Carbohydrates are an excellent source of energy and carbon in feed formulations. They can be easily distinguished from the other energy yielding nutrients in terms of their abundance and low price. To illustrate, the collective global production of the major cereal grains i.e., maize, wheat and rice amounted to a colossal 2.5 billion tonnes in the year 2013 (FAO). The total carbohydrate content and the digestible fraction of starch and sugars in these grains can be roughly estimated to be about 2.1 and 1.75 billion tonnes, respectively (www.feedipedia.org). Besides, the unit cost of carbohydrate sources is almost three to five fold less than that of the protein and lipid sources of interest. Therefore, the inclusion level of carbohydrates in commercial fish feed assumes direct economic significance i.e., in terms of lower feed cost per unit weight gain.

1. C
arbohydrates are an excellent
source of energy and carbon in
feed formulations. They can be
easily distinguished from the other
energy yielding nutrients in terms
of their abundance and low price.
To illustrate, the collective global
production of the major cereal grains
i.e., maize, wheat and rice amounted
to a colossal 2.5 billion tonnes in the year 2013 (FAO). The
total carbohydrate content and the digestible fraction of starch
and sugars in these grains can be roughly estimated to be about
2.1 and 1.75 billion tonnes, respectively (www.feedipedia.org).
Besides, the unit cost of carbohydrate sources is almost three to
five fold less than that of the protein and lipid sources of interest.
Therefore, the inclusion level of carbohydrates in commercial fish
feed assumes direct economic significance i.e., in terms of lower
feed cost per unit weight gain.
On the other hand, though not strictly essential in the biological
sense, optimal inclusion of dietary carbohydrates is known to
increase the retention of protein and lipid in farmed fishes and
reduce nitrogen discharge in farm effluents. These are factors that
are relevant to the sustainability of any aquaculture operation.
Moreover, the presence of carbohydrates in the ingredient
mixture during the process of cooking extrusion inevitably
helps in pellet binding, stability and floatability. These are
characteristics that minimize nutrient leaching and feed wastage.
Taken as a whole, carbohydrate is an often underrated but vital
cog in the fish feed manufacturing wheel.
In the evolving context of fish feed production, it is important to
note that increasing amounts of carbohydrates are inadvertently
added when competitively priced plant origin ingredients are
used to replace expensive and limited marine ingredients. Among
the different forms of carbohydrates that are abundant in plant
sources, only starch and sugars (energy reserves) have nutritive
value in fish nutrition and therefore they will be the focus of
this article. Whereas, structural non-starch polysaccharides
(fibre) mostly have negative nutritional value and so will not be
discussed further.
Farmed fishes have the entire biological machinery of digestive
and metabolic enzymes, hormones, glucose transporters and
glucose sensing components, which are essential to use glucose
as a cellular energy currency. Nevertheless, certain divergence
in regulatory mechanism makes them less able to use digestible
forms of carbohydrates to meet energy requirements, when
compared to other livestock.
There are remarkable differences in carbohydrate utilisation
between and even within fish species linked to their diverse
feeding habits, anatomical features, physiology and rearing
habitats. Particularly, farmed carnivorous fishes such as salmon
and trout are considered to be less tolerant to carbohydrate rich
meals mainly due to slow blood glucose clearance.
Consequently, the dietary inclusion level and appropriate source
of carbohydrate is decided based on protein sparing without
any adverse effect on growth and physiology of the fish. The
maximum recommended levels of dietary carbohydrate inclusion
fall within 15-25 percent for salmonids and marine fish, while it
can go up to 50 percent for herbivorous and omnivorous species
(NRC, 2011).
What could decide carbohydrate utilisation in fish?
A complex array of biological, dietary and environmental
factors determines the capacity of a fish to use a carbohydrate
rich meal (Fig. 1). Among the biological factors, natural feeding
habit and the resultant evolutionary adaptation is considered
as the primary determinant. For instance, omnivorous and
herbivorous fishes like carp, tilapia and catfish are known to have
superior amylase activity, intestinal glucose uptake capacity and
control of glycaemia as compared to carnivorous trout, salmon
and seabass. At the same time, it is important to note that the
optimum inclusion level of carbohydrates varies with the cultured
size or age of the fish, irrespective of its feeding habit.
The existence of genotypic differences within species also
remains possible in fish, as shown in terms of glucose tolerance
and metabolism in two experimental lines of rainbow trout.
Likewise, transgenic salmon with growth hormone gene construct
Carbohydrates in fish nutrition
An overview of what could decide, limit and improve
the use of nutritive carbohydrates in fish
by Biju Sam Kamalam and Stephane Panserat
20 | March | April 2016 - International Aquafeed
FEATURE

2. reportedly have an enhanced ability to digest and metabolically
utilise dietary levels of carbohydrates well above those known
to be used by their non-transgenic counterparts. Interestingly,
sustained swimming exercise can possibly be used as a metabolic
promoter to abolish the glucose intolerant phenotype of rainbow
trout fed carbohydrate rich meals, by augmenting glucose uptake
and use in skeletal muscle.
In fish feed, the nutritional and technical value of a starch
constituent depends on its characteristics such as starch granule
shape, size, distribution and amylose to amylopectin ratio, which
in turn are linked to their botanical origin. For example, the
surface area available for digestive enzymes to act differs with
the starch granule size of wheat (22 µm) and potato (40-100 µm),
resulting in significantly different digestibility estimates of 58 and
5 percent in rainbow trout. Similarly, the physical quality of the
feed pellet is also influenced by the starch source.
With respect to the degree of polymerisation, the apparent
digestibility and intestinal uptake generally decreases with
increasing complexity (glucose > starch), whereas the vice versa
is mostly true in case of protein sparing and economic viability.
However, the net energy value of simple sugars and complex
starch varies in a species-dependent manner. Altering the physical
state of starch through the hydrothermal process of gelatinisation
substantially improves its digestibility and use by fishes, more
significantly in carnivores like trout, seabass and seabream. In
technical terms, more addition of process water in the extruder
augments the degree of starch gelatinisation and digestibility.
Further, several studies have ascertained that the best use
of energy from dietary carbohydrates in fish depends on the
macronutrient composition of the diet. High level of dietary lipids
was found to reduce starch digestibility, elevate postprandial
glycaemia and prolong blood glucose clearance. In rainbow
trout and Senegalese sole, this phenotype was metabolically
characterised by an increase in the hepatic activity of the
gluconeogenic enzyme glucose 6-phosphatase, concomitant
decrease in the activities of glycolytic and lipogenic enzymes,
and impaired insulin signalling. Similarly, high level of amino
acids can elicit a cellular signalling response that can weaken
insulin action and attenuate the insulin mediated down-regulation
of gluconeogenesis, indicating that alterations in dietary protein
content can impair glucose homeostasis. These findings reinforce
the necessity to consider dietary macronutrient interface when
optimising carbohydrate usage levels.
Moreover, meal timing was found to have a significant effect on
carbohydrate utilization in gilthead seabream, i.e., carbohydrates
from a morning meal was used more efficiently than from an
afternoon meal, resulting in considerable protein sparing. On a
cautionary note, in any case, inclusion of carbohydrates beyond
tolerable limits causes decrease in starch digestibility, hepatic
dysfunction, impaired growth and even undesirable epigenetic
changes.
Being ectotherms, changes in temperature can modify the
processing of dietary inputs in fish. Within the optimal range, an
increase in the temperature of the rearing water is often known
to improve amylase activity and starch digestibility, leading to
a differential time course of blood glucose i.e., relatively rapid
rise and fall, higher activity of glycolytic enzymes and ultimately
better protein sparing regardless of the feeding habit of the fish.
The common understanding of warmwater fish having an edge
in carbohydrate utilisation over coldwater fish is also apparently
true.
In euryhaline fishes like rainbow trout and salmon, changes
in salinity was found to interact with the regulation of glucose
metabolism and starch digestibility was lower in seawater
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FEATURE

3. than in freshwater, both possibly related to the osmoregulatory
adaptation of the fish. Other environmental variables such as
photoperiod also have an effect on glucose tolerance and possibly
carbohydrate utilisation in fish. For example, Atlantic salmon
reared under continuous light showed higher glucose regulation
capacity than those fish exposed to simulated winter photoperiod.
What could limit carbohydrate utilisation in fish?
Logically, the slow glucose turnover and hyperglycaemic
phenotype in fish can be related to low body temperature, oxygen
consumption and metabolic rate. But as mentioned earlier,
carnivorous fishes have evolutionarily adapted their anatomy,
physiology and metabolism according to their natural diet that
contains very limited or no nutritive carbohydrates (Fig. 2).
Consequently, they are not able to regulate their intestinal glucose
uptake capacity and efficiently clear the glucose influx after a
carbohydrate rich meal, resulting in a prolonged high level of
glucose in the blood and earning them the ‘glucose intolerant’ tag.
All the fish investigated to date has the ability to hydrolyse and
absorb simple and complex carbohydrates in their gastrointestinal
tract. However in carnivores, starch digestion and glucose
absorption is limited by low activity levels of α-amylase
and disaccharidases, their inhibition by high level of dietary
carbohydrates and low capacity of intestinal glucose uptake
due to lower densities of transporters and smaller amounts of
absorptive tissue. For instance, compared to omnivorous tilapia,
the total carbohydrase activity in carnivorous Atlantic salmon,
rainbow trout, European seabass and gilthead seabream was 9,
22, 31 and 33 percent, respectively. More importantly, in the wild,
carnivores do not switch diets variedly like omnivores, so they
lack the phenotypic flexibility to modulate digestive enzymes
and glucose transporter levels to match dietary starch levels.
Nevertheless, the utility of carbohydrates as an energy source is
not only linked to digestibility.
After digestion and absorption, most of the glucose uptake
from the bloodstream into the cells of different tissues occurs
passively through the members of the facilitative glucose
transporter family. Among the four members of the class 1 sub-
family of glucose transporters (GLUT1-4) hitherto cloned and
characterised in different fish species, GLUT4 is the only insulin
sensitive member that possibly plays an important role in glucose
homeostasis. However, trout GLUT4 was found to have relatively
lower affinity for glucose and poor sequestration characteristics
i.e., insulin stimulated recruitment to cell surface for glucose
uptake. Moreover at the transcriptional level, GLUT4 expression
in the white muscle of rainbow trout was reportedly inert to a
carbohydrate rich meal, consistent with the poor ability of the
peripheral tissue to adapt to a high influx of glucose.
Insulin and glucagon are the two major pancreatic endocrine
hormones that regulate glycaemia and the underlying metabolism
in fish, as in higher vertebrates. Even in carnivorous rainbow
trout, the existence of insulin sensitivity, intact functional
mechanisms and classic metabolic adjustments has been
demonstrated through several studies. Plasma insulin levels in
fish can rise as high as 8.6 nM after a carbohydrate rich meal,
along with an increase in the number of muscle insulin receptors.
However, it is apparent that secretion and physiological action
of insulin may depend on a maze of complex interactions with
other hormones. For instance, insulin secretion is inhibited by
hypersomatostatinemia even at the transcriptional level. Besides,
very low number of insulin receptors is present per microgram
of membrane protein in trout muscle, possibly limiting insulin
action in peripheral tissue metabolism even when plasma insulin
levels are high. As such, the potency of inherent insulin secretion
to ameliorate hyperglycaemia remains enigmatic in carnivorous
fish. On the other hand, postprandial glucagon levels in rainbow
trout were found to be inversely related to the carbohydrate
content of the diet. But, this adaptive response was independent
of insulin secretion, indicating that the regulation of glucagon and
insulin may be dissociated in fish.
In the metabolic context, the net hepatic glucose flux resulting
from the simultaneous regulation of glucose-disposal and
glucose-producing pathways is a key determinant of blood
glucose concentration. Disparity in the regulation of these
metabolic pathways is linked to poor carbohydrate utilisation in
some fish species. The hypothesis concerning limited glucose
phosphorylation was refuted when the existence of an inducible
hepatic glucokinase with adaptive response to carbohydrate rich
diets was evident in all the examined fishes. However, there is
uncertainty over its capacity to regulate glucose homeostasis in an
insulin dependent manner. Further the lack of coherent regulation
of the rate limiting glycolytic enzymes and sluggish flux may
underlie poor glucose use in some fish after a carbohydrate rich
meal.
More importantly, the uncontrolled hepatic endogenous
glucose production in carnivorous fish through gluconeogenesis,
regardless of the dietary carbohydrate content, trigger the
glucose intolerant phenotype that eventually leads to poor use of
Figure 2: Summary of biological limitations for carbohydrate utilisation
in carnivorous fish (Source: Kamalam et al., Aquaculture (2016), http://
dx.doi.org/10.1016/j.aquaculture.2016.02.007)
Figure 1: Illustration of the various factors known to influence
carbohydrate utilisation in fish (Source: Kamalam et al.,
Aquaculture (2016),
http://dx.doi.org/10.1016/j.aquaculture.2016.02.007)
22 | March | April 2016 - International Aquafeed
FEATURE

4. carbohydrates for energy. Particularly, the absence of inhibition
in the activity/expression of glucose 6-phosphatase was possibly
due to functional reorientation of the evolutionarily duplicated
genes. Changes in blood glucose levels are also correlated
to deposition and mobilisation of hepatic glycogen reserves.
Nevertheless, excessive glycogen deposition that accompanies
a carbohydrate rich meal can compromise the overall function
of the liver. Carbohydrates consumed in excess of energy needs
could be stored as lipid in the liver and adipose tissue through
the process of de novo lipogenesis (DNL), a kind of metabolic
safety valve or glucose sink. However, the amount of DNL
from glucose is apparently limited in carnivorous fish and the
regulation of the glucose-fatty acid cycle is yet to be completely
understood.
Poor utilisation of glucose in the principal insulin sensitive
peripheral sites such as skeletal muscle and adipose tissue could
probably be another key limitation for carbohydrate utilisation in
carnivorous fish. For instance, the contribution of skeletal muscle
disposal of glucose was less than 15 percent of the total glucose
turnover in rainbow trout. The underlying reason can be low
insulin sensitivity and glucose uptake with possible consequences
for the regulation of glucose metabolism. In fact, the activities
of enzymes involved in glucose oxidation/disposal are not
responsive to the presence or levels of carbohydrates in the diet.
What could improve carbohydrate utilisation in fish?
There are certain promising strategies that are being
investigated to overcome the challenges in carbohydrate utilising
in farmed carnivorous fishes. The possibility of tailoring
metabolic pathways or functions to improve carbohydrate use is
being tested applying the concept of nutritional programming.
The hypothesis is that high carbohydrate stimulus exerted
at critical developmental stages in early life may imprint an
adaptive ability to cope with high carbohydrate diets in later life.
This strategy was found to potentially improve starch digestibility
in rainbow trout and glucose oxidation/disposal in gilthead
seabream.
However, the success rate depends greatly on choosing the
appropriate duration, source and magnitude of the stimulus and
the point of application (early developmental stage). It is also
equally important to understand the biological mechanisms
(e.g. epigenetic changes) that imprint the nutritional event until
adulthood. Another relevant prospect is the use of supplementary
enzymes, when cost implications are duly considered. The idea is
to catalyse the hydrolysis of complex carbohydrates by increasing
enzyme accessibility to substrates. But in practice, the exogenous
enzyme should withstand the rigours of feed processing, be less
susceptible to proteolysis inside the digestive tract of fish and
precisely dosed/delivered.
Based on observed genetic variability and phenotypic
plasticity in glucose tolerance and metabolism in carnivorous
fishes, specific genotypes that can adapt better to carbohydrate
rich diets can be selected and propagated. For instance,
selection for the ability to adapt to a totally plant based diet has
been proven to be successful in rainbow trout. The availability
of whole genome sequence can further facilitate the recognition
of relevant quantitative trait loci. However, the feasibility and
efficacy of non-destructive selection criterions is yet to be
explored. Other critical aspects that can improve carbohydrate
use is finding a fine balance between dietary macronutrients
in evolving feed compositions and acquiring a symbiotic gut
microbiome that can functionally contribute to carbohydrate
digestion and metabolism.
International Aquafeed - March | April 2016 | 23
FEATURE