Carbohydrate Requirements of Fish

Carbohydrates are a large organic group that contains glucose, starch and cellulose. Its common feature is that it contains only carbon, hydrogen, and oxygen, and their combustion produces carbon dioxide and one or more molecules of water. The simplest carbohydrates are the three carbons that play an important role in metabolism at the secondary level. Polysaccharides are more complex carbohydrates that exist naturally. It is primarily plant-based.

The following two types of polysaccharides are very important in the diet of animals and fish:

(A) Structural polysaccharides: These carbohydrates can be digested by herbivorous fish species. The polysaccharides included in this class are cellulose, lignin, dextran, mannas, inulin, pentosans, pectic acid, algic acid, agar and chitin.

(B) Universally digestible polysaccharide: This type of polysaccharide is mainly starch. 3/4 part of the plant is made up of such carbohydrates. However, small amounts of carbohydrates are present in the body as glycogen, sugar and its constituents. Glycogen is called animal starch because it is not found in plants. Monosaccharide constituents such as carbohydrates, amino sugars and D-oxy sugars are the constituent elements of all organisms.

General Properties of Carbohydrate

  • Carbohydrates act as stores of energy.
  • Ribose and deoxyribose sugars form the structure of the hereditary elements DNA  and RNA.
  • Polysaccharides such as cellulose act as cell wall components of bacteria and plants.
  • Carbohydrates combine with proteins and lipids to play important roles in various cell reactions.
  • It is a group of different types of hydroxy aldehydes or ketones.

Importance of Carbohydrate

Carbohydrates are the main nutrients. The body gets 45-65% calorie energy from carbohydrates. The requirements for carbohydrates in animals are mentioned below:

  • It plays a major role as the main source of fuel in the body.
  • The body can use carbohydrates very easily for energy.
  • All kinds of tissue and cell in animals can use glucose for energy.
  • Carbohydrates play a very important role in the proper functioning of the central nervous system, kidneys, brain and muscles.
  • Muscle tissue and liver can store carbohydrates which the body can later use for energy.
  • It plays an important role in removing waste from the body and in the functioning of the intestines.
  • Hexosamine acts as an antibiotic.
  • Monosaccharides are important components of nucleotides and nucleic acids.
  • Disaccharide plays a role in food digestion.
  • Starch acts as a temporary storehouse of glucose in plants and animals.

You might also read: Amino Acid Requirements of Fish

Sources of Carbohydrate

  • Different types of grains
  • Wheat bran
  • Rice husk
  • Potato Leaf meal
  • Cotton seed meal
  • Potatoes
  • Sweet potatoes
  • Sugarcane
  • Milk
  • Different types of fruits
  • Nuts
  • Bean seed meal

Classification of Carbohydrate

(A) Carbohydrates can be divided into two based on taste, viz

1. Sugar: Carbohydrates that are sweet in taste and granular and soluble in water are called sugars. Such as: glucose, fucose, sucrose etc.

2. Non-sugar: Carbohydrates that are not sweet in taste and insoluble in water are called non-sugar. For example: starch, cellulose, glycogen etc.

(B) Carbohydrates can be divided into three based on their composition, viz

1. Monosaccharides : Monosaccharides are derived from the Greek, mono = one and saccharin = sugar. Monosaccharides are carbohydrates that consist of a single carbon chain. Monosaccharides consist of a free aldehyde (- CHO) or ketone (= CO) group and one or more hydroxyls (- OH). They are soluble in water but insoluble in ethanol. It contains 3-6 carbon atoms. It can be divided  again according to the number of carbons, e.g.

(A) Triose: A carbohydrate that contains 3-carbon monosaccharides is called triose. Its molecular symbol is C3H6O3. It includes as glyceraldehyde, dehydroxy acetone etc.

(B) Tetrose: A carbohydrate that contains 4-carbon monosaccharides is called tetrose. Its molecular symbol is C4H8O4. Such as: erythrose, erythrolose etc.

(C) Pentose: A carbohydrate that contains 5-carbon monosaccharides is called pentose. Its molecular symboll is C5H10O5. Such as: ribose, deoxyribose, ribolose etc. Such carbohydrates are rarely found free in nature. In plants it resides as a polymer and is known as pantosans. The components of pentosans are xylose and arabinose. Plant fibers, vegetables, gum (glue) contain such pentasons.

(D) Hexose: A carbohydrate that contains 6-carbon monosaccharides is called hexose. Its molecular symbol is C6H12O6. Many types of carbohydrates belong to this group. Notable among these are glucose, fructose, galactose and mannose. Glucose and fructose are found free in nature.

(E) Heptose: The carbohydrate which contains 6-carbon monosaccharide is called heptose. Its molecular symbol is C7H14O7. E.g., sedoheptulose.

Carbohydrates are further divided into two groups based on aldehydes or ketone groups, viz.

(1) Aldose: The carbohydrate that contains aldehyde group (-CHO)  is called aldose. Such as glycerol, glyceraldehyde

(2) Ketose: The carbohydrate that contains ketone group (= CO) is called ketose. Such as dihydroxy acetone

All types of fruits, fruit juices and honey contain small amounts of glucose. Therefore, glucose is produced commercially by hydrolyzing grain and root starch with the help of acids or enzymes. Glucose is the last product of carbohydrates in all types of non-ruminant animals, including fish, which attracts special attention. Fructose is the only ketohydrogen found in glucose-free fruit and honey. Fructose is slightly sweeter than glucose.

2. Oligosachharides: Greek, Oligos = less and sachharin = the origin of the word Oligosachharide from sugar. The carbohydrates that, when hydrolyzed, contain up to 2-10 monosaccharide molecules are called oligosaccharides. Its molecular sign- (CnH2nO2)2-10. Oligosaccharides decompose to produce 2 to 10 monosaccharide molecules. Oligosaccharides can again be of different types depending on the number of monosaccharides in the carbohydrate. E.g.

1. Disaccharides:The carbohydrate that, when hydrolyzed, contains two molecules of monosaccharide, is called disaccharides. Such as sucrose, cellobiose etc. Sucrose is a disaccharide that is free. The main ingredient in sugarcane and sugar worms is sucrose. It is formed during the germination of legume seeds. Other common disaccharides are maltose and lactose. Maltose is the dimer of glucose and lactose is the co-polymer of galactose and glucose. Maltose is formed by the addition of two molecules of glucose.

2. Trisaccharides: The carbohydrate that is hydrolyzed to produce three molecules of monosaccharide is called trisaccharide. E.g. raffinose. Thus, if there are 4,5,6 monosaccharides, they are named as tetrasaccharides, pentasaccharides, hexacaccharides etc.

3. Polysachharides:  Greek, Poly = many and sachharin = the origin of the word Polysachharide from sugar. Polysaccharides are a large group of complex carbohydrates that are produced by condensing numerous sugar molecules. Polysaccharides are carbohydrates that are found in many monosaccharide molecules when analyzed. Its molecular sign is (C6H10O5) n. The number of monosaccharides in polysaccharides can range from a few hundred to a few thousand such as starch, cellulose etc.

Most polysaccharides are water soluble. Moisture analysis with acids or enzymes produces their structural monosaccharides. Starch is a polymer of high molecular weight D-glucose. It is the main carbohydrate stored in plants. Most starches contain a mixture of two types of polymers called acylase and amylopectin. In this case, the ratio of amylose and amylopectin is 1: 3. The enzyme required for catalysis is secreted in the digestive juices of animal and fish cells. Starch is not water soluble and is stained by iodine.

Glycogen is the only animal complex carbohydrate. The liver and muscles act as a source of energy that is readily available in small amounts. Dextin is produced as a result of incomplete hydrolysis or digestion of starch. It is thought that some shellfish contain extra cellulose enzymes. This enzyme affects the hydrolysis of cellulose. Cellulose-producing microorganisms exist in the intestines of herbivores animals and fish. These animals can use the indigestible cellulose of the host as food. Other complex polysaccharides are hemicellulose and pentosans. Hemicellulose contains many carbohydrates such as Araban, xylan, some hexsans and polyurinids. All of these ingredients are generally less tolerant to chemical reactions. Pentosans is a polymer of xylose or arabinose that is found mainly in plant constituents and vegetables.

There are two types of polysaccharides based on biological function, viz

(1) Structural: The polysaccharides that take part in the formation of cells and provide cell strength are called structural polysaccharides such as cellulose, chitin, etc.

(2) Metabolic: All the polysaccharides that take part in the metabolism of cells are called metabolic polysaccharides. For example: starch, glycogen etc.

(C) Carbohydrates are divided into two two on the basis of deoxidation capacity, viz

(1) Reducing carbohydrates: Reducing carbohydrates are carbohydrates that can deoxidize other compounds such as glucose, fructose, etc.

(2) Non-reducing carbohydrates: Carbohydrates that cannot deoxidize other compounds are called non-reducing carbohydrates. For example: sucrose.

Biological Functions of Carbohydrates:

  • It is the main source of energy in animals.
  • Glucose acts as storage of energy. In animals, it is stored as glycogen and in plants it is stored as starch.
  • Stored carbohydrates play a role as an alternative energy source of protein.
  • It controls the nervous system and acts as a source of energy for the brain.
  • It combines with proteins and lipids to produce antigens, vitamins and antibiotics.
  • It acts as a structural component of microorganisms and plant cells.
  • It acts as an important component of connective tissue in animals.
  • It makes connections between different cells.
  • Carbohydrates are rich in fiber which helps in relieving constipation in animals.
  • It builds immunity against various diseases.

Carbohydrate Requirements in Fish

Carbohydrates specifically refer to the nitrogen-free extract in the diet that is physiologically digestible. Every 1 gram of carbohydrate produces 4 kcal of energy (Hastings, 1979). Fish have no special need for carbohydrates, but they are an affordable source of energy in the diet. Many carnivorous species are less efficient in using carbohydrates than herbivorous and omnivorous species (Wilson, 1994). Some carbohydrates are stored as glycogen in fish tissues such as the liver and muscles, and these carbohydrates can be used as a quick energy source. Some carbohydrates are converted into lipids and stored in the body as a source of energy.

In the process of photosynthesis, different types of carbohydrates are produced in plants. Cellulose and other fibrous carbohydrates act as structural components of plants that are not digested in the intestines of animals, especially fish. In fact, less than 7% of crude fiber in fish diet makes it difficult to digest indigestible material.

Soluble carbohydrates are stored as primary energy in starch seeds, tubers and other plants. Animal tissues such as the liver and muscles contain small amounts of dissolved carbohydrates such as glycogen, which is structurally similar to starch. When the body needs glucose, this stored glycogen is released quickly. Foods prepared for carnivorous fish contain less than 20% soluble carbohydrates whereas omnivorous fish species usually contain 25-45% soluble carbohydrates.

Carbohydrates are an affordable source of food energy but they are not well used by all animals. Eating digestible carbohydrates for excessive energy production stores them as fats and disrupts normal bodily functions (Hastings, 1979). Chinook salmon can tolerate high levels of carbohydrates (30% of the diet) in the diet (Buhler and Halver, 1961), and feeding eel fish with 30% carbohydrate-rich food results in rapid growth similar to a 50% protein-rich diet (Degani, 1987).

Carbohydrates act as the main source of energy. The following table lists the carbohydrate requirements of some farmed fish:

Fish Species

Carbohydrate Requirements of Fish (%)





Labeo rohita




Catla catla




Cirhinus cirhosus




Cyprinus carpio




Ctenopharyngodon idellus




Cyprinus carpio var. specularis




Clarias batrachus




Heteropneustes fossils




Clarias gariepinus




Macrobrachium rosenbergii




Pinaeus monodon




Carbohydrate Metabolism in Fish

Most of the carbohydrates used in animal feed, especially fish, are plant-derived. Carnivorous fish such as Atlantic salmon and Japanese yellowtail consume small amounts of carbohydrates in their diet. In fact, experiments have shown that a small amount of raw carbohydrates are provided in the diet of these fish species. On the other hand, omnivorous fish such as common carp and channel catfish are able to digest a significant amount of carbohydrates in their diet. Grass carp are herbivores that survive mainly on plant foods.

Carbohydrate Digestion, Absorption and Storage

Starch assimilation by animals depends on their amylase excretion efficiency. All fish species secrete α-amylase. Studies have shown that the activity of this enzyme is higher in herbivores. In carnivorous fish such as rainbow trout and sea perch, amylase is mainly produced from the pancreas. However, in herbivores, this enzyme is present throughout the alimentary canal. Increased activity of amylase secreted by the pancreas of the upper intestine of the upper extremities is observed. In the case of carnivorous rainbow trout, if the level of carbohydrates is more than 20%, the digestion of starch and dextrin will be gradually reduced, but fish can use up to 00% of glucose, sucrose and lactose in their diet. Contrary to popular belief, carnivorous fish are able to use simple carbohydrates as a primary source of energy.

Starch assimilation by animals depends on their amylase excretion efficiency. All fish species secrete α-amylase. Studies have shown that the activity of this enzyme is higher in herbivores. In carnivorous fish such as rainbow trout and sea perch, amylase is mainly produced from the pancreas. However, in herbivores, this enzyme is present throughout the alimentary canal. Increased activity of amylase secreted by the pancreas of the upper intestine of the upper extremities is observed. In the case of carnivorous rainbow trout, if the level of carbohydrates is more than 20%, the digestion of starch and dextrin will be gradually reduced, but fish can use up to 00% of glucose, sucrose and lactose in their diet. Contrary to popular belief, carnivorous fish are able to use simple carbohydrates as a primary source of energy.

There are not enough data on glucose absorption by fish. Studies of goldfish have shown that the active transport of glucose, like most mammals, is involved in the transport of Na +. It is generally thought that such absorption occurs on the mucosal surface of intestinal cells. The monosaccharides produced as a result of digestion in carbohydrates are mainly glucose, fructose, galactose, manose, xylose and arabinose. Although the absorption rate of these sugars can be determined in the case of many terrestrial mammals, similar data are not readily available in the case of fish.

Carbohydrates from protein and fat are not the best sources of energy for fish. Although carbohydrates can be used as an alternative to proteins for tissue formation. The metabolism of amino acids from glucose to fish is more efficient for energy. The fish expels the nitrogenous waste as amine through the gills.

Other Factors Affecting Carbohydtare Metabolism

In addition to genetic adaptation, climatic factors play an important role in the carbohydrate metabolism of fish. Physiological adaptation of fish, especially enzyme adaptation, is very important. Since the ability of an animal to survive largely depends on its normal metabolic function. While some enzymes play a role in metabolic adaptation, other enzymes cannot. The enzymes involved in energy release (enzymes involved in glycolysis, pentosans, tricarboxylic acid cycle, electron transport and fatty acid oxidation, etc.) show adaptation to temperature. In contrast, enzymes that are heavily involved in the breakdown of metabolic substances play a small role in physiological adaptation.

Table Metabolic or physiological adaptation  of some enzymes 1

Name of Enzymes that take part in physiological adaptation

Enzymes that do not participate in bilateral or physiological adaptation





Lactic dehydrogenase

Acid phosphatase

6-phosphog luconase dehydrogenase

D-amino acid oxidase

Succinic dehydrogenase


Cytochrome oxidase

Acetyl choline esterge

Malic dehydrogenase

Choline acetyl transferase

Succinate-cytochrome c-reductase

Alkaline phosphate

Amino acyl transfer


Cytochrome C-Reductase






Malic acid

Glucose-6-phosphate dehydrogenase

1Adapted from: Comparative Animal Physiology, edited by C.L. Prosser, 1973

Surprisingly, two powerful enzymes called amylase, and glucose-6 phosphate dehydrogenase, are involved in carbohydrate metabolism. On the other hand, when the lipase enzyme is added to the enzyme, they participate in the digestion of fats, which again cannot show adaptation to temperature. The molecular mechanism of heat adaptation of a particular enzyme is still not well known. Variations in kinetic energy at specific temperatures can also lead to significant changes in co-factors such as lipids, co-enzymes, or other factors such as pH and ions for the adaptation of animals with differences in the ratio of iso-enzymes and temperature.

Energy Transformation

Despite the racial differences in the tolerance of carbohydrates in the diet, it is generally believed that glucose is the main and final product of carbohydrate digestion and that glucose metabolism occurs in the same way in all cells. Major metabolic changes have been shown through the bilateral Emden-Meyerhoff pathway. Glucose is the only starting point in this process. Through phosphorylation it is converted into glucose 6-phosphate. Free energy is wasted in all conversion processes. Thus free energy is required to produce 2 molecules of lactate from glucose 6-phosphate. In this case, 22000 cal / mole energy is used and in the final stage 4 molecules of ATP are produced.

Emden-Meyerhoff pathway

Emden-Meyerhoff pathway

[Bilateral arrows show the step-by-step reaction of the same enzyme on both sides. The reaction of the secondary step is shown by the fragile arrow mark. The different enzymes involved in the two-way reaction of the reaction are shown by the joint solid arrow marks. (Adapted from: Principles of Biochemistry, by A. White, et al., 1978)]

Deficiency Signs of Carbohydrate in Fish

  • Contains black / red color on body and fins
  • The body becomes colorless
  • Fins erosion
  • Causes skin lesions
  • Increasing bacterial infections
  • Lateral line erosion
  • Lower jaw erosion
  • Gallbladder bleeding
  • Aversion to food
  • Convulsions
  • Exhale quickly
  • Losing balance
  • Abnormal swimming
Factors Influencing Nutritional Status, Health and Immune Function of Fish

Factors Influencing Nutritional Status, Health and Immune Function of Fish