Metabolism of Food

Following digestion and absorption in the gut, the base components of the nutrients which constitute the fishes' diet are used in different ways. Chemical reactions result in either liberation of their energy through final breakdown - this is known as catabolism - or production of new molecules and tissues - known as anabolism. The sum of these catabolic and anabolic reactions is referred to as metabolism.

It is the metabolism of the main dietary ingredients of protein, fat and carbohydrate that is of prime importance in terms of energy for maintenance and growth. Through digestion proteins are broken down into amino acids, carbohydrates to simple sugars, and fats to glycerol and free fatty acids (FFA). On absorption these smaller components end up in the liver, although fats take a circuitous route through tissue fluids and part of the lymphatic system known as the lacteals.

A number of metabolic processes occur in the liver. Amino acids are removed from the blood and stored temporarily prior to transport through the body for the synthesis of new fish proteins. There is a continual breakdown and synthesis of proteins which are used in cell and tissue structures, and enzymes. Any excess will be used for maintenance/movement energy as it is broken down to carbon dioxide, water and ammonia. Absorbed sugars (from dietary carbohydrates) are converted to glucose in the liver which can then either be burned to supply energy or recombined to form glycogen which is stored in the liver as an energy reserve. The processes involved in the synthesis of proteins, fats and carbohydrates are shown in each of the diagrams below.

Nutrient metabolism showing inter-relationships and links into Glycolysis, Krebs, etc

Each of these links is shown in more detail below.

SOURCE: De Silva S.S & Anderson, T.A. (1995)
Fish Nutrition in Aquaculture
Chapman & Hall

Glycolysis

Krebs/Citric acid cycle

Fats are readily catabolised as a source of daily maintenance and movement energy needs. If there is too much fat in the diet, or fish are overfed for a long period of time, fat deposition will occur. Fats can be synthesised from amino acids and glucose as well as from glycerols and free fatty acids - it is thus easy for fish to accumulate fat from their diet. Fat is also the most economical way to store energy as it has an inherent higher energy latency. The above metabolic pathways are shown in the diagram.

Feed Metabolism

	Amino Acids		Glucose		Triglycerides
	Protein		Glycogen		Fatty Acids
			Carbohydrate		Free Fatty Acids