Energy balance – ch. 10

Energy The ability to do work Quantitatively the most important nutrient in the diet Derived from the oxidation of organic molecules – Protein, carbs, fats

Energy in the body Stored in the body as fat Transferred in the body in the bonds of ATP Examples of energy transfers: – Chemical to heat – Chemical to mechanical – Chemical to electrical

Energy transfers in the body are never 100% efficient! Heat is always being lost!

Measuring dietary energy Calories (cal) – Energy to heat 1 g of water from 14.5° to 15.5° C – Kilocalorie (Kcal or Cal) = 1000 cal – Megacalorie (Mcal) = 1 therm = 1000 Kcal Most other countries use joules

GE DE ME NE

Gross energy (GE) Total amount of heat resulting from complete oxidation of a feedstuff – burning it Bomb calorimeter No distinguishing available from nonavailable Human food labels in GE

Absorption / digestibility Food Animals are not 100% efficient! Food Body Loses Microbes

Digestible energy (DE) GE – fecal energy = DE Eliminates that amount of energy the animal did not get to absorb Single largest loss of nutrient in feces – waste Apparent is not accurate – Try to account for endogenous losses with fasting measurements, parenteral feeding, 100% used diet

Metabolizable energy (ME) DE – urine - gases of digestion = ME Ruminants - large energy loss through gases – Mostly methane, some others Nonruminants have some gases of digestion (fermentation), but they are normally ignored

Corrections for N N excreted accounts of energy used to catabolize protein ME of diet can decrease as N content increases Can correct ME for N in diet – Done with poultry

Net energy (NE) ME – heat increment – heat of fermentation = NE Energy truly available for maintenance, growth, & production HI & HF can heat animals like other mechanisms – Useful in cold weather – Detrimental in hot weather – energy used to dissipate heat

Heat increment (HI) AKA: specific dynamic effect Heat produced by digestion & metabolism of nutrients – Inefficient capture of energy from oxidation – Oxidation not coupled to energy capture – Work done to excrete waste products – Increased work by the GI tract, respiratory, & circulatory systems from digestion & metabolism

Increased metabolism rate after eating – Liver accounts for most HI HI changes with situation – Increases as FI increases – Increases when accretion & utilization is limited by other nutrients (e.g. AA’s, minerals) Not the same as total heat production – Animals produce heat when fed or fasting

Heat of fermentation (HF) Heat from microbial metabolism – Fermentation, growth Hard to estimate

Basal metabolism Condition when the minimum amount of energy is needed to sustain the body – Postabsorptive state HI and HF do not affect measurement – Muscular repose – Thermoneutral environment Circulation, respiration, secretions, muscle tonus, chemical & electrical gradients, tissue repair and maintenance

Factors affecting Basal Metabolism – Age: heat production drops through most of life – Neuroendocrine factors: hormones and sex – Species and breed – Adaptation to fasting – Muscle training – Mental effort

Maintenance Condition when a nonproductive animal neither gains nor loses energy reserves Factors affecting maintenance: – Basal metabolism – Any energy needed to perform daily tasks – Environmental factors

Most maintenance energy for ATP Maintenance is largest energy cost Maintenance requirement can be estimated by metabolic BW Typically: Metabolic BW = BW 0.67 to 0.75

Environmental expenditures Energy needed to conserve or dissipate heat Homeotherms produce energy to maintain body temperature Thermoneutral zone: temperatures where little energy is needed to maintain body temp

Heat production related to: – Profile – Size / surface area – Exposure to environment Skin, wool, blood vessels dilating or constricting Acclimatization: adaptive changes to environment – behavioral and physiological – Easier for animals to warm up than cool down

Energy efficiency Gross energy vs. other measure of energy per unit of production Amount of energy in that gain – Fat vs. protein – Fat - more energy stored – Protein - more energy for deposition & maintenance

Efficiency: maintenance > milk/repro > growth HI and HF can help with maintenance Systems with NE m, NE g, NE l

Nutrient requirements & energy Appetite is largely a reflection of energy needs Energy density & FI inversely related – Gut fill may limit FI at very low densities 2 ways to handle this fact – Base requirements at a set energy density e.g. standard diet of 3200 kcal ME / kg feed – Base requirements per unit energy e.g. gram Lys / kcal ME

Total digestible nutrients (TDN) Similar to DE Calculation based on feed analysis to estimate energy value of feed

Measuring heat production Evaporation, heat from skin, excretion, respiration Because of previous measurement trials, lots of energy values for different feedstuff are calculated by equations

Comparative slaughter – Slaughter animals before and after trial to calculate the amount of energy gained (retained energy; RE) during the trial period Direct calorimeters – trap animals and measure heat and humidity coming off of them Indirect calorimeters – measure gases (CH 4, CO 2, O 2 ) in & out and of animal along with excretions (N) to estimate oxidation

C-N balance – – Based on most energy is trapped as protein & fat with little carbohydrate in the body – Frequently used in conjunction with calorimetry – Measure C and N in and out of body feed, feces, urine, gases – Protein is 16% N & 51.2% C on avg. – Calculate amount of N & C stored as protein & fat – Calculate energy accretion from protein & fat accretion