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close this bookActivity, Energy Expenditure and Energy Requirements of Infants and Children (International Dietary Energy Consultative Group - IDECG, 1989, 412 pages)
close this folderAssessment and significance of body composition in infants and children
View the document(introductory text...)
View the documentAbstract
View the document1. Techniques for estimating body composition
View the document2. Changes in body composition during growth
View the document3. The companionship of lean body mass and fat
View the document4. Maintenance energy need is related to body size and composition
View the document5. The energy cost of weight gain
View the documentAcknowledgements
View the documentReferences
View the documentDiscussion (summarized by A. Ferro-Luzzi)

3. The companionship of lean body mass and fat

While the composition of the LBM is assumed to be fairly constant from mid-adolescence through most of the adult years, its size is not invariant. For example, tall individuals of all ages (including infants) have on average a larger LBM than those who are short; boys a larger LBM than girls (there is even a slight sex difference in infants), men a larger LBM than women.

It should be recognized that in many situations involving significant changes in body weight, both lean and fat participate in the weight change. With but few exceptions, a change in one component is accompanied by a change in the other, and in the same direction, though not always in the same proportion. In a sense, then, lean and fat behave as true companions.

The data in Table 1, compiled from the literature, show that the companionship rule holds for a number of situations: intentional underfeeding and overfeeding; spontaneous changes in body weight, i.e., free-living individuals on self-selected diets; and infants born to diabetic mothers. Whenever there is a significant change in body weight, LBM and fat both participate in the change; and of course normal growth involves a change in both LBM and fat.

Table 1. Composition of weight change


N, sex




Overfed adults

30 M, 18 F

+3 to 14 kg


density, K-40, N balance

Spontaneous weight gain (obese subjects)

1 M, 7 F

+6 to +22 kg


density, K-40

Spontaneous weight change (adults)

66 F, 60 M

-16 to +31 kg

0.38 *


Spontaneous weight change (lactating women)

37 F

-7 to +7 kg

0.29 *


Seasonal weight change (adults)

50 F

-5.0 to +2.1 kg


total body water

Induced weight change (adults)

14 M

-3.7 to +3.5 kg

0.38 *

N balance

Infants of diabetic mothers vs controls


+700 9

0.31 **

carcass analysis

* regression slope
** calculated for 38th week of gestation
References are given in FORBES, 1990.

However, not all individuals exhibit the same compositional response to weight changes induced by energy deficit or surfeit. Generally speaking, thin individuals lose proportionally more LBM and less fat during weight reduction than do obese individuals. This is why obese people have a greater tolerance for fasting. Likewise, thin people tend to gain a larger proportion of lean in response to high-energy diets with adequate protein content than do those who are obese. While the evidence for the companionship concept has been adduced from studies of adolescents and adults, it is entirely reasonable to assume that it pertains also to infants and children. Certainly, recovery from infant and childhood malnutrition involves an increase in both LBM and fat.

The companionship rule also applies to established states of under- and overnutrition. The vast majority of obese individuals, children and adults alike, have a supranormal LBM, which constitutes a quarter to a third of their excess weight. Individuals with cystic fibrosis and those with anorexia nervosa have a reduced LBM as well as less body fat (FORBES, 1987a, 1990).

However, there are a few pathological exceptions to the companionship rule. Obese children with the Prader-Willi syndrome have a reduced LBM in the face of an increased body fat content (FORBES, 1990). In this respect, this condition differs from the usual type of exogenous obesity. Such patients are known to have muscle hypotonia and hypogonadism, and it has been found that weight reduction can be achieved only with very low energy diets.

Rats who develop obesity as a result of experimentally produced lesions of the hypothalamus also exhibit a reduced LBM, reduced bone growth, and reduced body length in the face of massive accumulations of body fat; hence they do not provide a good model for the usual form of human obesity.

Another exception is the effect of anabolic steroids. These compounds promote nitrogen retention, and when given in large doses serve to increase LBM and to reduce body fat content (FORBES, 1987a). This is why athletes take steroids.