🪾 Lecture 8: Energy Partitioning

Food Energy: Lecture Objectives

Energy Concepts

• Understand energy as the capacity of a physical system to perform work

• Recognize different forms of energy, including mechanical, chemical, and light

Food Energy

• Understand energy terminology

• Understand energy partitioning → how energy is divided into different uses

• Use Atwater Factors to perform energy partitioning calculations

Energy in Animal Nutrition

Definition of Energy

• Energy → capacity of a physical system to perform work

• Exists in many forms, including mechanical, chemical, and light

Food Energy

• Chemical energy animals obtain from certain nutrients in food

• Derived from carbohydrates, proteins, and fats

• During metabolism, energy is released or absorbed as nutrients are broken down or used to build molecules

Units of Food Energy

Calories and Energy Conversion

• Calorie (cal) → heat required to raise 1 g of water by 1°C

• 1 kilocalorie (kcal) = 1,000 calories (cal)

  • Also called 1 Calorie (big C)

• 1 megacalorie (Mcal) = 1,000 kcal

• 1 cal = 4.184 joules (J)

• 1 kcal = 4.184 kilojoules (kJ) = 4,184 J

Energy Partitioning in Animals

From Gross Energy to Net Energy

• Gross Energy (GE) → total energy in food

• Fecal energy loss → energy lost in feces

• Digestible Energy (DE) → GE − fecal energy loss

• Gaseous and urinary energy loss → energy lost in gases and urine

• Metabolizable Energy (ME) → DE − gaseous and urinary energy loss

• Heat Increment of Feeding (HIF) → heat produced during digestion and nutrient metabolism

• Net Energy (NE) → ME − HIF; energy available for body functions and production

How Net Energy Is Used

Maintenance (NE_m, Net Energy for Maintenance)

• BMR → supports essential body functions

• Voluntary activity → movement

• Thermoregulation → maintaining body temperature

Production (NE_P, Net Energy for Production)

• Growth → building body tissues

• Gestation → supporting pregnancy and fetal development

• Lactation → producing milk

Gross Energy (GE)

Definition and Measurement

• Gross energy (GE) → heat released when a food item is completely combusted (burned with oxygen)

• Not all GE is available to the animal for use

• Measured using bomb calorimetry

  • Measures the heat released during combustion by detecting the temperature change compared with a calibration temperature

Gross Energy (GE) by Nutrient Type

Energy Released During Combustion

• Fat → 8.7–9.5 kcal/g

• Protein → 5.3–5.8 kcal/g

• Carbohydrates → 3.3–4.3 kcal/g

Memory Cue

- Fat > Protein > Carbohydrates in gross energy per gram

Standardized Gross Energy Values

GE per Gram of Nutrient

• Fat → 9.4 kcal/g

• Protein → 5.7 kcal/g

• Carbohydrates → 4.1 kcal/g

Important Note

- These are standardized values, not the middle of the previously listed ranges

Gross Energy (GE) Limitations

Why GE Values Are Not Exact

• Standardized GE values are estimates, not exact values

• Carbohydrates = 4.1 kcal/g

  • Includes heat of combustion from starch, sugars, and structural/insoluble carbohydrates

  • Can overestimate carbohydrate GE available to some animal species

Effect of Microbial Fermentation

• With microbial fermentation → microbes can break down structural/insoluble carbohydrates, allowing animals to use their energy

• Without microbial fermentation → animals cannot effectively utilize structural/insoluble carbohydrates

Memory Cue

- Microbial fermentation → structural carbohydrates can be used for energy

Digestible Energy (DE)

Definition and Calculation

• Energy available after digestion

• DE = GE − Energy Lost in Feces

• Fecal energy loss → energy contained in undigested food components

How DE Is Determined

- Measured through animal experiments using total fecal collection

Metabolizable Energy (ME)

Definition and Calculation

• Energy available after digestion and nutrient absorption

• ME = DE − Energy Lost in Urine and Gases

• Urinary and gaseous energy losses → energy not retained for the animal to use

How ME Is Determined

- Measured through animal experiments using total fecal and urine collection

Measuring Metabolizable Energy (ME)

Metabolic Chamber

• Used to determine energy use after a meal

• Measures:

  • Oxygen consumption

  • Carbon dioxide production

Gross Energy (GE) to Metabolizable Energy (ME)

Energy Lost from Protein

• Protein GE = 5.7 kcal/g

• Energy lost through nitrogenous waste = 1.25 kcal/g

• Protein ME = 5.7 − 1.25 = 4.45 kcal/g (GE - fecal and gaseous/urinary loss)

Nitrogenous Waste Excretion

• Mammals → urea

• Birds and reptiles → uric acid

• Aquatic animals → ammonia

Gross Energy Values

• Fat → 9.4 kcal/g

• Protein → 5.7 kcal/g

• Carbohydrates → 4.1 kcal/g

From Gross Energy (GE) to Metabolizable Energy (ME)

Accounting for Energy Losses

• Energy is lost through feces and urine (and gases)

• GE > DE > ME

  • GE → total energy in food

  • DE → GE minus energy lost in feces

  • ME → DE minus energy lost in urine and gases

Energy Values Before Accounting for Fecal Loss

• Fat → 9.4 kcal/g

• Protein → 4.45 kcal/g after urinary energy loss

• Carbohydrates → 4.1 kcal/g

Key Point

- These values do not fully account for digestibility yet; DE must be considered to determine ME accurately

Atwater Factors: GE to ME

Accounting for Digestibility

• Fat: 9.4 kcal/g × 96% digestibility = 9.0 kcal/g

• Protein: 4.45 kcal/g × 90% digestibility = 4.0 kcal/g

• Carbohydrates: 4.1 kcal/g × 98% digestibility = 4.0 kcal/g

Energy Conversion

• GE → DE → ME

• Digestibility accounts for energy lost in feces when converting GE to DE

• Further energy losses, such as urinary and gaseous losses, are accounted for when converting DE to ME

Atwater Factors for Metabolizable Energy (ME)

Standard Energy Values

• Fat → 9 kcal/g

• Protein → 4 kcal/g

• Carbohydrates → 4 kcal/g

Calculating ME

• Used to predict the metabolizable energy content of food

• ME (kcal) = (9 × g fat) + (4 × g protein) + (4 × g carbohydrates)

Calculating Metabolizable Energy (ME)

ME in kcal/g

- ME = (9 x g fat) + (4 x g protein) + (4 x g carbs)

ME in kcal/kg

- [ME = (9 x % fat) + (4 x % protein) + (4 x % carbs)] x 10

Key Difference

• kcal/g → uses grams of each nutrient

• kcal/kg → uses nutrient percentages and multiplies the result by 10

Converting Nutrient Grams to Percentages for ME Calculations

Converting Grams to Percentages

• % = (g ÷ total g of food) × 100

• Percentage represents grams of nutrient per 100 g of food

• Example: 5% protein = 5 g protein per 100 g of food

Metabolizable Energy (ME) Formulas

• ME (kcal/g) = (9 × g fat) + (4 × g protein) + (4 × g carbohydrates)

• ME (kcal/kg) = [(9 × % fat) + (4 × % protein) + (4 × % carbohydrates)] × 10

Converting Nutrient Content to Metabolizable Energy (ME)

ME Calculation Formulas

• ME (kcal/g) = (9 × g fat) + (4 × g protein) + (4 × g carbohydrates)

• ME (kcal/kg) = [(9 × % fat) + (4 × % protein) + (4 × % carbohydrates)] × 10

• ME (kcal/kg) = [(9 × g fat/100 g) + (4 × g protein/100 g) + (4 × g carbohydrates/100 g)] × 10

Why Multiply by 10?

• Converts nutrient amounts per 100 g of food to amounts per 1 kg of food

• 1 kg = 1000 g

• 100 g x 10 = 1000 g = 1 kg

Atwater Factors: When to Use Them

Standard ME Values

• Fat → 9.0 kcal/g

• Protein → 4.0 kcal/g

• Carbohydrates → 4.0 kcal/g

When to Use Atwater Factors

• Use to estimate metabolizable energy (ME) only when an analyzed ME value for the food item cannot be found in the literature

• Prefer analyzed ME values when available, as Atwater factors provide estimates

Net Energy (NE)

Definition and Calculation

• Energy available for maintenance and production

• NE = ME − Heat Increment of Feeding (HIF)

• HIF → energy lost as metabolic heat during digestion and nutrient metabolism

Heat Increment of Feeding (HIF)

- Difference in heat loss between an animal after feeding and a fasting animal

Measuring Net Energy (NE)

Metabolic Chamber

• Used to measure energy expenditure and heat production

• Helps determine NE by accounting for heat lost after feeding compared with fasting conditions