Why study nutrition

1
Nutrition (Chapter 8)

• Why study nutrition?
• Proper supply of nutrients is basic to achieving
  optimal growth and production efficiency
• Fishes can be herbivores, carnivores,
  detritivores, omnivores. It is important to know
  what their proper diet should be.
• Dietary needs may also change during development.
• Broodstock and larval nutrition are often
  critical considerations for embryo and larval
  growth and survival.
• Topics
• Digestion
• Nutrient absorption
• Digestibility
• Energetics
• Protein
• Carbohydrates
• Lipids
• Vitamins and minerals
• Diet quality

2
Nutrition (Chapter 8)

• Some definitions
• Ingestion (consumption)
• The taking of food into gastrointestinal (GI)
  tract
• Digestion
• The breaking down of ingested food items into
  smaller particles or units capable of being taken
  in across the GI wall (epithelium)
• Absorption (assimilation)
• The taking of digested food into the circulatory
  system across the GI wall
• Elimination
• The elimination of undigested or unabsorbed food
  items by defecation

3
Nutrition

• Digestion
• Different segments of the GI system have
  different digestive enzyme composition
• Esophagus
• Stomach (some fish have no stomach)
• Intestine (including pyloric caeca)
• Hepatopancreas
• Types of enzymes
• Proteases
• Lipases and esterases
• Carbohydrases

4
Nutrition

• Digestion (continued)
• Several factors may influence the rate of
  digestion and movement of food through the GI
  tract
• Species
• Age
• Size
• Sex
• Temperature
• Stocking density
• Meal size
• Type of food
• Feeding rate
• Rate of digestion of feeds is important in
  aquaculture
• If too fast burst of energy is used
  inefficiently
• If too slow nutrients are voided (excreted)
  before they can be fully utilized

5
Nutrition

• Nutrient absorption
• Two processes are involved in absorption of
  nutrients across the wall of the GI tract into
  the blood stream
• Diffusion simple - against a gradient across the
  GI epithelium or facilitated by a transport
  system across cell membranes
• Active transport relies on transport system that
  utilizes energy for its function. Transport
  system may utilize carrier proteins, or be based
  on pinocytosis.
• Examples
• Fatty acids simple diffusion
• Amino acids facilitated diffusion
• Macromolecular protein active transport by
  pinocytosis
• Carbohydrates (glucose) active transport by
  carrier protein.

6
Nutrition

• Digestibility
• Not all food ingested is digested and absorbed.
  Left over is voided (eliminated) as feces.
• Digestibility is a measure of the extent to which
  food is digested and absorbed into the animal.
• Digestibility is an important measure of diet
  quality. Namely, although a balanced nutrient
  diet is important, also is its ability to be used
  by the animal.
• Digestibility is usually measured as
• Total digestibility of the complete diet
• Dry matter digestibility
• Specific nutrient digestibility (e.g., for a
  specific protein or carbohydrate, etc.)
• Its units are expressed in terms of percentage of
  the original ingested material that is digested
  and absorbed.
• Digestibility of protein from fish meal is high,
  close to 90 but for protein from plant material
  (soybean meal, etc.) percentages are variable.
  Thus, source of protein in feedstuff can affect
  its quality.

7
Nutrition

• Digestibility (continued)
• Digestibility can be measured directly or
  indirectly
• Direct method quantity ingested and quantity of
  fecal matter voided are measured digestibility
  100 (quantity ingested fecal
  matter)/quantity ingested
• Indirect method relies on the use of markers.
  Markers can be,
• Indigestible material introduced in small
  quantities and mixed evenly in the diet (external
  marker e.g., chromic oxide), or
• Indigestible component of the diet (internal
  marker e.g., crude fiber)
• Marker is indigestible so it will concentrate in
  feces relative to the diet. The degree of
  concentration of the marker provides an estimate
  of the digestibility of the diet or individual
  nutrient
• These measurements are only estimates because
  feces also include non-diet (endogenous)
  material. Thus, the values obtained should be
  considered measures of percent apparent
  digestibility.

8
Nutrition

• Energy
• Remember bioenergetics equation?
• Expanded equation C (Mr Ma SDA) (F U)
  (Gs Gr)
• C rate of energy consumption
• Mr standard metabolic rate (rate of energy use
  by a fasting animal at rest)
• Ma metabolic rate increase (over standard) due
  to activity
• SDA metabolic rate increase (over standard) due
  to digestion and assimilation of food
• F waste due to egestion (feces)
• U waste due to excretion (urine)
• Gs somatic growth rate
• Gr reproductive or gonadal growth rate
• Which of the equation components is the primary
  interest of aquaculture?
• Somatic growth rate (Gs)

9
Nutrition

• Energetics and feeding
• Energy in feeds must be provided in adequate
  amounts e.g., too much energy is wasteful and
  can lead to health impairment (obesity) or flesh
  deterioration
• Composition of diet and supply of essential
  nutrients are important considerations (essential
  nutrients those that cannot be synthesized by
  animal but are needed for its growth e.g.,
  essential amino acids)
• Energy sources are protein, carbohydrates, and
  lipids. However, different species differ in
  their ability to process each of these types of
  nutrients (why?)
• Lipids contain more energy per unit weight than
  the others and also increase palatability of
  feeds. They are excellent source of non-protein
  energy for fishes.
• Carbohydrates are less efficient sources of
  energy than lipids, but they are inexpensive.
  See table 8.2 (MJ Mega Joules)

10
Nutrition

• Energetics and feeding - protein
• Protein digestion
• Proteins are composed of amino acids
• Proteases (e.g., pepsin - works in acidic
  environment) break down proteins into its
  component amino acids
• Most protein digestion occurs in the stomach
  (which produces pepsin and acid)
• (Acidic environment may also assist in breaking
  up plant cell walls in macrophyte feeders such as
  tilapias)
• Stomach-less fishes do not produce pepsin so
  they produce alkaline proteases (work best in
  basic environments)
• Protein as source for energy versus growth
• Dietary protein is the largest and most expensive
  component of aquaculture feeds. Objective in
  diet formulation is thus to reduce the protein
  that is used for energy and maximize its
  incorporation into flesh.
• The protein-sparing capability of a diet refers
  to its ability to spare proteins for growth and
  utilize other nutrients for energy. In salmonids,
  lipids are used for sparing whereas in
  anguillids, carbohydrates can be used. Dietary
  protein/energy (P/E) ratios must be carefully
  evaluated for optimal use of protein. See Figure
  8.3

11
Nutrition

• Energetics and feeding protein (continued)
• Recommended protein levels in fish diets range up
  to 56
• Protein sources may include
• Fish meal - best in term of growth results, but
  expensive (and potentially environmentally
  unfriendly)
• Plant material such as soybean (most common) or
  cottonseed meal - cheap, but results are
  variable.
• Usually, mixture of fish meal and plant meal is
  used.
• Consumer attitudes toward possibility of disease
  transmission through fish meal, the cost of fish
  meal, and the excessive use of trash fish are
  driving research to replace fish meal with
  plant-based protein. Knowledge of the
  nutritional physiology of the animal (e.g.,
  essential amino acid requirements) is needed to
  achieve this objective.

12
Nutrition

• Energetics and feeding carbohydrates
• Carbohydrates are polysaccharide chains (e.g.,
  glycogen) made of monosaccharide units (e.g.,
  glucose)
• Produced by photosynthesis (plants) or
  gluconeogenesis (animals)
• There are two basic forms of carbohydrates that
  differ in how monosaccharides are linked together
  in the chain
• ? linkages (loose structure)
• ? linkages (flat, layered structure such as
  cellulose)
• Most organisms produce enzymes (carbohydrases)
  that degrade ? linkages (e.g., amylase,
  chitinase). In animals, these carbohydrases are
  usually produced by the pancreas.
• Only bacteria produce carbohydrases that can
  degrade ? linkages.
• Carnivorous fishes do not respond as well as
  omnivorous fishes to increased dietary
  carbohydrates.
• Most important carbohydrates for diet
  formulations in fish aquaculture are starch,
  chitin (? linkages) and cellulose (? linkages
  known as fiber).
• Depending on species, recommended levels of
  carbohydrate in diets range from 10 (yellowtail)
  to 40 (carp).

13
Nutrition

• Energetics and feeding lipids
• Main functions of lipids in animals are
• Energy storage
• Cell membrane components
• Digestion of ingested lipid is accomplished by
• Bile action
• Lipase action
• Bile produced by the liver emulsifies lipids and
  makes them more accessible to lipases (breaks
  down large lipid droplets into smaller ones)
• Lipases are produced in pancreas and breakdown
  the lipids in the small intestine. They work
  best in basic environment.
• Fatty acids (carboxylic acids) are a component of
  lipids
• saturated (no double bonds)
• monounsaturated (one double bond)
• polyunsaturated (PUFAs multiple double bonds)
• Cxyn-z (w often used instead of n)
• PUFAs with 4 double bonds are known as highly
  unsaturated fatty acids (HUFAs). See Table 8.7
  (this table is shown FYI no need to memorize)

14
Nutrition

• Energetics and feeding lipids (continued)
• In most species, lipids are better source than
  carbohydrates for protein sparing, but care must
  be exercised not to provide too much lipid in
  diet
• Generally, levels of 15-18 seem to be adequate
• During fasting, fishes utilize lipids as energy
  source preferentially over protein or
  carbohydrates
• n-3 and n-6 PUFAs all originate from dietary
  sources and thus are essential fatty acids (EFAs)
• Fish need particularly long chain n-3 and n-6
  PUFAs in their diets since they have difficulty
  synthesizing these lipids

15
Nutrition

• Vitamins and minerals
• Vitamins
• Vitamins serve as cofactors or substrates in
  certain metabolic reactions and are required in
  the diet in small amounts
• There are two general groups of vitamins
• Fat-soluble A, D, E, and K
• Water-soluble B group, C and some other
  cofactors
• Lack of vitamins can result in nutrition-related
  disease, poor growth, and increased
  susceptibility to infections (concern feed
  processing and storage time depress vitamin
  levels in feeds)
• Too much fat-soluble vitamins results in
  vitaminosis (vitamin poisoning) since they are
  difficult to excrete

16
Nutrition

• Vitamins and minerals
• Minerals
• Minerals are also required for some metabolic
  reactions and as structural elements (e.g.,
  calcium for bones remember hardness requirements
  for most fishes?)
• Normally provision of minerals in the diet is not
  of concern for aquatic animals since minerals are
  found dissolved in water
• Two groups of minerals are required
• Major minerals Calcium, phosphorous, magnesium,
  sodium, potassium, chlorine and sulfur
• Trace minerals include iron, iodide, and several
  others

17
Nutrition

• Diet quality
• Food conversion ratio (FCR) is an important
  parameter of diet quality
• FCR mass of food consumed (dry weight)/increase
  in mass of animal produced)
• The lower the value of FCR, the better the diet
• Usual values, 1.2-1.5
• Other measures protein efficiency ratio (PER)