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Bioenergetics: Calculating Energy Values in Food. Introduction. Energy is required by all animals to sustain life Sources : food, natural productivity, body stores (times of environmental stress or feed deprivation)

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Bioenergetics calculating energy values in food

Bioenergetics:Calculating Energy Values in Food


  • Energy is required by all animals to sustain life

  • Sources: food, natural productivity, body stores (times of environmental stress or feed deprivation)

  • Lecture objectives: How much energy is needed by aquatic organisms?, How does it varies from terrestrials?, What are the sources, how is energy partitioned for various uses

Lecture objectives
Lecture objectives

  • How much energy is needed by aquatic organisms?

  • How it varies from terrestrials?

  • What are the sources?

  • How is energy partitioned for various uses?


  • Lavoisier first demonstrated that oxidation of nutrients was some form of combustion (burning)

  • Rubner (1894) first demonstrated that fundamental Laws of Thermodynamics also applied to intact living animal systems

  • Organic matter  processes  CO2 + H2O + energy (released)

  • Understanding energy transforms is only possible when it is converted from one form to another


  • Energetics is the study of energy requirements and the flow of energy within systems

  • bioenergetics is the study of the balance between energy intake in the form of food and energy utilization by animals for life-sustaining processes

  • processes?: tissue synthesis, osmoregulation, digestion, respiration, reproduction, locomotion, etc.


  • the original energy source for food energy is the sun (See…I knew what I was talking about for once!)

  • energy from the sun is converted by photosynthesis into the production of glucose

  • glucose is the hydrocarbon source from which plants synthesize other organic compounds such as COH, protein, lipids

  • as previously mentioned, one must consider the quality of these sources


  • Animals are not heat engines

  • They can’t use the multitude of sources of energy we have (e.g., flywheels, falling objects, the tide, etc.)

  • Must obtain their energy from chemical bonds of complex molecules

  • How do they do it? In a nutshell, they oxidize these bonds to lower energy states using oxygen from the air

  • Trick: some bonds have more energy than others


  • most aquaculture animals obtain their energy from feeds

  • As mentioned, some bonds have more energy associated with them than others

  • when you have many nutrients comprising a feed, the energy level of that feed can vary substantially

  • availability of energy varies according to feed ingredient and species

  • growth is the endpoint of net energy

Glycogen molecule
Glycogen Molecule

major COH storage form of energy

Lipid molecule
Lipid Molecule

another major storage form

Introduction cont
Introduction (cont.)

  • Energy goes through many cycles and transformations, always with loss of heat

  • can be released at various rates: gasoline can exploding vs. compost pile

  • nutritional energetics involves the study of the sources and transformations of energy into new products (mainly we are concerned with growth or tissue deposition)

  • of all dry matter we consume, 70-90% goes to synthesis of new products

Energy forms
Energy Forms

  • Matter and energy are basically the same

  • it is often convenient to consider energy a property of matter (kcal/g feed)

  • nutritive value of food items is often reflected by calories

  • what you are used to seeing in the store is not calories, but kilocalories (kcal’s), or Calorie

  • common form of energy in the cell is ATP

Energy forms1
Energy Forms

  • All processes in the animal body involve changes in energy

  • the word “energy” was first introduced in 1807, and defined as “ability to work”

  • found in many forms: heat, kinetic, electromagnetic, radiant, nuclear and chemical

  • for our purposes, chemical energy is the most important (e.g., ATP)

Heat energy
Heat Energy

  • The measurement of energy requires converting it from one form to another

  • what we typically measure is heat (why?)

  • according to the first law of thermodynamics, all forms of energy can be converted quantitatively into heat energy

  • heat energy is represented by the various constituents of the diet

Heat energy1
Heat Energy

  • however, the body is not a heat engine, heat is an end product of reactions

  • it is only useful to animals to keep the body warm

  • chemical reactions either generate heat (+H) or require heat (- H)

Units of heat energy
Units of Heat Energy

  • The basic unit of energy is the calorie (cal)

  • it is the amount of heat required to raise the temperature of 1g of water 1 degree Celsius (measured from 14.5 to 15.5oC)

  • it is such a small unit, that most nutritionists prefer to use the kcal (or 1,000 calories)

  • REM:the kcal is more common (supermarket Calories)

Other units of heat energy
Other Units of Heat Energy

  • BTU (British Thermal Unit) = amount of heat required to raise 1 lb of water 1oF

  • international unit: the joule - 1.0 joule = 0.239 calories or 1 calorie = 4.184 joule

  • a joule (J) is the energy required to accelerate a mass of 1kg at a speed of 1m/sec a distance of 1m

Energy terms from de silva and anderson
Energy Terms (from De Silva and Anderson)

  • Energy flow is often shown as a diagram: every text has its own idea of a suitable diagram:

Energy terms
Energy Terms

  • Gross energy (GE): energy released as heat resulting from combustion (kcal/g)

  • Intake Energy (IE): gross energy consumed in food (COH, lipid, protein)

  • Fecal Energy (FE): gross energy of feces (undigested feed, metabolic products, gut epithelial cells, digestive enzymes, excretory products)

  • Digestible Energy (DE): IE-FE

Energy terms cont
Energy Terms (cont.)

  • Metabolizable energy (ME): energy in the food minus that lost in feces, urine and through gill excretion:

    ME = IE - (FE + UE + ZE)

  • urinary energy (UE): total gross energy of urinary products of unused ingested compounds and metabolic products

  • gill excretion energy (ZE): gross energy of products excreted through gills (lungs in mammalian terrestrials), high in fish

  • surface energy (SE): energy lost to sloughing of mucus, scales, exoskeleton

Energy terms cont1
Energy Terms (cont.)

  • Total heat production (HE): energy lost in the form of heat

  • heat lost is sourced from metabolism, thus, HE is an estimate of metabolic rate

  • measured by temperature change (calorimetry) or oxygen consumption rate

  • divided into a number of constituents

  • as per energy flow diagram 

Energy terms total heat production
Energy Terms (total heat production)

  • Basic metabolic rate (HeE): heat energy released from cellular activity, respiration, blood circulation, etc.

  • heat of activity (HjE): heat produced by muscular activity (locomotion, maintaining position in water)

  • heat of thermal regulation (HcE): heat produced to maintain body temp (above zone of thermal neutrality)

  • heat of waste formation (HwE): heat associated with production of waste products

  • specific dynamic action (HiE): increase in heat production following consumption of feed (result of metab), varies with energy content of food, especially protein

Energy utilization
Energy Utilization

  • Energy intake is divided among all energy-requiring processes

  • Magnitude of each depends on quantity of intake plus animal’s ability to digest and utilize that energy

  • Can vary by feeding mode: carnivorous vs. herbivorous

From Halver (page 7)

Focus gross energy
Focus: Gross Energy

  • Energy content of a substance (i.e., food) is typically determined by completely oxidizing (burning) the compound to carbon dioxide, water and other gases

  • the amount of energy given off is measured and known as gross energy

  • gross energy (GE) is measured by a device known as a bomb calorimeter

Gross energy of feedstuffs1
Gross Energy of Feedstuffs

  • Fats (triglycerides) have about twice the GE as carbohydrates

  • this is because of the relative amounts of oxygen, hydrogen and carbon in the compounds

  • energy is derived from the heat of combustion of these elements: C= 8 kcal/g, H= 34.5, etc.

  • typical heat from combustion of fat is 9.45 kcal/g, protein is 5.45, COH is 3.75

Available energy
Available Energy

  • Gross energy only represents the energy present in dry matter (DM)

  • it is not a measurement of its energy value to the consuming animal!!

  • the difference between gross energy and energy available to the animal varies greatly for different foodstuffs

  • the key factor to know is how digestible the food item is

  • digestible energy also varies by species

Digestible energy
Digestible Energy

  • The amount of energy available to an animal from a feedstuff is known as its digestible energy (DE)

  • REM: DE is defined as the difference between the gross energy of the feed item consumed (IE) and the energy lost in the feces (FE)

  • two methods of determination: direct or indirect

  • by the direct method, all feed items consumed and feces excreted are measured

Digestible energy1
Digestible Energy

  • The indirect method involves only collecting a sample of the feed and feces

  • digestion coefficients are calculated on the basis of ratios of energy to indicator in the feed and feces

  • indicator?: an inert indigestible compound added to the feed

  • indicators: natural (fiber, ash) or synthetic (chromic oxide)

De calculations
DE Calculations

Direct Method

Feed energy - Fecal energy

% DE =

X 100

Feed energy

Indirect Method

Feed energy

Fecal indicator


% DE = 100 -


Fecal energy

Feed indicator

Metabolizable energy
Metabolizable Energy

  • Even more detailed!

  • Represents DE minus energy lost from the body through gill and urinary wastes

  • More difficult to determine! Why?

  • REM: all urinary wastes in water!!! How do you collect that????

Intake energy - (E lost in feces, urine, gills)

%ME = -------------------------------------- x 100

Feed energy

Metabolizable energy1
Metabolizable Energy

  • Use of ME vs DE would allow for a much more absolute evaluation of the dietary energy metabolized by tissues

  • however, ME offers little advantage over DE because most energy is used for digestion in fish

  • energy losses in fish through urine and gills does not vary much by feedstuff

  • fecal energy loss is more important

  • forcing a fish to eat involuntarily is not a good representation of actual energy processes

Energy balance in fish
Energy Balance in Fish

  • Energy flow in fish is similar to that in mammals and birds

  • fish are more efficient in energy use

  • energy losses in urine and gill excretions are lower in fish because 85% of nitrogenous waste is excreted as ammonia (vs. urea in mammals and uric acid in birds)

  • heat increment (increase) as a result of ingesting feed is 3-5% ME in fish vs. 30% in mammals

  • maintenance energy requirements are lower because they don’t regulate body temp

  • they use less energy to maintain position

Terrestrials vs aquatics
Terrestrials vs. Aquatics

  • This section concerns the requirements for energy by aquatic animals, how energy is partitioned, what it is used for and how it is measured

  • a major difference in nutrition between fish and farm animals is the amount of energy required for protein synthesis

  • protein synthesis refers to the building of proteins for tissue replacement, cell structure, enzymes, hormones, etc.

  • fish/shrimp have a lower dietary energy requirement

Factors affecting energy partitioning
Factors Affecting Energy Partitioning

  • Factors either affect basal metabolic rate (e.g., body size) or affect other changes

  • those affecting BMR are the following:

  • body size:non-linear, y = axb, for most physiological variables, b values usually range between 0.7 and 0.8

  • oxygen availability: have conformers (linear) and non-conformers (constant until stressed)

O 2 consumption by size
O2 Consumption, by Size

(Fig. 2.1 from De Silva and Anderson)

Factors affecting energy partitioning1
Factors Affecting Energy Partitioning

  • temperature: most aquaculture species are poikilotherms, significant effect, acclimation required, aquaculture situation may mean rapid temp changes

  • osmoregulation: changes in salinity result in increased cost of energy

  • stress: increased BMR resulting from heightened levels of waste, low oxygen, crowding, handling, pollution, etc. (manifested by hypoglycemia)

  • cycling: various animal processes are cyclic in nature (e.g., reproduction, migration)

Factors affecting energy partitioning2
Factors Affecting Energy Partitioning

  • Those factors not affecting BMR are:

  • gonadal growth: most energy diverted from muscle growth into oogenesis, deposition of lipid, can represent 30-40% of body weight, implications????

  • locomotion: major part of energy consumption, varies due to body shape, behavior and size, aquatic vs. terrestrial issues

Another index gross conversion efficiency k
Another Index: Gross Conversion Efficiency (K)

  • Referred to as “K”, often used as an indicator of the bioenergetic physiology of fish under various conditions

  • does not refer to an energy “budget”

  • measures growth rate (SGR) relative to feed intake over similar time periods

  • both factors are related to body size:

  • SGR = (ln Wtf-lnWti)/(Tf - Ti) x 100

  • RFI = (feed intake)/((0.5)(Wtf -Wti)(Tf-Ti))

K = (SGR/RFI) x 100

Energy and growth
Energy and Growth

  • Dietary excesses or deficiencies of useful energy can reduce growth rate

  • this is because energy must be used for maintenance and voluntary activity before it is used for growth

  • dietary protein will be used for energy when the diet is deficient in energy relative to protein

  • when the diet contains excessive energy, feed intake is typically don’t want to be fat????

  • this also reduces intake of protein and other nutrients needed for growth

Dietary sources of energy proteins
Dietary Sources of Energy:proteins

  • Considerable interaction between major nutrient groups as energy sources

  • protein can be used as an energy source

  • not typically used because of cost and use for protein synthesis (growth)

  • optimal ratio of protein:energy is around 22 mg PRO/kJ (45 kJ/g PRO; old info)

  • species variation: 17 (59) for tilapia, 29 (35)for catfish, 29 (34) for mutton snapper (Watanabe, et al., 2001);

  • digestibility variation

  • temperature variation

Energy and growth1
Energy and Growth

  • Consumption of diets with low protein to energy ratios can lead to fat deposition (fatty acid synthetase)

  • this is undesirable in food fish because it reduces the dress-out yield and shortens shelf life

  • undesirable in shrimp due to build-up in hepato- pancreas (midgut), ultimately affecting cooking

  • low protein:energy diets can be useful for maturation animals, hatchery animals raised for release

Energy requirments of fish
Energy Requirments of Fish

  • Determining the energy requirement of fish has been a difficult task, slow in coming

  • most research has been devoted to identifying protein requirements, major minerals and vitamins

  • in the past, feeds were formulated letting energy values “float”

  • excess or deficiency of nutritional energy does not often lead to poor health

Energy requirements of fish
Energy Requirements of Fish

  • Further, if feeds are formulated with practical feedstuffs (ingredients), their energy levels are not likely to be off

  • it is really a matter of cost: protein is the most expensive component of the diet, COH sources are cheap, why use protein as an energy source????

  • In terrestrials, feed is consumed to meet energy requirements

  • thus, as energy level of the feed goes up, protein level is also designed to go up

Energy requirements of fish1
Energy Requirements of Fish

  • This is because terrestrial animals are typically fed on an ad libitum basis

  • fish, on the other hand, aren’t fed this way

  • they are fed on a feed allowance basis (we estimate feed fed)

  • various studies have shown that the digestible energy (DE) requirement for channel catfish and carp was around 8.3-9.7 kcal DE/100 g fish/day

  • in terms of age, dietary level of DE and protein typically drop with age

Energy requirements of fish2
Energy Requirements of Fish

  • DE and protein requirements typically follow each other, so the DE:P ratio (kcal/g) is fairly similar with age (if anything, a small increase)

  • this is partially due to the fact that fish grow faster when young (higher tissue turnover rate, demand for protein)

  • however, the influence of energy is stronger than that of protein relative to growth (Cuzon and Guillaume, 1997)

  • energy levels in crustacean diets usually range similar to those of fish

Energy requirements of aquatics
Energy Requirements of Aquatics

  • The objective in formulating diets for most aquatic species is the same: finding a cheap energy source that is digestible and will spare protein

  • glucose is not acceptable in that it causes high blood sugar levels, poor growth, poor survival

  • complex dietary COH’s prove better

  • COH typically spares protein for growth

  • increase in dietary energy tends to increase performance when a diet low in protein is fed

Energy problems
Energy Problems

  • Lipids and carbohydrates are typical energy sources for crustaceans

  • unfortunately, crustaceans are unable to tolerate diets having greater than 10% lipid (also hard to manufacture the feed!)

  • this means that the major energy source must be derived from COH

  • various COH are used to various degrees by crustaceans, making it difficult to calculate the true energy value of diets