1 Lipids Readings: Chapter 5. 2 What are Lipids? organic compounds that dissolve readily in organic solvents but poorly in aqueous solutions –“hydrophobic”

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Lipids

Readings: Chapter 5

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What are Lipids?• organic compounds that dissolve readily

in organic solvents but poorly in aqueous solutions– “hydrophobic” = water fearing– “lipophilic” = fat loving

• three main classes of lipids:• triglycerides• phospholipids• sterols

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Fatty Acids: Building Blocks of Lipids• fatty acids are molecules composed of

a chain of carbon and hydrogen molecules with a methyl group at one end and an acid group at the other end

METHYL group “omega” end

ACID group“alpha end”

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Classification of Fatty Acids

Fatty acids are classified based on:

- LENGTH of the carbon chain

- degree of SATURATION

- location of DOUBLE BONDS

- ISOMERIC forms

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Classification of Fatty Acidsa) Length• the # of carbon molecules in the chain• can be 4 - 24 carbons long• most common in food are 18 carbon

fatty acids• short chain fatty acids are more liquid

and are more soluble in water

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Classification of Fatty Acids cont.

b) Degree of Saturation• refers to the # of hydrogen atoms (H+) bound to

the carbon atoms• “double bonds” reduce the # of H+ making a

fatty acid LESS SATURATED• refer to as:

– SATURATED– MONOUNSATURATED– POLYUNSATURATED

• foods contain a mixture of fatty acids

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Saturated Fatty Acids• no double bonds between

carbon atoms

• solid at room temperature

• example: stearic acid

• common sources: animal fats (butter, lard, beef tallow, etc…), cocoa butter, palm oil, coconut oil

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Monounsaturated Fatty Acids (MUFA)

• have a single double bond

• liquid at room temperature

• examples: oleic acid

• common sources: olive oil, canola oil, peanut oil

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Polyunsaturated Fatty Acids (PUFA)

• two or more double bonds• liquid at room temperature• examples: -linolenic acid &

linoleic acid• essential nutrients• common sources: flax seed oil,

safflower oil, soy bean oil, cottonseed oil, fish

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H H

Classification of Fatty Acids cont.

c) Location of Double Bonds• measured from the methyl end (also called

the omega end, abbreviated “”) to the first double bond

H–C–C–C=C–C–C=C–C–C=C–C–C–C–C–C–C–C–C–O–H1 2

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methyl end

H H

HH H H

H H H H H H H H H

O =

HH H H H H H

HHH H

= omega 3 fatty acid

double bond

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Classification of Fatty Acids cont.

c) Location of Double Bonds cont.

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Classification of Fatty Acids cont.

d) Isomeric Forms• molecules that have the same atoms but have

different geometric or positional arrangements

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Non-Essential & Essential Fatty Acids

• the body is able to synthesize most fatty acids as needed– desaturation: e.g. removes H+ from stearic acid to

make oleic acid– elongation: e.g. adds carbons to build storage and

structural fats

• the body cannot make fatty acids that have double bonds before the 9th carbon– omega 3 & omega 6 must be provided in the diet =

ESSENTIAL FATTY ACIDS

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Essential Fatty Acids

Omega 3 Fatty Acids• polyunsaturated fatty acids with 1st double

bond three carbons from the methyl end

Examples: linolenic acid, EPA, DHASources: canola, soybean, flaxseed oil, walnut oils,

fatty fish (tuna, salmon, sardines, etc..)

15Table 5.3, page 176

Omega 3 Fatty Acids in Selected Foods

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Omega 6 Fatty Acids• polyunsaturated fatty acids with 1st double bond

six carbons from the methyl end

Examples: linoleic acid, arachidonic acidSources: sunflower, corn, peanut, cottonseed,

soybean oils

Essential Fatty Acids

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Functions of the Essential Fatty Acids

• most fatty acids used to provide energy

• small proportion used to make eicosanoids

Examples:– thromboxanes– prostaglandins– leukotrienes

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Functions of EicosanoidsIf derived from -3 fatty acids…

– vasodilation– inhibition of blood clotting– reduce inflammation– lower blood cholesterol

If derived from -6 fatty acids…– vasoconstriction– promote blood clotting– promote inflammation

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Classes of Lipids: Triglycerides

• most abundant form of lipid both in our DIET and in our BODY

• formed from 3 FATTY ACIDS attached to a GLYCEROL backbone

GlycerolFatty Acids

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Formation of Triglycerides• occurs via a condensation reaction,

produces ester bonds

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Triglyceride Composition

• most triglycerides contain a mixture of more than one type of fatty acids, such that they may contain saturated, MUFA, and PUFA

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Functions of Triglycerides

a) energy source– in normal conditions supplies 60% of energy

needs during rest– preferred fuel of heart, resting muscles

b) Satiety value - provides satisfaction from meal

c) energy reserve– stored in adipocytes to provide energy during

times of energy deficit– 1 g of fat stores provides > 6x as much energy as

1 g of glycogen!

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d) insulation & protection– visceral fat cushions & shields organs– subcutaneous fat protects & insulates the body

e) transport of fat soluble vitamins– Vitamins A, D, E, K, carotenoids, other

phytochemicals

f) sensory qualities– contributes to the flavour,

texture, & odour of food

Functions of Triglycerides cont.

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Classes of Lipids: Phospholipids

• formed from a backbone of glycerol with 2 fatty acids, a phosphate, and a nitrogen containing group (e.g. choline)

• synthesized in body by the liver

• soluble in both water and lipids

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a) form cell membranes

b) lipid transport– form surface of lipid transport molecules

called lipoproteins, allowing lipids to dissolve in the blood

Functions of Phospholipids

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Functions of Phospholipids cont.

c) emulsifiers– facilitate mixing of lipids and water due to

ability to dissolve in both water and oil

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• NOT essential nutrients!

Examples: phosphatidyl choline, lecithins

• common food sources:– eggs– liver– soybeans– wheat germ– peanuts

Phospholipids in the Diet

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Classes of Lipids: Sterols

• carbon atoms form RINGS

• do not contain fatty acids

• both lipophilic and hydrophobic

• not essential nutrients

• example: cholesterol– found only in animal foods

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Functions of Sterols

a) Component of cell membranes– provide structure– abundant in brain and nervous tissues

b) Precursor to other substances– steroid hormones (e.g. cortisol,

testosterone, aldosterone, estrogen)– Vitamin D– bile acids

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Lipid Digestion & AbsorptionMouth & Stomach• lingual lipase secreted in saliva, gastric lipase

secreted by stomach- small contribution to digestion

• mechanical digestion of chewing and gastric mixing keep fat dispersed

• after 2 - 4 hours in the stomach, about 30% of lipids broken down to diglycerides and free fatty acids

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Lipid Digestion & AbsorptionSmall intestine

• presence of fat in the small intestine stimulates secretion of:- CCK

- secretin

- GIP

• bile contains bile salts & phospholipids which emulsify fats

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Lipid Digestion & Absorption• pancreatic lipase cleaves 1 fatty acid off

triglycerides at a time, produces free fatty acids & monoglycerides

• sterols remain unchanged by digestion or are esterified

• bile salts surround the products of lipid digestion forming micelles and transport digested lipid to intestinal cell surface

• bile salts are recycled• monoglycerides & fatty acids are absorbed by

passive diffusion

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Lipid Digestion & Absorption• monoglycerides & long chain fatty acids

reform triglycerides• triglycerides, cholesterol, and

phospholipids combine with proteins to form chylomicrons, a type of lipoprotein

• glycerol, short & medium chain fatty acids are absorbed directly into the blood

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Lipids in the Body: Transport• some short chain fatty acids (& glycerol)

travel freely in blood

• most lipids require transporters called lipoproteins

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Classes of LipoproteinsChylomicrons• transport diet derived lipids from small

intestine to body tissues• contain mostly triglyceride (~ 90%) and very

little protein, lowest density• the enzyme lipoprotein lipase splits off

triglycerides from the chylomicron • fatty acids may also be taken up by muscle

cells

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Classes of LipoproteinsVery Low Density Lipoproteins (VLDL)• synthesized by liver & intestines• lipoprotein lipase splits off and hydrolyzes

triglycerides from VLDL as it circulates• as it loses triglycerides becomes an

intermediate density lipoprotein (IDL)– IDL are about 40% triglycerides– pick up cholesterol from HDL– return to the liver & converted to LDL

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Classes of LipoproteinsLow Density Lipoproteins (LDL)• contain more than 50% cholesterol• deliver cholesterol to the body cells• LDL binds to a receptor on the surface of cell

membranes, triggering the cell to engulf the LDL molecule

• contain a protein called Apo B– facilitates cell uptake of cholesterol and

triglycerides

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Classes of Lipoproteins

Low Density Lipoproteins (LDL) cont.• other LDL receptors found on liver cells

– regulate blood levels of cholesterol– receptors are blocked by saturated and trans fatty

acids

• immune cells called macrophages also have scavenger receptors for LDL– uptake of LDL by macrophages implicated in

development of atherosclerosis

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Classes of LipoproteinsHigh Density Lipoproteins (HDL)• made in the liver & intestines• contains highest proportion of protein of all

lipoproteins• scavenges cholesterol and transfers the

cholesterol to other lipoproteins (IDL) for return to the liver

• contains a protein called Apo E which activates receptors in the liver for uptake

41Figure 5-25, page 190

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Blood Lipids & CVD• elevated blood lipids are a risk factor for

cardiovascular disease (CVD)• assess risk by examining blood levels

of:– triglycerides– total cholesterol– LDL– HDL

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Blood Lipids & CVD

Elevated Triglycerides

• linked to excess body fat, physical inactivity, cigarette smoking, Type II Diabetes, others

• most often seen in individuals with other lipid abnormalities

• optimal levels below 150 g / dL blood

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Blood Lipids & CVD

Elevated Total Cholesterol

• also linked to excess body fat, physical inactivity, cigarette smoking, Type II Diabetes, others

• optimal levels below 200 mg / dL blood

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Blood Lipids & CVD

LDL = “BAD” Cholesterol

• transport lipids from liver to the tissues

• elevated levels a risk factor for cardiovascular disease

• excess LDL in blood is susceptible to:

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Blood Lipids & CVDLDL = “BAD” Cholesterol cont.• oxidized LDL (oxLDL) causes damage to

blood vessel walls, initiating the development of atherosclerosis

• oxLDL taken up by macrophages through scavenger receptors

• macrophages become filled with lipids and are called foam cells

• results in build up of a fatty plaque on the blood vessel wall

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Blood Lipids & CVDLDL = “BAD” Cholesterol cont.• LDL particles may vary in size and density

– small dense LDL risk of CVD

• risk is also dependant on the specific proteins found within the LDL particle– example: risk for apo B and apo A-1

• optimal levels less than 100 g / dL

Think “L” = “less healthy” or “lethal”

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Blood Lipids & CVDHDL = “GOOD Cholesterol”• transports lipids from tissues to the liver• high levels risk of CVD• low levels risk of CVD• ratio of HDL:LDL (or HDL:total cholesterol) also

important ( 1:2 = low risk)• regular physical activity helps HDL• optimal levels greater than 60 g / dl

Think “H” = “healthy”

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Note: you cannot eat either LDL or

HDL!

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How does DIET influence blood lipids?

Monounsaturated Fatty Acids• associated with a risk of CVD

– may help lower triglycerides, total cholesterol & LDL

– may make LDL particles less susceptible to oxidation

– may help increase HDL– may also reduce other CVD risk factors

• know rich food sources of MUFA!

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How does DIET influence blood lipids?

Omega 3 Fatty Acids

• associated with a risk of CVD– lower LDL & triglycerides– decrease blood pressure– decrease blood clotting– reduce inflammation

• know rich food sources of PUFA!

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How does DIET influence blood lipids?

Saturated Fatty Acids• high dietary intake associated with ’d risk of

CVD• may increase CVD risk by HDL, LDL, &

total blood cholesterol• know rich food sources!Dietary Cholesterol• does not increase blood cholesterol as

dramatically as saturated or trans fatty acids

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How does DIET influence blood lipids?

Trans Fatty Acids• unsaturated fatty acids that have an abnormal

arrangement of hydrogen atoms about a carbon double bond– hydrogen atoms are opposite each other

• produced by the process of hydrogenation

transcis

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How does DIET influence blood lipids?

Hydrogenation• addition of H+ to

unsaturated fats

Benefits • less susceptible to

spoilage• changes textureRisks• produces trans fatty

acids

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Risks of Trans Fatty Acids LDL & HDLSources

– most margarines– cakes, cookies, doughnuts, pastries, crackers– meats & dairy products– chips & other snack foods– many brands of peanut butter– deep-fried foods

How does DIET influence blood lipids?

ANY food that contains HYDROGENATED or

PARTIALLY HYDROGENATED

VEGETABLE OIL or SHORTENING on its

ingredient list will likely contain trans fatty acids.

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Dietary Lipids & Cancer• link between dietary fat & cancer is tenuous

– linked to excess consumption of saturated fat

• appear to act as a cancer promoter• strong link between dietary fat intake &

prostate cancer• studies linking dietary fat & risk of breast

cancer are inconclusive• eating fish may protect against some cancers

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Dietary Lipids & Obesity• fat contributes 9 kcal/g• a high fat diet may be linked with excess energy

consumption thus contributing to weight gain

Dietary Lipids & Diabetes Mellitus

a diet high in MUFA may be beneficial for managing blood glucose levels

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Current Dietary Intakes• > ¼ of Canadians aged 31-50 obtain more

than 35% of their total calories from fat • average fat intake of Canadians in 2002 was 34%

of total energy intake (dropped from 36% in 1992)BUT…• total food energy consumed by Canadians rose

18% between 1992 & 2002• this increase in average energy intake means

Canadians are actually consuming more total grams of fat!

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Recommended IntakesAMDR for Total Fat• 20-35% of total energy intake

Daily Values• Total = 65 g, Saturated = 20 g • Cholesterol = 300 mg

Linoleic Acid• AI = 17 g/day (men), 12 g/day (women)• AMDR = 5-10% of total energy intake

-Linolenic Acid• AI = 1.6 g/day (men), 1.1 g/day (women)• AMDR = 0.6-1.2% of total energy intake

“Saturated fatty acids, trans fatty acids, & cholesterol have no

known beneficial role in preventing chronic disease and are not

required at any level in the diet”

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Dietary Tips…

Reducing Total Fat Intake• remove fats from foods• reduce use of fat in cooking• use fat containing condiments in moderation• replace high fat foods with low fat alternatives• choose low fat milk and dairy products• look for “invisible” fat

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Dietary Tips…

Reduce Saturated & Trans Fat Intake

• reduce intake of animal fats

• reduce intake of hydrogenated vegetable oil and products that contain it

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Fat Replacements• can be made from carbohydrate (including fiber),

protein, or synthetic

Olestra (Olean)• derived from sucrose and fatty acids (called a

sucrose polyester)• too large to be digested by the enzymes of the

human digestive tract therefore does not contribute any kilocalories

• Problems?

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