7.2 Carbohydrates

Carbohydrates (starches and sugars) differ by varying the combination of carbon and water. Carbohydrates are usually plentiful, widely available, and the least expensive energy source for fish diets. While carbohydrates are major energy sources in mammals and some other vertebrates, fish are not as efficient in metabolizing carbohydrates. As a comparison, mammals typically gain around four calories per gram. Fish may only take around 1.6 calories per gram. Carbohydrates are included in fish diets to reduce diet costs and give some “protein sparing” (explained below) to circumvent metabolic energy being taken from the dietary amino acids. Carbohydrates are less essential to carnivorous fish species. Dietary starches also fill the role of binders when heated to create extruded floating feeds, and the temperatures used in the production process make the starches more biologically available.

Excess carbohydrates are stored as glycogen in the liver. In fish, glycogen can be mobilized to satisfy part of the animal’s energy demands. Many fish species can utilize up to about 20 percent of dietary carbohydrates.^[1] Beyond the useful range of carbohydrates, fish create enlarged livers from the excess glycogen storage in an increased hepato-somatic ratio. This tendency is quicker to develop in carnivorous fish and presents pale-colored livers when harvested for slaughter or seen at necropsy.

Digestion of carbohydrates in fish is a process of enzymatic degradation resembling the better-understood mammalian enzymes and processes. The temperature has a direct effect on enzymatic action. Lower temperatures slow the function and efficiency of enzymes. Analysis of enzymatic efficiency in fish is beyond this level of discussion. The majority of the digestive processes are conducted in the midgut, which is mildly alkaline. The pancreas, gut wall, and hepatic bile enzymes are known to be present in fish. Specific enzymes attack parts of the three energy classes – carbohydrates, fats/lipids, and proteins.^[2] The assumption about enzymes being universal throughout fish species needs caution. Predators such as salmonids may not have sufficient carbohydrate enzymes. In terms of specific sugars, salmon can use glucose but not fructose because of the enzyme menagerie they are equipped with. On a comparative basis, the starch in corn could be a readily available nutritional candidate, but significant differences in digestibility exist between vertebrate species. For comparison, swine can get 85% of corn’s starch value,^[3] carp can get 60-70%, but trout (a salmonid) might only utilize 40%. Grain starches are typical carbohydrate standards for metabolic studies. Other starches from other sources may be utilized differently in specific situations. More research is needed.

Levels of dietary carbohydrate inclusion are recommended to be within 15-25 percent for salmonids and marine fish. The recommended level of carbohydrates can go up to 50 percent for herbivorous and omnivorous species.^[4] In this comparison of generalities, fresh or warm water species are better digesters of carbohydrates. Coldwater and marine species lose digestive efficiency if much above 20% dry-weight carbohydrates in the formulation. Beyond the water and temperature distinctions, fish have the major enzymes for carbohydrate digestion. For coldwater species like salmonids, higher levels of dietary carbohydrates have been reported to cause enlarged livers, as described above.

Fish are noted to develop hyperglycemia on high carbohydrate diets. Hyperglycemia is not a failure to produce enough insulin. It has been found that blood sugar elevation in fish also includes a response from somatostatins that alter the insulin release response. Somatostatin intervention in carbohydrate metabolism is mentioned to avoid the repeated statement that fish are diabetics. The resting levels of insulin closely match the homeothermic species but may have fewer insulin receptors to respond.^[5]

Reviewing the role of carbohydrates in fish diets, fish digest their diet at rates according to the environmental temperature and the interrelationship of the energy components, i.e., carbohydrates & protein. While fish have lower tolerance and digestibility of CHO in certain instances, a diet with some CHO content is useful for manufacturing purposes. The binding characteristic in the pelletization process during the manufacturing process was listed. Another area is called “protein sparing .” When fish are held in suboptimal temperatures and fed maintenance diets, much of the protein-derived peptides are deaminated for energy which consumes metabolic energy. Using small amounts of CHO as an energy source will reduce the protein (protein sparing) used for the energy from the deamination of certain amino acids.^[6]