birds and Fish need energy to maintain basic metabolic activities and to support growth, reproduction, activity, and health. Proteins, carbohydrates, and lipids (the macronutrients) provide this energy and also some essential nutrients.
Protein is a major energy source for all fish and birds, and it appears that blood glucose may derive more readily through gluconeogenesis than directly from dietary carbohydrates.
Whit this information we can conclude that Protein is used for fish growth if adequate levels of fats and carbohydrates (energy) are present in the diet. If not, the more expensive protein can be used for energy and life support rather than growth.
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