CARBOHYDRATES

C. COWEY

CARBOHYDRATES

Generally speaking native carbohydrates are not very well utilized by fish. In a natural environment fish are located at the top of the food pyramid, consequently most fish are carnivorous; they are adapted to eating diets high in protein and their carnivorous diet contains very few carbohydrates.

Some Japanese authors have carried out experiments, utilizing purified diets where they substituted dextrin for protein. Their conclusion were that optimum dextrin level for carp, which is perhaps an omnivorous fish, is 30 to 40% for marine carnivorous fish, such as red sea-bream, it is 10% to 20% and for yellow-tail it is about 10%

FURUICHI and YONE, 1980

Dextrin substituted for protein on a weigh basis in diets.

Optimal dietary levels:

For all 3 specifies, liver glycogen concentration was elevated.

The experimentation carried out by FURUICHI and YONE is shown below.

FURUICHI and YONE, 1981

Oral glucose loads of 167 mg/100 g body weight to yellowtail, carp and red sea-bream previously fed diets with 0.10 or 40% dextrin for 30 days.

Glucose tolerance was lowest in yellowtail, followed by red sea-bream then carp.

Plasma insulin, initially 20 uU/100 ml rose to 70 uU (carp), 63 uU (red sea-bream) and 51 uU(yellowtail)

The levels of plasma insulin indicate that fish have little response to glucose in comparison to what is found in omnivorous animals.

It has been repeatedly shown reasonably high levels of carbohydrates given in the diet of fish result is swollen liver (hepatomegaly).

Levels of amylase activity in yellowtail and carp are shown below. Amylase is, of course, the enzyme primarily responsible for attacking starch in the digestible tract of fish; other enzymes will be involved in breaking down the smaller carbohydrate fragments to glucose which is presumably absorbed.

SHIMENO, HOSOKAWA, HIRATA and TAKEDA, 1977

Amylase, pepsin and trypsin activity in the digestive tract of carp and yellowtail

(Umoles liberated/min/g tissue)

The analyse activity of yellowtail, in the digestible tract, is very low, if compared to that of carp.

The apparent digestibility or raw starch by yellowtail decreases as the level of starch in the diet increases (see below), in all cases, it is low,

SHIMENO, HOSOKAWA, HIRATA and TAKEDA, 1977

Apparent digestibility of starch by yellowtail

The same applies for rainbow trout. The relatively old data by SINGH and NOSE illustrates this point very nicely.

SINGH and NOSE, 1967

Apparent digestibility of starch for rainbow trout

If we recall again the problem of substitution of dextrin for protein (in studies on protein requirement) to maintain the energetic level of the diet constant it can be argued that isoenergetic levels are maintained only for substitutions of small quantities protein.

Treatment of raw starch may improve the digestibility of it.

CHO and SLINGER, 1978

Apparent digestibility of starch by rainbow trout

Autoclaving of wheat middlings increases its digestibility from O to 62%. These methods (heat treatments, autoclaved, etc…) which change the physical status of starch making it more acceptable. Pre-gelatinized starch is now known to be well utilized by salmonids and other fish.

The following table refers to channel catfish which utilize starch better than salmonids, but here again, it must be stressed that cooking starches before putting them in the diet appreciably increases their digestibility (from 26, 1% to 58, 5%).

Percent digestibility of gross energy for channel catfish in certain feedstuff.

Liver glycogen is very slowly utilized by fish. Japanese workers carried out the experiment shown below some years ago.

Glycogen utilization from fish liver is very low

Carp starved 22 days, glycogen in liver was 10.65%

Initially it had been 8.51%

Even after 100 days starvation 1.55% glycogen remains.

This contrasts very markedly with, for example, the omnivorous rat, whose liver glycogen disappears after 24 h of starvation.

Another example taken from a natural environment confirms the low utilization of liver glycogen. Salmonids during spawning migration. Canadian workers took measurements of liver glycogen in salmonids during their up river migration and they showed that for this long period of starvation during which fish form their gonads, the level of liver glycogen was almost constant and that the glycogen utilized was on occasion exceeded by gluconeogenesis.

We have already seen that activities of amylotic enzymes in fish, especially in marine carnivorous fish, enzymes that break down complicated carbohydrates to glucose, are low in activity. This is one explanation of the low utilization of carbohydrates by fish.

Another factor concerns glucose phospharylation. Nothing will happen metabolically to glucose within animals (fish) until it is phosphorylated.

GLUCOSE PHOSPHORYLATION

ATP ------ ADP + P       (Sum of equations (1) and (2))

Another carbohydrates which fish may meet in quite high quantities, in natural environments, is chitin. Chitin is a homogenous unbranched polymer of N-acetyl glucosamine. Chitinase is found in fish tissues but apparent digestibility studies (Cr₂O₃) showed no significant digestion of chitin at 10 % or 30 % in the diet of rainbow trout.

Amino sugars are however respired (utilized) by trout (14 C substrate; I.P.Injection;14 CO₂ out).

ANNEX I

Glucose utilization in fish measured isotopically

Kelp base (BEVER et al., 1977)

Coho salmon (LIN et al., 1978)

Fish given tracer dose (6–3H) glucose replacement rate from indwelling arterial cannula at zero time; serial blood samples taken at intervals thereafter.

1. It is then possible to determine glucose replacement rate R 3 (Production of glucose in post-absorptive animal G 6 P - G) from 6-H glucose.

2. Apparent glucose replacement rate R_afrom (6- 14 C) glucose

3. % glucose recycling.

4. Minimal transit time(average sojourn of a glucose molecule)

5. Minimal glucose mass

6. Glucose space

ASSUMPTION

Tritium (3H) in C-6 glucose is irreversibly lost to body waters in certain gluconeogenic / glycolytic actions.

14C glucose under-estimates glucose replacement rate because of recycling of labelled C into newly synthetized glucose.

RESULTS