1. Techniques for estimating body composition

Changes in body composition can be assessed by means of the metabolic balance technique, using nitrogen balance as an estimate of change in lean body mass. Many years ago BENJAMIN (1914) used this technique in infants, but was disappointed to find that the calculated increase in body protein during growth was far greater than expected. The problem is that retention of nitrogen, and also other elements, tends to be overestimated due in part to failure to take into account cutaneous losses. If, however, the balance is strongly positive, as in rapidly growing prematurely born infants or in infants and children recovering from malnutrition, such errors become less important though they never disappear completely. Examples of the use of the balance technique can be found in the publications of WATERLOW (1961), REICHMAN et al. (1981) and FJELD et al. (1989). Using energy balance as well as nitrogen balance, Reichman's group, for instance, was able to deduce the relative contributions of fat, carbohydrate, and protein to the weight gain of prematurely born infants.

However, the metabolic balance method can only provide information on the change in body content, not body content per se. Recent decades have witnessed the development of techniques for estimating body content of lean and fat, of body fluid volumes, and of skeletal mass in living humans. Estimates of lean and fat can be made by potassium-40 counting, by densitometry, and by assessment of total body water. While the last method, employing either deuterium or oxygen-18 dilution, can be applied to infants and children as well as adults, the first two are beset with technical difficulties in their use for the very young. A newborn infant, for instance, contains only about five grams of potassium, which means that the ⁴⁰K signal is but a small fraction of the background noise, so that there is lack of precision. Satisfactory assays have been made, however, in 4- to 5-year-old children (FLYNN et al., 1970). The underwater weighing technique for determining body density, so commonly used in studies of adolescents and adults, is obviously unsuited to infants and young children. Attempts to determine body volume in young infants by the application of Boyle's law have not been successful thus far.

All of these methods carry the assumption that the composition of the lean body mass (potassium, water, and density) is known. The fact that all three moieties change during growth, and especially in young infants, raises problems in calculating the results.

A new technique is now under investigation. This is dual photon absorptiometry, which involves scanning the body with two gamma rays differing in intensity which emanate from a radioactive source. The claim is that computer analysis of the transmitted gamma rays can generate values for non-osseous lean tissue, skeletal mass, and total body fat. Adult subjects are required to lie still for 60 minutes, but PETERSEN et al. (1988,1989) have reported reasonable results using a 20-minute scan. The radiation dose is about 5 mR.

The ratio of extracellular fluid volume to intracellular fluid volume also changes during growth. As shown in Figure 1, this ratio is high in the fetus, and falls progressively during infancy and childhood to reach its lowest point in the early adult years, to be followed by a gradual increase during aging.

Figure 1. Ratio of ECF/ICF volume, plotted against the logarithm of postconception age, from the 6-month fetus to the 80-year-old adult. Combined sexes. B = birth. Data compiled from several sources. From FORBES (1987a) with permission.

COKINGTON et al., (1963) have studied the changes in body fluid volumes of infants during recovery from malnutrition. The ratio of ECF/total water is high at first, and falls to normal during recovery. The rate of fall in the ratio is more rapid on feedings providing 15% of energy from protein than on those providing 9%. On the other hand, MANNA (1963) in his studies of normal infants found no difference in body fluid volumes between infants given human milk (which has a relatively low protein content) and those fed artificial formulas. KAGAN et al. (1963) found that prematurely born infants fed a high-protein-high-ash formula gained more weight and had a higher ECF/ICF ratio than those fed human milk.

Two new techniques for estimating total body water are now being applied to the study of young infants. The total body electrical conductivity (TOBEC) method measures the changes in an electromagnetic field (5-10 MHz, 7 mW/cm²) when an object which conducts electricity, such as the human body, is placed in the field (COCHRAN et al., 1986). The bioimpedance method measures the resistance to the passage of a weak electrical current (100mA, 50 kHz) through the body (FJELD et al., 1990). Both measurements can be made very quickly, and neither carries an appreciable hazard. Estimates of the density of individual bones can be made by the technique of photon absorptiometry. The particular bone to be studied (usually the radius and/or ulna) is scanned by gamma rays emanating from a radioactive source, whence the attenuation of the rays is proportional to the amount of bone mineral present. Single photon absorptiometry, i.e., one monoenergetic gamma ray, has been used for single bones of very young infants (GREER et al., 1983), and the dual photon technique, i.e., two gamma rays of different energies, has been used to scan the entire skeleton of young infants (PETERSON et al., 1989). The former technique requires only a minute or so, while the latter requires the subject to lie still for 20 minutes. The radiation dose is 5-13 mR.

Using this technique it has been shown that prematurely born infants, and in some studies term infants, fed human milk with its relatively low Ca and P contents, have a lower bone density than those fed infant formula (CHAN et al., 1986). Thus, the early work of STEARNS (1939), who used the metabolic balance technique, has been confirmed.

Mention should also be made of the techniques of anthropometry, especially the measurements of subcutaneous fat-plus-skin thicknesses by means of special calipers, providing a gross and far from precise estimate of body fat. Urinary creatinine excretion provides an estimate of skeletal muscle mass (for references see FORBES, 1987a).