Proteins make up 16 per cent of body weight, half intracellular and half extracellular. Proteins are composed of 22 amino acids of which nine (histidine, threonine, lysine, phenylalanine, leucine, isoleucine, valine, methionine, and tryptophan) are essential. Normal protein requirement is 0.6±0.15 g/kg/day (mean ± 2 SD) for adults.

Amino acid metabolism

Amino acids absorbed from the gastrointestinal tract enter the portal circulation and pass through the liver. Glutamine is taken up by the intestine where it is a source of energy. Most of the amino acids, except those with branched chains (leucine, isoleucine, and valine), are removed by the liver. The liver also receives alanine for gluconeogenesis from muscle and the intestine, where glutamine is transaminated to alanine.

The branched-chain amino acids entering the systemic circulation are taken up by muscles and other peripheral tissues for protein synthesis. Some nitrogen is released from muscle in the form of alanine and glutamine. Thus the muscle is a major reservoir of glutamine for immunocytes and the intestine.

Amino acids in excess of requirements are not stored but are used as metabolic fuel. The ultimate disposal of nitrogen from excess amino acids depends upon urea formation.

Obligatory nitrogen losses and nitrogen balance

Fasting nitrogen excretion rates are proportional to metabolic rate. Endogenous nitrogen excretion is about 2 mg/kcal/day. This value, which is called the obligatory nitrogen loss, varies from 41 to 69 mg/kg/day in humans.

Protein-energy interrelationship

In humans, increasing energy intake as either fat or carbohydrate increases nitrogen retention by 4 mg and 2 mg respectively per extra kilocalorie fed. Nitrogen retention increases when either nitrogen or energy intake are increased. The ability of proteins to modulate nitrogen retention independent of other factors is known as anabolic drive (Miljw,ard...1.999).

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