Energy partitioning

What is energy?

• Energy = the capacity of a physical system to perform work.

• Energy can exist in many forms:

  • Mechanical

  • Chemical

  • Light

Food energy

• Food energy = chemical energy animals get from nutrients in food.

• Comes from:

  • Carbohydrates

  • Proteins

  • Fats

• Nutrients are metabolized, and energy is released/absorbed. Lecture 8 - Energy partitioning

Energy Units

Calorie

• 1 calorie (cal) = heat required to raise the temperature of 1 g of water by 1°C.

• 1 kilocalorie (kcal) = 1,000 cal

• 1 megacalorie (Mcal) = 1,000 kcal

• 1 cal = 4.184 J

• 1 kcal = 4.184 kJ Lecture 8 - Energy partitioning

Energy Partitioning

Energy from food gets divided into different stages:

Gross Energy (GE)
↓ subtract energy lost in feces
Digestible Energy (DE)
↓ subtract energy lost in urine + gas
Metabolizable Energy (ME)
↓ subtract heat increment of feeding
Net Energy (NE)
↓
Maintenance OR Production

Easy way to remember:

GE → DE → ME → NE

Feces → Urine/gas → Heat → Useful energy Lecture 8 - Energy partitioning

What is GE?

• Gross energy = heat released when food is completely burned/combusted.

• Not all of this energy is available to the animal.

• Measured using bomb calorimetry.

• The measurement is based on temperature differences. Lecture 8 - Energy partitioning

GE values of nutrients

Nutrient	GE
Fat	9.4 kcal/g
Protein	5.7 kcal/g
Carbohydrates	4.1 kcal/g

These are the standardized values used in the lecture.

Important!

GE values are not perfect/exact.

For example:

• Carbohydrates = 4.1 kcal/g

• This includes energy from starches, sugars, and structural/insoluble carbohydrates.

• Therefore, it can overestimate the GE of carbohydrates for some animals.

What is DE?

Digestible energy = energy available after digestion.

Some energy is lost in the feces.

Formula:

DE = GE − energy in feces

So:

Gross Energy − Fecal Energy Loss = Digestible Energy

• Determined through animal experiments involving total fecal collection. Lecture 8 - Energy partitioning

Think of it as:

"How much energy is left after the animal gets rid of the undigested food?"

Metabolizable Energy (ME)

After digestion, there are still energy losses.

Energy is lost through:

• Urine

• Gas

What is ME?

Metabolizable energy = energy available after digestion and nutrient absorption.

Formula:

ME = DE − urinary and gaseous energy

So:

GE → DE → ME

• First remove fecal energy.

• Then remove urinary + gaseous energy. Lecture 8 - Energy partitioning

Metabolic chamber

A metabolic chamber can be used to determine energy after an animal consumes a meal.

It measures:

• Oxygen consumption

• Carbon dioxide production Lecture 8 - Energy partitioning

Protein and Energy Loss

When protein is metabolized, nitrogenous waste must be excreted.

The lecture gives:

• Mammals → urea

• Birds and reptiles → uric acid

• Aquatic animals → ammonia Lecture 8 - Energy partitioning

This means some energy from protein is lost in the urine.

For example:

Protein GE = 5.7 kcal/g

After accounting for urinary energy loss:

5.7 − 1.25 = 4.45 kcal/g

Atwater Factors

Atwater values:

Nutrient	ME
Fat	9 kcal/g
Protein	4 kcal/g
Carbohydrates	4 kcal/g

Basic formula

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

Calculating ME in kcal/kg

If the food composition is given as percentages:

Formula:

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

Why multiply by 10?

Because:

100 g × 10 = 1 kg

Example Calculation

Food contains:

• 6% fat

• 10% protein

• 40% carbohydrate

Step 1: Calculate each nutrient's contribution

Fat:

9 × 6 = 54

Protein:

4 × 10 = 40

Carbohydrates:

4 × 40 = 160

Step 2: Add them

54 + 40 + 160 = 254 kcal/100 g

Step 3: Convert to kg

254 × 10 = 2540 kcal/kg

Final answer:

ME = 2540 kcal/kg food Lecture 8 - Energy partitioning

Reading Food Composition

Example from the oatmeal crisp slides:

• Moisture = 5%

• Protein = 8%

• Fat = 12%

• Ash = 4%

• Fiber = 5%

• NFE = 66%

NFE = nitrogen-free extract

The lecture identifies this as soluble carbohydrates.

For the ME calculation, the relevant values are:

• Protein = 8 g/100 g

• Fat = 12 g/100 g

• NFE/carbohydrate = 66 g/100 g Lecture 8 - Energy partitioning

Net Energy (NE)

After getting ME, there is still energy lost as metabolic heat.

What is NE?

Net energy = energy available for maintenance or production.

Formula:

NE = ME − Heat Increment of Feeding (HIF)

Heat Increment of Feeding (HIF)

• HIF = heat loss from an animal after feeding compared with heat loss from a fasting animal.

14. What Does Net Energy Get Used For?

Once we reach NE, the energy can be used for:

Maintenance (NEm)

• BMR

• Voluntary activity

• Thermoregulation

Production (NEP)

• Growth

• Gestation

• Lactation