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In the UK the adult daily RNI for protein is 0.75 g/kg, with protein representing at least 10% of the total energy intake. Most affluent people eat more than this, consuming 80-100 g of protein per day. The total amount of nitrogen excreted in the urine represents the balance between protein breakdown and synthesis. In order to maintain nitrogen balance, at least 40-50 g of protein are needed. The amount of protein oxidized can be calculated from the amount of nitrogen excreted in the urine over 24 hours using the following equation:
Grams of protein required Urinary nitrogen × 6.25 (most proteins contain about 16% of nitrogen).
In practice, urinary urea is more easily measured and forms 80-90% of the total urinary nitrogen (N). In healthy individuals urinary nitrogen excretion reflects protein intake. However, urine N excretion does not match intake either in catabolic conditions (negative N balance) or during growth or repletion following an illness (positive N balance).
Protein contains many amino acids, of which nine are indispensable (essential). These amino acids cannot be synthesized and must be provided in the diet. The dispensable (non-essential) amino acids can be synthesized in the body, but some may still be needed in the diet unless adequate amounts of their precursors are available. Animal proteins, such as in milk, meat and eggs, are of high nutritional value as they contain all indispensable amino acids. Conversely, many proteins from vegetables are deficient in at least one indispensable amino acid.
In developing countries, adequate protein intake is achieved mainly from vegetable proteins. By combining foodstuffs with different low concentrations of indispensable amino acids (e.g. maize with legumes), protein intake can be adequate provided enough vegetables are available.
Loss of protein from the body (negative N balance) occurs not only because of inadequate protein intake, but also owing to inadequate energy intake. When there is loss of energy from the body, more protein is directed towards oxidative pathways and eventually gluconeogenesis for energy. Of all the amino acids, glutamine is quantitatively the most important one in the circulation and in inter-organ exchange. Alanine is also an important amino acid released from muscle; it is deaminated and converted into pyruvic acid before entering the citric acid cycle. Homocysteine is a sulphur-containing amino acid which is derived from methionine in the diet. A raised plasma concentration is an independent risk factor for vascular disease.
Amino acids may be utilized to synthesize products other than protein or urea. For example:
- haem requires glycine
- melanin and thyroid hormones require tyrosine
- nucleic acid bases require glutamine, aspartate and glycine
- glutathione, which is part of the defence system against free radicals, requires glutamate, cysteine and glycine.
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