Table 1.

Methods to measure fat

	General principle	Method	Acronym	Methodology	Method-specific principle	Further comments
Density-based methods	If the density (weight per unit volume) of a human body is known, then the relative proportions of fat and fat-free mass can be estimated using an equation such as those of Siri [72] or Lohman [73]. While mass can be easily determined by weighing, volume measurements are more difficult [74].	Hydrodensitometry (underwater weighing)	UWW	Weighs the subject while submerged in a large tank (having exhaled maximally) and also outside the tank [29].	Based on Archimedes' principle (buoyancy law) that if the density of an object exceeds that of water, it will sink. Given two people of equivalent weight outside the tank, the one with more fat, which is less dense than water, will weight less in water than the one with more fat-free tissue (such as bone and muscle) which is more dense than water [75]. (In fact, it is unnecessary to actually weight the subject underwater, since their volume can also be assessed via the amount of water displaced when they are submerged.)	Often described as 'the gold standard', but time-consuming and requires the subject to submerge themselves, so particularly unsuitable for certain populations, such as children, and limited to research settings [76].
		Air Displacement Plethysmography	ADP	Measures the volume of air the subject displaces inside an enclosed chamber.	Given the subject's volume and weight, their density can be calculated.	Early plethysmographs were complex, inconvenient and required temperature-controlled surroundings. A simple, quick automated plethysmograph [77] has been available since mid 1990s, but is still limited to research settings [74].
Scanning methods	Can assess not just overall fat mass, but also its regional distribution.	Computerised Tomography; Magnetic Resonance Imaging	CT; MRI	CT – a series of x-rays pass through the body at different angles. MRI – uses a strong magnetic field and a radio wave antenna which sends signals to the body and then receives them back. These are used to produce internal images.	Both allow for the creation of cross-sectional high-resolution internal images.	Expensive, involve radiation exposure (CT) and limited to research settings [27-29].
		Dual-Energy X-ray Absorptiometry	DEXA or DXA	A series of transverse scans, via low energy x-ray beams, progress inch-by-inch across the body and are collected by an external detector.	The beams are differentially absorbed by the various different tissues (fat, bone, etc) in the body.	Can be used to calculate fat and fat-free mass, and both total and regional body composition in subjects over a wide range of ages and body sizes. Relatively low radiation dose. Validated against UWW and comparison with animal carcasses in the pediatric weight range. Use limited to research settings [27, 28, 46, 78, 79].
Bioelectrical impedance methods	Electric currents pass more easily through body fluids in muscle and blood, but encounter resistance ('biolectical impedance') when they pass thorugh fat, since it contains little water.	Bioelectrical Impedance Analysis	BIA	Conductors are attached to the subject's body, and a low, safe, current is sent through. Electrodes are generally placed at wrist and ankle; an increasingly commonly used analyser requires subjects to stand on it in bare feet and hold a handgrip in each hand. Foot-to-foot BIA measures the impedance of the lower body and only requires the subject to stand on pad electrodes.	The resistance between the conductors provides a measure of body fat.	Although less accurate than more sophisticated measurements, some current analysers are relatively inexpensive, portable, simple and quick, meaning BIA can now be used in the field and with large samples [20, 80].
Anthropometric methods	Direct measurements of various body parameters.	Skinfold measurements	SF	Subcutaneous (but not internal) fat is measured by firmly grasping a fold of skin with callipers and raising it, with no muscle included. Single site measurements, e.g. triceps skinfolds [16] are simplest. An alternative is to add skinfolds from a variety of sites, generally representing both peripheral and trunk areas [17].	Subcutaneous fat may be taken as an indicator of total fat. Fat distribution can also be determined via the ratio of trunk to peripheral skinfolds [81]. It is also possible to calculate total body fat via equations: Slaughter's equations predict percent body fat from the sum of triceps plus subscapular, or triceps plus calf in children and young people [82]; more recent equations by Dezenberg use triceps skinfolds plus body weight, sex and ethnicity [83].	Cheap and fairly simple, but the need to partially undress may put some subjects off, leading to bias. Also difficult to measure reproducibly, particularly if the subject is fat [84].
		Waist circumference	WC	Ideally measured using a flexible plastic tape with a sprung handle to ensure reproducible levels of tension [29]. Since a potential source of error is incorrectly positioning the tape, the measurement site is generally specified by reference to specific anatomic landmarks [85].	WC reflects total and abdominal fat levels, and as an indicator of adiposity is not greatly influenced by height [86].	WC centiles for children have been developed in a number of countries [86-89]. It has also recently been suggested that the ratio of waist to height could be used as a rapid screening tool [90].
		Waist-hip ratio	WHR		A larger WHR in adults indicates relatively larger amounts of abdominal fat and has been used to describe body fat distribution. However it is influenced by several other bodily factors and there is some evidence that it is a poorer measure of body fat distribution in children [91].	Infrequently used in studies of children and adolescents.