development of air-displacement plethysmography (ADP) in 1995 [7, 8]. Commercially available ADP devices have now made densitometry a tool usable not only in nutrition research but also in clinical practice.
Many different techniques for body composition analysis have been explored; most have fallen by the wayside as it is recognised that disadvantages outweigh their usefulness (Table 1). A core set of methods remain in wide-spread clinical use: anthropometry, tracer dilution, densitometry, dual-energy X-ray absorptiometry (DXA), and bioelectrical impedance analysis (BIA)—all predominantly devoted to characterising the human body in terms of a 2C model of FFM and fat mass. The methods vary in precision and accuracy.
Two technologies for body composition assessment deserve particular mention. BIA became popular from the mid-1980s when a simple to use impedance device became commercially available [9, 10]. BIA systems measure the opposition to the flow of an harmless electric current through the body and, since electricity is conducted through body water, can provide an estimate of TBW which may then be transformed to a prediction of FFM based on an assumed hydration constant as for the deuterium dilution technique. This same period has seen wide-spread application of DXA for body composition analysis. Originally developed for the measurement of bone mineral density and content (BMC), DXA systems are now widely used to measure not only BMC but also lean and fat. DXA systems thus straddle the line between a 3-compartment model and the 2C model (FFM = BMC + lean) of the body.
Finally, imaging techniques such as nuclear magnetic resonance imaging (MRI) and computed tomography (CT) are become powerful tools in the armamentarium of the of the clinician and nutritionist. Still primarily considered as research tools because of cost and complexity of use, they are the closest we can get to emulate cadaver analysis in vivo by visualising and quantifying tissues, organs, or constituents such as muscle and adipose tissue.
The body compartments/constituents that can now be routinely measured include FFM, fat mass, TBW, and its sub-compartments intracellular water and extracellular water and BMC. This information can be gained for the whole body and for some technologies, e.g., BIA and DXA for body regions. Both of these techniques may be used in both research and routine clinical practice. In a research setting, where MRI and CT are available, these can provide actual tissue volumes of, for example, adipose tissue or muscles. In a handful of centres around the world, whole body potassium counting is available that can measure the metabolically active body compartment, the body cell mass .
Pierson  identified two recent “epochs” in the development of body composition methods: the first from 1963, the publication of Moore’s seminal text The Body Cell Mass and Its Supporting Environment, to 1986, a period marked by the search for precision in measurement, and, the second, from 1986, the year of the 1stInternational Symposium on In Vivo Body Composition Studies to the present, marked by the search for accuracy in measurement. The quest for accuracy and precision has proved worthwhile. Methods in current use typically vary in precision from 2 to 5% with similar degrees of accuracy depending upon technique and the body compartment being measured. Efforts continue to improve accuracy and precision of methods and the literature is replete with publications comparing different methods in attempt to determine which is the more accurate. Unfortunately, many of these do not compare a test method to an accepted reference method ideally a 4-compartment model. Equally, there is no universal consensus on what is a clinically acceptable degree of accuracy. Body composition analysis lags here compared to clinical chemistry for which reference methods, protocols, and quality control procedures exist specifying acceptable levels of precision and accuracy.
Where has body composition information proved useful?
Even a cursory survey of the literature finds a wide range of applications for body composition analysis in nutrition. These include describing growth and development from birth through to adulthood, understanding the developmental origins of health and disease, understanding nutrition in public health and the design of population level nutritional strategies, the physiology of aging, the impact of disease, and the monitoring of therapeutic interventions [13, 14].
Today: what we are getting to know now?
The emphasis in body composition research is evolving from simple assessment of body compartment sizes to attempts to link body composition to function. Functional body composition aims to existing quantitative knowledge of body compartments with their functional roles within the body’s regulatory environment . The most notable advances in this area is the ability to link body composition to energy balance and regulation .
Great strides are being made in the use of imaging techniques, notably MRI, for quantitative body composition analysis. We now have, for example, the ability to measure with great fidelity muscle volumes and infiltration of adipose tissue .
Traditional anthropometry is receiving a new lease of life with the development of automated optical scanning systems. These can provide in a very short time many more body dimensions (lengths, breadths circumferences) than could be feasibly measured manually with a measuring tape . Early validation studies show that optical methods compare favourably with reference methods but further refinements of the methods are required.
Tomorrow: what do we still need to know about body composition?
For the last century body composition research has been driven by technologically driven with research striving for ever increasing accuracy and precision. In 2005, Heymsfield and colleagues in an editorial review provocatively titled “The end of body composition research?”  argued that many of the now over 3000 publications in the field per year are largely due to application of current technologies to understanding physiological processes rather than new advances in techniques of measurement. In large part this is due to increased access to inexpensive and easy to use technologies. Body composition analysis was once the province of a relatively small number of research centres in universities and medical research institutions. The advent of portable technologies such as BIA and the expansion of DXA as a routine clinical tool have now made body composition analysis readily available to the wider clinical community. This will only continue.
Body composition analysis and the smartphone revolution
Where are advances likely to occur? The early 21st century has been marked by the ever increasing miniaturisation and spread of personal electronic devices exemplified by the ubiquitous smartphone. The possibilities offered by the sensor technologies in such devices have quickly been recognised for providing a platform for personalised health monitoring [20,21,22]. Body composition analysis has not been immune to this possibility. Already smartphone-based BIA measurement systems are available [21, 23] and optical scanning of the whole body to provide body volume and anthropometric data using the smartphones camera is in the early stages of development . The use of consumer mobile technologies for body composition analysis opens up many possibilities. For the first time, body composition data can be readily obtained in real-life settings rather than in specialised laboratories . Different physiological processes (heart rate, blood pressure, oxygen saturation, activity level, energy expenditure) can be simultaneously monitored along body composition. This is likely to develop further as innovation continues in sensor technologies.
The tyranny of low sample size
We are now a data-connected society. Health data can be collected on an hitherto unforeseen scale . No longer need body composition research be limited to studies with sample sizes of a few hundred at most. This provides opportunity to explore connections between physiological processes and body composition in ways that were not previously possible. Large data analytics are likely to become the mainstay of nutritional epidemiology.
Standardisation of methodology
There is currently no internationally accepted quality assurance framework for body composition assessment . Individual research laboratories develop their own standard operating procedures and quality control processes. Data are not always comparable between laboratories despite them ostensibly using identical methods leading to the need for inter-laboratory cross-validation. This problem is exacerbated where different methods purporting to measure the same body compartment produce different results. This issue is exemplified by the BIA technique where BIA devices from different manufacturers will rarely provide the same estimate of FFM and FM. This inconsistency undermines confidence in the technique. At least 30 factors have been identified that might influence BIA [28, 29]. This potential for variation demands standardisation of measurement to minimise error . It is to be hoped that bodies such as the International Society for Body Composition Research can take the lead in developing quality control procedures for body composition assessment.
Functional body composition—the missing link
There is still an overwhelming need to bridge the gap between knowing the size of body compartments and their function [31, 32]. Indeed, the current paradigm of using conceptual models, driven by what we currently have the ability to measure, constrains our ability to relate form with function. The overwhelming application, particularly in nutritional practice, of the simple 2C model of fat and FFM in reality bears weak relationship with function. In this model, “fat” is chemical fat not the metabolically active biological compartment of adipose tissue. FFM is composite compartment that includes everything bar fat; it provides us with little information on the relative metabolic and physiological function of its constituents including individual muscles and the visceral organs. Imaging techniques can provide tissue level composition information but are not routinely available. Wider use of these technologies is required but is impeded by their cost and relatively limited availability.
Despite the power of imaging and other technologies, they are still unable to provide us with a reliable measurement of the nutritionally important chemical compartment of body protein. Arguably, this is the single most important compartment not routinely measured. The ability to measure protein has been available since the late 1970s with the development of in vivo neutron activation analysis (IVNAA) . Always limited to a few centres, IVNAA for protein determination has fallen into decline in recent years owing to lack of facilities and little inclination to introduce the technology due to concerns over the radiation hazard that the method presents. Recently, a method to estimate body protein from a combination of DXA measurements of body volume and bone mass with TBW measured by BIA has been proposed . Errors compared to IVNAA were small (1.22 kg) and this method holds great promise for the future.
Heymsfield and colleagues’  concerns that we are at the end of body composition methodology research seem unfounded. Undoubtedly, the quest for ever simpler, cheaper more accurate methods of body composition analysis will continue. The focus is likely to shift to finding methods that can be used in populations en masse, harnessing the ever-increasing power of electronic technologies, sensor development, and signal processing coupling body composition directly with function. Perhaps, with deference to Mark Twain, reports of the imminent death of body composition methodology are greatly exaggerated and it would be better to suggest that we are at the dawn of new body composition methodology research.