Protein synthesis and breakdown and amino acid catabolism in protein-calorie malnutrition.

As protein-calorie malnutrition (PCM) develops, the protein content of the body falls, despite the many mechanisms which operate to limit the further loss of nitrogen from the body (Waterlow, 1968). Studies of the composition of children dying of PCM have shown that in both kwashiorkor and marasmus there is a reduction in total body protein to about two-thirds of the normal amount for a child of the same height (Garrow, Fletcher & Halliday, 1965). This fall in body protein clearly results from an imbalance between synthesis and breakdown, but the effects will depend not only on the size of the imbalance, but also on the mass of protein involved and the rate at which the protein turns over. Collagen has a slower turnover-rate than most other proteins in the body, so that any discrepancy between its synthetic and breakdown rates will have little effect on collagen mass before malnutrition develops. This preservation of collagen tends to mask the large fall in non-collagen proteins to half their normal amounts (Picou, Halliday & Garrow, 1966). As the body loses protein, some tissues show a disproportionate loss in weight. Muscle mass in particular is rcduced, so that it may amount in a marasmic child to less than a third of the normal. The fall in concentration of non-collagenous proteins in muscle also matches the fall in the protein content of liver, kidney and heart (Alleyne, Halliday, Waterlow & Nichols, 1969). Thus muscle, which in a healthy child contributes about 30% of total body protein, can be considered as a reserve tissue which can provide a large supply of essential amino acids for the rest of the body as well as substrates €or gluconeogenesis.