The body can be considered as composed of two compartments - the energy-dense fat tissue, and the lean body mass, which consists largely of muscles and visceral organs plus supporting tissues. For their height, women’s bodies have a higher percentage fat content and a lower muscle mass than men’s, and women’s urinary creatinine-height index (43) is lower than that of men.1 When weight is lost, both adipose tissue and lean tissue (muscle) are used for fuel, but the proportion of lean tissue lost depends on the amount of fat stored (44): the greater the mass of adipose tissue, the smaller the loss of lean tissue on starvation. Ferro-Luzzi, Branca & Pastore (45) have described this relationship. Because women have a greater fat mass but smaller muscle mass than men of equivalent weights, they lose less lean tissue; Fig. 59 shows this preferential loss of fat in women and the increasing amounts of lean tissue lost as body weight and BMI fall.
1 Defined as the individual’s 24-hour urinary creatinine excretion as a fraction of the value for a normal individual of the same height.
The proportion of lean tissue, and specifically muscle tissue, in the body is determined by both genetic and environmental factors. Ethnic differences are apparent, with Papua New Guinean men and women having higher values of lean body mass (LBM) and a smaller percentage of fat than Ethiopians or Indians (1, 46). Whether the LBM of adult Ethiopians or Indians is affected by early nutritional conditions is unclear; there has been insufficiently detailed analysis of LBM of, for example, well nourished Indian children growing on the NCHS 50th percentile and well nourished Indian adults with a BMI of 22-23. Under nutritional stress, populations with a smaller body fat mass lose more LBM and can thus be expected to lose weight more rapidly than others.
Figure 59 - Proportion of body weight lost (or gained) represented by lean tissue according to body mass index at the beginning of the weight loss (or gain) perioda
a Adapted from reference 45 with the permission of the publisher.
Δ FFM/Δ WT = (charge in fat-free mass)/(weight change)
Training with isometric techniques leads to hypertrophy of muscle, a feature of body-builders and of athletes involved in sports that require the application of intense power over short periods of time (e.g. weight-lifters, sprinters). Isotonic training, on the other hand, leads to very modest changes in muscle mass, although some increase in mid-arm muscle circumference (AMC) can be observed in most athletes compared with non-athletes. Deconditioning is readily induced by cessation of training and in the event of prolonged bed-rest.
In clinical and public health settings, the preferential loss of lean tissue that results from tissue catabolism and gluconeogenesis in both acute and chronic infections is of particular significance. Individuals with a high fat content may lose substantial amounts of lean tissue - particularly muscle - during illness, and it is this loss of the protein-rich tissues, which are responsible for control and maintenance of organ metabolism, that is the determining factor in the individual’s survival at low body weight.
When an individual is ill, e.g. with an infection, not only does the muscle mass begin to fall but there is also a dramatic change in the fatiguability of muscle and in the maximum power that can be achieved (47). It is relatively simple to test muscle power and endurance by measuring the strength and sustainability of the handgrip (47, 48).
Starvation and semistarvation studies on humans and experimental animals have convincingly shown that most organs contribute, in variable proportion, to the loss of body weight; the brain and the spinal cord are notable exceptions (3, 5). Animal experiments indicate that atrophy of the organs occurs as early as that of the muscle and in parallel with loss of body weight (see Fig. 60). The organs of concentration camp prisoners and famine victims, estimated to have lost between 25% and 45% of their original weight, weighed between 52% (spleen) and 80% (heart) of normal (5).
The weight loss of most organs is accompanied by cytological changes, ranging from cloudy swelling and degenerative changes to mitochondrial brown atrophy. The heart is compromised and becomes susceptible to arrhythmia, anaemia develops because of reduced erythropoiesis, and the capacity of the liver to handle drugs, metabolites, hormones, or toxic substances in the diet becomes impaired. In addition, while the mucosa and other physical barriers to microbial or parasitic entry are remarkably well preserved, the immune system itself is depressed. With a defective immunological response, the stress of even a mild infection is magnified, and there is progressive development of widespread life-threatening conditions, such as septicaemia, parasitaemia, or miliary tuberculosis. The interaction between nutritional status and immune competence is also clearly seen in individuals infected with human immunodeficiency virus (HIV), who display marked nutritional deterioration as their disease progresses; malnutrition exacerbates the disease and is often the determinant of death when 50% of the normal lean tissue has been lost (3, 49).
Figure 60 - Effects of semistarvation on organ weight in the rata
Note: Semistarvation was produced by lowering daily food intake by two-thirds for 6 weeks. The animals were then allowed to recover by gradually restoring food intake. Selected organ weights are plotted as a percentage of baseline weight vs. percentage of initial body weight.
a Reproduced from reference 3 by permission of Blackwell Scientific Publications, Inc.