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Section Two: Specific metabolic roles of amino acids, and the paradigm of optimal, as opposed to minimal dietary protein and amino acid requirements for performance, long-term health and optimal organ function
Introduction
For an adult in N balance there will be an amount of amino acids to be catabolized equal to the dietary intake of protein, less faecal losses of undigested dietary protein, undigested endogenous proteins (including mucin in gastro-intestinal mucus, remnants of digestive enzymes and shed mucosal cells) and bacterial proteins. It is only in growth, pregnancy and recovery from tissue protein losses that there is a net increase in whole body protein, and even for someone in positive N balance there will be a considerable amount of amino acids to be catabolized because they are "surplus" to requirements for net protein synthesis.
It is well established that the diet-induced thermogenesis (DIT) associated with protein is considerably higher than that associated with carbohydrate or fat. Some 20-30 % of the energy consumed as protein is dissipated in DIT, compared with 5-10 % of that from carbohydrate and 0-3 % of that from fat(1,2). Allowing for this thermic effect of protein suggests that the net metabolizable energy of protein is nearer 13 kJ/g than the Atwater factor of 17 kJ/g.(3)
The thermic response to a high protein meal or diet (in longer-term studies) has been reported in many studies, some using liquid meals or diets and others meals or diets comprising normal foods, but designed to give different proportions of the macronutrients. Many such studies have used extreme proportions of protein, fat and carbohydrate. For example, Crovetti et al.(4)fed three different isocaloric meals of ordinary foods to normal body weight women and measured heat output by indirect calorimetry over 7 hours. The meal providing 68 % of energy from protein gave an increase in energy output of 261 kJ, compared with 69 % carbohydrate at 92 kJ and 70 % fat at 97 kJ. Dauncey & Bingham(5)fed diets providing 37 % protein or 3 % protein (replaced by sugars) and 48 % fat, and measured total energy expenditure over 24...