To determine the optimum indispensable (I) amino acid (AA) balance in Atlantic salmon (Salmo salar L.) fry, a single protocol established for the pig was adapted. The balance was calculated from the reduction in N gain after replacing about 45% of a single IAA by a mixture of dispensable AA in isonitrogenous diets. We confirmed that the mixture of AA simulating the AA pattern of cod-meal protein and gelatine (46:3, w/w) was used with the same efficiency as cod-meal protein and gelatine. From the deletion experiment an optimum balance between the IAA was derived. Expressed relative to lysine=100, the optimal balance was: arginine 76 (se 0·2), histidine 28 (se 2·2), methionine+cystine 64 (se 1·7), phenylalanine + tyrosine 105 (se 1·6), threonine 51 (se 2·4), tryptophan 14 (se 0·7), valine 59 (se 1·7). No estimates were made for isoleucine and leucine. Expressed as g/16g N, the optimal balance was: arginine 4·0 (se 0·0), histidine 1·5 (se 0·1), lysine 5·3 (se 0·2), methionine+cystine 3·4 (se 0·1), phenylaline+tyrosine 5·6 (se 0·1), threonine 2·7 (se 0·1), tryptophan 0·7 (se 0·0), valine 3·1 (se 0·1). This AA composition is close to that of the Atlantic salmon whole-body, but using it as an estimation of the IAA requirements may lead to an overestimation of the branched-chain AA requirements and an underestimation of aromatic and S-containing AA requirements. The results are discussed in accordance with the key assumptions associated with the model used (broken-line model, IAA efficiencies and maintenance requirements).

The activity of feeding and the metabolism caused by digestion and assimilation of food increase an animal's heat production. This increased heat production has been referred to as specific dynamic effect, specific dynamic action or heat increment (HI). HI is much larger when protein is a source of energy than when carbohydrate or fat are the sources of energy. HI for protein is much greater when the animal's ambient temperature is high than when it is low. Metabolisable energy is used more efficiently, thus having less HI, for maintenance than for production. A suggested explanation is that heat resulting from digestion and assimilation of food can substitute for the heat production in fasting when food provides energy equal to, or less than, maintenance requirements. When energy intake is large enough to support production, HI from anabolic processes becomes a waste product that cannot substitute for a fasting or maintenance function. The same rationale can be applied to the observation that dietary protein has a greater HI when an animal is at high temperatures. At low temperatures protein would be used for maintenance. At high temperatures the same amount of energy from protein would be enough to support production. As a result, HI of the protein would be increased at high temperatures. The high HI of protein or amino acids when at a high level in the diet can be at least partially explained. Protein synthesis requires a large amount of energy. Some energy is required to excrete nitrogenous waste. In addition, dietary protein stimulates protein turnover. Research findings have suggested that HI should be lowered by decreasing dietary protein. For non-ruminant animals this could be accomplished by discovering the essential amino acid requirements for an ideal protein. The ideal protein should result in a minimum HI. Research to date generally fails to document improved feed efficiency as a result of feeding an ideal protein.