lecture 2 - carbohydrates, fats and energy

1. Lipids: the basics

• Contain mainly C, H and O, sometimes N and P.

• Soluble in non-polar organic solvents.

• Fats and oils are mostly triglycerides.

• Fat = solid at room temperature.

• Oil = liquid at room temperature.

• Lipids are more reduced than carbohydrates → therefore contain more potential energy.

• Fat provides about 9 kcal/g and approximately 2.25× as much energy as carbohydrates. L_2. Carbohydrates, fats and en…

Why do fats contain more energy?

Fatty acids contain more hydrogen per carbon and are therefore more reduced.

More reduced → more potential for oxidation → more energy can be released.

Carbohydrates are already partially oxidized, so they contain less potential energy per carbon.

2. Major functions of fats

• concentrated energy source

• long-term energy reserve

• excess energy can be stored as triglycerides in adipose tissue

• thermal insulation → maintains body temperature

• mechanical protection → protects organs

• electrical insulation → e.g. myelin sheath

• supply essential fatty acids

• component of cell membranes

• precursor for prostaglandins

• help transport fat-soluble vitamins

• improve palatability/aroma

• increase satiety because fats are digested more slowly

3. Fatty-acid structure

A typical natural fatty acid has:

Methyl group — carbon chain — carboxyl group

• methyl end = CH₃

• carboxyl end = COOH

• usually an even number of carbon atoms

• usually an unbranched chain

Chain length

Type	Carbon atoms
Short-chain	2–6 C
Medium-chain	8–12 C
Long-chain	14–24 C

Most fatty acids synthesized by plants and animals are long-chain; 16C and 18C are especially prevalent.

Important relationship:

↑ chain length → ↑ melting point

4. Saturated vs unsaturated fatty acids

Saturated fatty acids

• No double bonds

• Fully saturated with hydrogen.

• Generally solid at room temperature.

• Examples in lecture: butter, lard, coconut oil, palm oil.

Monounsaturated fatty acids

• 1 double bond

• Generally liquid at room temperature.

• Examples: olive, canola and peanut oils; avocado, almonds, cashews.

Polyunsaturated fatty acids

• 2 or more double bonds

• Include omega-3 and omega-6 fatty acids.

Key rule:

↑ number of double bonds → ↓ melting point.

5. Unsaturated fats, oxidation & rancidity

Unsaturated fatty acids are more susceptible to oxidation.

Oxidation of double bonds can cause:

oxidation → peroxides/free radicals → rancidity → reduced nutritional value

Antioxidants mentioned in the lecture:

• Vitamin E

• Vitamin C

• selenium

• carotenoids such as beta-carotene and lycopene

Hydrogenation

Hydrogenation increases saturation and stability, making fats more resistant to oxidation.

In ruminants, unsaturated fats entering the rumen can be naturally biohydrogenated by rumen microbes.

The lecture also discusses conversion from cis → trans configurations and notes that trans configurations can alter biological availability/effects.

6. Omega fatty acids ⭐

Animals can synthesize fatty acids with a double bond in the omega-9 position, but not omega-3 or omega-6.

Therefore omega-3 and omega-6 must come from the diet.

Linoleic acid

• 18:2

• omega-6

• essential

Linolenic acid

• 18:3

• omega-3

• essential

Arachidonic acid

• 20:4

• omega-6

• can normally be synthesized from linoleic acid

• therefore generally not essential

Exception: cats.

Cats cannot adequately make arachidonic acid from linoleic acid, so arachidonic acid is an essential dietary nutrient for cats.

Functions of essential fatty acids

They are involved in:

• phospholipids/cell membranes

• prostaglandins

• smooth-muscle contraction

• platelet aggregation

• inflammation

Deficiency can cause:

• growth retardation

• reproductive problems

• skin lesions

• kidney and liver disorders.

7. Triglycerides ⭐

Most common structure in dietary lipids.

A triglyceride consists of:

1 glycerol + 3 fatty acids

They are connected by ester bonds.

Triglycerides are a major storage form of lipids, especially in adipocytes.

Most lipids consumed are triglycerides.

Membrane lipids need to remain fluid, so they contain relatively more unsaturated fatty acids.

8. Other lipid types

Phospholipids

Similar to triglycerides, except one fatty acid is replaced by a compound containing a phosphate group (or occasionally nitrogen).

Important for:

• cell membranes

• emulsification

Most prevalent phospholipid mentioned: lecithin.

Derived lipids

Prostaglandins

• synthesized from arachidonic acid

• metabolic functions

Steroids

• cholesterol

• ergosterol

• bile acids

Terpenes

• produced by plants

• carotenoids

• xanthophylls

Sterols

• multi-ring structures

• insoluble in water

• major sterol = cholesterol

• cholesterol is found in animal products and manufactured in the liver.

9. Carbohydrates: basics

Carbohydrates = CHO

Approximate elemental ratio:

C:H:O = 1:2:1

They are the most abundant organic molecules in nature.

Plants are the major dietary source.

In plants:

• starch = energy storage

• cellulose = structural

Animals contain very little stored carbohydrate: <1% of body weight in the lecture's example. L_2. Carbohydrates, fats and en…

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10. Classification of carbohydrates ⭐

Classified according to the number of sugar units:

Type	Number of units
Monosaccharide	1
Disaccharide	2
Oligosaccharide	3–10
Polysaccharide	>10

L_2. Carbohydrates, fats and en…

Monosaccharides by carbon number

• 3 C = triose

• 4 C = tetrose

• 5 C = pentose

• 6 C = hexose

Pentoses and hexoses are nutritionally important.

11. Important monosaccharides ⭐

Glucose

A hexose (6C).

Very important because it:

• is part of starch, cellulose and glycogen

• is the major end product of carbohydrate digestion in monogastrics

• is a primary sugar used for energy.

Fructose

• hexose

• found in fruit, honey and cane sugar.

Galactose

• component of lactose

• can be metabolized to glucose.

Ribose

Found in:

• ATP/ADP

• DNA/RNA

• riboflavin.

L_2. Carbohydrates, fats and en…

12. Important disaccharides ⭐

Lactose = glucose + galactose

• milk sugar

Maltose = glucose + glucose

• intermediate in starch hydrolysis

• α-1,4 linkage

Sucrose = glucose + fructose

• common table sugar

• sugar cane/sugar beet

Cellobiose = glucose + glucose

• β-1,4 linkage associated with cellulose

• does not exist freely in nature.

L_2. Carbohydrates, fats and en…

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13. Starch ⭐

1. Amylose

• α-D-glucose

• α-1,4 bonds

• straight chain

• about 14–30% of plant starch

• water soluble

2. Amylopectin

• α-1,4 bonds

• α-1,6 bonds at branch points

• branched

• about 70–85% of plant starch

• not water soluble

3. Glycogen

= animal starch

• highly branched

• stored in small amounts in liver and muscle

• water soluble.

14. Cooking and starch

Raw starch is not well digested.

Heating causes starch granules to swell:

Gelatinization

This increases access for digestive enzymes.

After cooling, starch can form an indigestible crystalline form:

Retrograded starch

15. Cellulose, hemicellulose, pectin & lignin

Cellulose

• major structural polysaccharide in plants

• made from β-D-glucose

• straight chain

• β-1,4 bonds

• highly stable

Hemicellulose

Mixture including:

• glucose

• mannose

• arabinose

• galactose

Principal component of plant cell walls and degraded only by microbial enzymes.

Pectin

• found primarily between plant cell walls

• soluble in water

• described as a type of "plant glue"

• degraded by microbial enzymes.

Lignin

Important distinction:

Lignin is NOT a carbohydrate.

It surrounds cellulose and hemicellulose and increases plant rigidity.

As plants mature → more lignification → lower digestibility.

According to the lecture, animals and bacteria cannot break its bonds.

16. FOS & beta-glucans

Fructooligosaccharides — FOS

• indigestible by animals

• degraded by microbial enzymes

• used as prebiotics

• promote Lactobacillus and Bifidobacteria

• can suppress E. coli

• improve GI health.

Beta-glucans

• contain β-1,4 and β-1,3 bonds

• absorb water and form a gel

• reduce passage rate

• can prevent digestive-enzyme access

• found mainly in barley, oats and yeast.

17. Dietary fibre

Soluble fibre

Includes:

• pectins

• gums

• some hemicellulose

Functions:

• slows movement through stomach/small intestine

• binds fatty acids

• helps regulate blood cholesterol.

Insoluble fibre

Includes:

• cellulose

• hemicellulose

• lignin

Helps move undigested food through the large intestine.

18. Bioenergetics ⭐

Energetics = study of energy requirements and energy flow within systems.

Bioenergetics = balance between:

• energy intake from food

• energy utilization by animals for life-sustaining processes.

Animals need energy for:

• tissue synthesis

• osmoregulation

• digestion

• respiration

• reproduction

• locomotion.

Ultimate source of food energy:

☀ Sun → photosynthesis → glucose → other organic compounds

The common cellular form of energy is ATP.

19. Energy units ⭐

Energy can be measured in:

• cal

• kcal

• Mcal

• J

• kJ

• MJ

Important conversion:

1 cal = 4.184 J

One calorie is the amount of heat required to raise 1 g of water by 1°C.

And:

1 kcal = 1,000 cal

The "Calories" normally used for food are actually kilocalories (kcal).

20. Maintenance vs production

Animals use energy for two broad purposes:

Maintenance

Energy needed to maintain an adult animal without:

• production

• reproduction

• weight gain.

Production

Energy beyond maintenance used for productive functions.

21. The energy hierarchy ⭐⭐⭐

This is one of the most important things to memorize.

Gross Energy — GE

Total potential energy of a feed.

Measured using a bomb calorimeter.

GE alone is not very useful for comparing usable feed energy because feeds can have similar GE but very different digestibility.

↓

Digestible Energy — DE

DE = GE − fecal energy (FE)

↓

Metabolizable Energy — ME

Subtract urinary and gaseous losses:

ME = DE − UE − GPD

UE = urinary energy

GPD = gaseous products of digestion, mainly methane.

↓

Net Energy — NE

Subtract heat increment:

NE = ME − HI

HI = heat increment from metabolism/fermentation.

So memorize the flow:

GE → subtract feces → DE → subtract urine + gases → ME → subtract heat increment → NE

22. Net Energy for Maintenance — NEm

Includes energy for:

• basal metabolism

• voluntary activity

• warming the body

• cooling the body.

There can also be net energy for production functions

23. Total Digestible Nutrients — TDN ⭐

Traditional method for expressing digestible energy concentration of feeds.

Formula:

TDN (% DM) = %DP + %DCF + %DNFE + (2.25 × %DEE)

Why multiply digestible ether extract/fat by 2.25?

Because fat contains roughly 2.25× the energy of carbohydrates.

Also memorize:

1 kg TDN = 4,400 kcal Digestible Energy

Limitations of TDN

It can overestimate usable energy, particularly in forages, because it does not fully account for:

• urinary energy

• methane

• work of digestion

• heat of fermentation

• heat of nutrient metabolism.

Methane from rumen fermentation can represent 3–10% of feed energy according to the lecture

24. Calculating a dog's energy requirement

Formula given:

ME (kcal/day) = K × T^0.67

T = body weight

K:

• 99 = inactive

• 132 = active

• 160 = very active

Example in lecture:

45 kg inactive dog:

99 × 45^0.67 = 1,268.49 kcal/day

If food = 3,800 kcal/kg:

1,268.49 ÷ 3,800 ≈ 0.33 kg food/day. L_2. Carbohydrates, fats and en…

25. Modified Atwater equation ⭐

Used to estimate ME of dog/cat food:

ME (kcal/kg) = 10 × [(8.5 × %EE) + (3.5 × %CP) + (3.5 × %NFE)]

Where:

• EE = ether extract/fat

• CP = crude protein

• NFE = nitrogen-free extract

The lecture states that this provides an approximate ME value and is adequate for dogs and cats

🧠 The absolute must-memorize facts

If you're making a quiz for yourself, I would make sure you can answer these without looking:

1. Fat = 9 kcal/g and ≈2.25× carbohydrate energy.

2. More reduced molecule → greater potential for oxidation/energy.

3. Saturated = 0 double bonds; monounsaturated = 1; polyunsaturated = ≥2.

4. More double bonds → lower melting point.

5. Longer fatty-acid chain → higher melting point.

6. Linoleic = 18:2, omega-6, essential.

7. Linolenic = 18:3, omega-3, essential.

8. Cats require arachidonic acid in the diet.

9. Triglyceride = 1 glycerol + 3 fatty acids.

10. Mono = 1 sugar; di = 2; oligo = 3–10; poly = >10.

11. Lactose = glucose + galactose.

12. Maltose = glucose + glucose.

13. Sucrose = glucose + fructose.

14. Amylose = straight α-1,4.

15. Amylopectin = α-1,4 + α-1,6 branches.

16. Glycogen = animal storage carbohydrate, highly branched.

17. Cellulose = β-D-glucose + β-1,4 bonds.

18. Lignin reduces plant digestibility and is not a carbohydrate.

19. Cellular energy form = ATP.

20. 1 cal = 4.184 J.

21. DE = GE − fecal energy.

22. ME = DE − urinary energy − gaseous losses.

23. NE = ME − heat increment.

24. Energy hierarchy: GE → DE → ME → NE.

25. 1 kg TDN = 4,400 kcal DE.

26. Dog requirement: ME = K × T^0.67.

27. Modified Atwater: 8.5 kcal/g fat; 3.5 protein; 3.5 NFE.

Those are the points I would turn into questions first because they capture the main definitions, relationships, formulas, and exceptions emphasized across the lecture.