12.1: An introduction to techniques used to measure body compo­sition (14.0) (2025)

Accurate methods for mea­sur­ing body compo­sition are required in inves­tiga­tions of obesity, mal­nu­trition, weight loss following bariatric surgery, muscle wasting, sarco­penia, osteopenia, and osteo­porosis. Body compo­sition infor­mation is also used to establish the appropriate prognosis and treatment of hospital patients, and with longitudinal assess­ment, to monitor the effects of inter­ventions on body compo­sition (Lemos & Gallagher, 2017).

Selection of the method to measure body compo­sition depends on the required precision and accuracy, the study objective, cost, conven­ience to the subject, their health, and equipment and tech­nical expertise avail­able(Lukaski, 1987). Methods based on multi-com­po­nent models that include analysis of protein and minerals, minimize assump­tions related to tissue density, hydra­tion, and structure. This is important because in mal­nour­ished indi­viduals, the elderly, and subjects with metabolic disturbances, the relative pro­por­tions of the body com­po­nents is often altered, and losses of protein, fat, and bone mineral content may occur, often in association with the rapid accumulation of water. Changes such as these invalidate the deter­min­ation of fat and the fat-free mass in the 2‑com­po­nent model.

Absolute validity cannot be assessed for any of the indirect in vivo body compo­sition methods because the gold stan­dard for body compo­sition analyses is cadaver analysis. Instead, only relative validity can be assessed, defined as the com­par­ison for each subject of the results from the “test” method with the results from another method, termed the “refer­ence or criterion” method; the latter having a greater degree of demonstrated validity. A 4‑com­po­nent model is now considered suffi­ciently accurate to act as a refer­ence or criterion method, but its use in many settings is limited because of the expensive and sophisticated technology required. Multiple statistical approaches can be used to establish the validity of the “test” method com­pared with a refer­ence method. They include regression and correlation analyses, paired t tests, and more recently, Bland-Altman analysis; see Earthman (2015) for further details.

The charac­ter­istics of the various pro­ce­dures used for mea­sur­ing body compo­sition are sum­ma­rized in Box 14.1. This list includes both non-scanning and scanning techniques. Detailed sections (14.2‑14.12) describe each of these indirect in vivo methods now avail­able to assess body compo­sition. Comments on these methods are also given, along with the assump­tions used, and the advantages and disadvantages of each method. Scanning techniques such as computer tomog­raphy (Section 14.9), magnetic resonance imaging (Section 14.10) and whole body dual energy X‑ray absorp­tiometry (DXA) (Section 14.11) are included together with a discussion of their clinical importance (Lee et al., 2019; Neeland et al., 2019). Each technique gen­er­ates body compo­sition data in dif­fer­ent ways, so the methods are not inter­changeable. Methods with the lowest cost are often the most imprecise.

Methods employing the 2‑com­po­nent model (i.e., body fat and fat-free mass) include total body potas­sium, total body water via isotope dilution or bio­elec­trical imped­ance, densit­ometry via hydro­static weighing or air-displace­ment pleth­ysmog­raphy, and total body elec­trical conduc­tivity. Such methods are not suitable for clinical popu­lations when the basic assump­tions of the 2‑com­po­nent model are often invalid. Instead, in these popu­lations, techniques using a 3, 4, or 5‑com­po­nent model should be applied. For example, (Section 14.11) has the capacity to gen­er­ate data that can be used with a 3‑com­po­nent model. See Lohman (1986) and Pietrobelli et al. (1996) for further details.

Three scanning techniques — comput­erized tomog­raphy (Section 14.9), magnetic resonance imaging (Section 14.10), and DXA (Section 14.11) — can be used to quantify components (e.g., skeletal muscle, bone, visceral ectopic fat) at the tissue-organ level of body composition as well as to assess the relative pro­por­tions of the fat-free mass, body fat, and bone mineral content. Of these, only DXA has been recom­mended for the assess­ment of fat mass in patients with a variety of disease states; the use of DXA for the assess­ment of fat-free mass is not recom­mended for clinical popu­lations because its validity for assess­ment of fat-free mass in any clinical popu­lation remains unknown (Sheean et al., 2020).