Lipids metabolism introduction

Lipids
 Category of compounds that includes
• Triglycerides
• Sterols
• Phospholipids
 Contain carbon, oxygen, and hydrogen
 Hydrophobic

Lipids
 Basic functions in the body
• Store and provide energy
- Fats provide 9 kcal per gram
• Provide insulation
• Help manufacture steroids and bile salts
• Play a role in transporting fat-soluble nutrients in the
blood
• Used to manufacture major sex hormones
• Key to the structure of cell membranes

Fatty Acids
 Building blocks for triglycerides and phospholipids
 A chain of carbon and hydrogen atoms with a carboxyl
group at the alpha end and a methyl group at the omega end

Fatty Acids
 Higher ratio of carbon and hydrogen to oxygen than in
carbohydrates and protein
• 9 kilocalories per gram of fat
 20 different fatty acids with varied length, saturation, and
shape

Fatty Acids Vary in Length
 Short-chain fatty acids
• Two to four carbons
• Weak attraction
• Liquid at room temperature
 Medium-chain fatty acids
• Six to ten carbons

Fatty Acids Vary in Length
 Long-chain fatty acids
• 12 or more carbons
• Most common type of fatty acid in foods
• Strong attraction
• Solid at room temperature

Fatty Acids Vary in Saturation
 Saturated
• All the carbons on the fatty acid are bound to hydrogen
• Solid at room temperature
• Higher melting point

Fatty Acids Vary in Saturation
 Unsaturated
• Some carbons on fatty acid form a double bond with
each other instead of binding to hydrogen
• Mon-ounsaturated fatty acids (MUFA)
- Has one double bond
• Polyunsaturated fatty acid (PUFA)
- Has two or more double bonds
• Liquid at room temperature
• Lower melting point

Saturated and Unsaturated Fatty Acids
Help Shape Foods
Figure 5.3

Fatty Acids Differ in Double-Bond Location
 The location of the first double
bond in unsaturated fatty acids
effect the omega-3 fatty acid
and omega-6 fatty acid
 Omega-3 fatty acid
• First double bond is between
the third and fourth carbon
from the omega end
• Example: Alpha-linolenic
acid
- One of the two essential
fatty acids

Fatty Acids Differ in Double-Bond Location
 Omega-6 fatty acid
• First double bond is between the sixth and seventh
carbon from the omega end
• Example: Linoleic acid
- One of the two essential fatty acids

The Omega Fatty Acids
Figure 5.4

© 2010 Pearson Education, Inc. Figure 5.5
Fatty Acids Vary in Shape
 Unsaturated fatty acids form two different shapes

Fatty Acids and Rancidity
 Rancidity: spoiling of fats through oxidation
• More double bonds therefore more susceptible to
oxidation and rancidity
PUFA > MUFA > Saturated fatty acids
 Enhancing stability of fatty acids by reducing rancidity
• Adding antioxidants
• Limiting food exposure to oxygen, heat, and light
• Hydrogenation

 Fatty acids
• Carbon and hydrogen chain, carboxylic acid, methyl
group
• Basic structural units of triglycerides and phospholipids
• Differ in chain length, degree of saturation, shape
- Saturated fatty acids: no double bonds
- Monounsaturated fatty acids: one double bond
- Polyunsaturated fatty acids: two or more double bonds
• Essential fatty acids
- Lenoleic
- Alpha-linolenic acid
• Food manufactures hydrogenate or add antioxidants to
fatty acids to reduce susceptibility to rancidity

© 2010 Pearson Education, Inc. Portion of Figure 5.7
Triglycerides
 Three fatty acids connected to a glycerol backbone

Caution:
High levels in
the blood are
a risk factor
for heart
disease
Triglycerides
 Most common lipid in both foods and the body
 Make up about 95% of lipids found in foods
 Functions
• Add texture
• Makes meats tender
• Preserves freshness
• Stores as adipose tissue
for energy

 Hydrophilic on one end; hydrophobic on the other
 Make up the phospholipid bilayer in the cell membrane
• Lecithin (aka phosphatidylcholine) (also aka Lecithin)
- A major phospholipid in the cell membrane
- Used as an emulsifier in foods
 Synthesized by the liver
Phospholipids
Portion of Figure 5.8

Phospholipids’ Role in Cell Membranes
Figure 5.9

Sterols
 More complex than phospholipids or triglycerides
• Four connecting rings of carbon and hydrogen
 Do not provide energy
 Cholesterol is the best known sterol
• Found in every cell in the body
• Helps with numerous body processes
 Phytosterols – major plant sterols
Figure 5.11

 Triglycerides
• Found in the body
• Stored in the adipose tissue
• Major source of energy
 Phospholipids
• An important part of cell membrane structure
• Lecithin
- Important to cell membrane
- Used as a food emulsifier
 Sterols
• Do not contain fatty acids
• Do not provide energy

What Happens to the Lipids You Eat?
 Lipids from foods
• Fat, phospholipids, and sterols
 Digested to
• Free fatty acids, glycerol, and monoglycerides
 With the aid of the enzyme lipase

Most Fat Is Digested and Absorbed in the
Small Intestine
 Fat tends to cluster in chyme
 Fat stimulates the secretion of cholecystokinin (CCK) in
the duodenum
 CCK stimulates the gallbladder to release bile acid

Small Intestine
 Phospholipids
• Emulsified by bile
• Dismantled into two free fatty acids and the phospholipid
remnant
• Packaged as micelles
• Transported through intestinal wall

Small Intestine
 Sterols
• Not digested
• Absorbed intact through intestinal wall
 If undigested and absorbed in small intestine
• Bind with fiber
• Eliminated in the feces

Chylomicrons Facilitate Lipid Absorption
 Lipids are absorbed based on structure and circulatory
system
• Glycerol and short- to medium-chain fatty acids
• Long-chain fatty acids
- Combine with phospholipids and cholesterol
chylomicrons

Chylomicrons Facilitate Lipid Absorption
 Chylomicrons are too large
to be absorbed directly into
the bloodstream
• Travel through lymph
fluid
• Enter blood stream
through thoracic duct
next to the heart

Absorption of Dietary Lipids
Figure 5.16

Lipoproteins Transport Fat Through the
Lymph and Blood
 Lipoproteins
• Chylomicrons
• Very low-density lipoproteins (VLDLs)
• Low-density lipoproteins (LDLs)
• High-density lipoproteins (HDLs)

Lymph and Blood
 VLDLs, LDLs, and HDLs
• Globular molecule with a lipid center surrounded by a
plasma membrane
• Density determines function
• More protein, higher density

The Ratio of Protein to Lipid Determines the
Density of the Lipoprotein
Figure 5.17

Lymph and Blood
 Very low-density lipoproteins (VLDLs)
• Become LDLs
 LDLs: “bad” cholesterol
 HDLs: “good” cholesterol
 HDL and LDL levels can be used to determine the health of
arteries

The Roles of VLDL, LDL, and HDL Lipoproteins
Figure 5.18

 Most fats are digested in the small intestine with the help of
bile acids and pancreatic lipase
 Short- and medium-chain fatty acids are absorbed directly
into the bloodstream
 Longer chain fatty acids and other remnants of fat digestion
• Packaged in chylomicron lipoprotein carriers
• Travel in lymph before entering the bloodstream
 Lipoproteins are globular-shaped transport carriers
• Outer shell high in protein and phospholipids
• Inner core carries insoluble fat, cholesterol, and other
lipids through lymph and bloodstream

 VLDLs and HDLs are synthesized in the liver
 VLDLs eventually become LDLs after depositing some
fatty acids in the body’s cells
 LDLs deposit cholesterol in the cells and arterial walls
 HDLs remove cholesterol from the arteries and deliver it to
the liver to be used in the synthesis of bile or excreted in
the feces

Uses of Fat and Cholesterol
 Source of energy
 Form body structures
 Regulate metabolism
 Enhance absorption of fat-soluble vitamins
 Provide insulation to help regulate body temperature
 Cushion major organs

Fat Is Used as Energy
 Fat
• Provides concentrated source of kilocalories
- 9 kilocalories per gram
• Readily available when the body needs energy
• Body’s main source of energy throughout the day

Fat Is Used as Energy
 Fat
• Body has an unlimited ability to store excess energy as
fat in adipocytes
- Fat cells have the capacity to enlarge as much as 1,000
times their original size
- Body has the ability to produce additional fat cells

Fat Is Used as Energy and Helps Absorb
Lipid Compounds
 Preferred source of energy for the heart, liver, and muscles
 Cannot sustain life alone
 Needs glucose
• Only glycerol can be used for gluconeogenesis
 Several essential nutrients require dietary fat for absorption
 20 grams per day are needed to stimulate chylomicrons that
transport fat-soluble vitamins

Fat Helps Insulate and Protect
 Fat in subcutaneous tissue
• Insulates body
• Maintains body temperature
 Fat protects bones and vital organs from trauma
• Too much fat eliminates the protective benefit

Essential Fatty Acids, Eicosanoids, and
Cell Membrane
 Linoleic acid can be elongated and converted to
arachidonic acid
 Alpha-linolenic acid
• Converts to eicosapentaenoic acid (EPA)
- EPA elongates to docosahexaenoic acid (DHA)
• Needed for healthy cell membranes

Essential Fatty Acids, Eicosanoids, and
Cell Membrane
 EPA and arachidonic acid are used to manufacture
eicosanoids
 Eicosanoids are hormonelike substances
• Prostaglandins, thromboxanes, and leukotrienes
• Regulate the immune system, blood clotting,
inflammation, and blood pressure

Cholesterol Is Used to Make Bile,
Hormones, and Vitamin D
 Cholesterol
• A structural part of the cell membrane
• A precursor to vitamin D
• A precursor to bile acid
• Precursor for sex hormones such as estrogen and
testosterone

 Fat
• An energy-dense source of fuel
• Cushions and protects bones, organs, and nerves
• Helps maintain body temperature
• Provides essential fatty acids
• Is needed for the absorption of fat-soluble vitamins and
carotenoids
 Essential fatty acids are precursors to EPA and DHA which
manufacture
• Prostaglandins
• Thromboxanes
• Leukotrienes
• Regulate the immune system
• Regulate blood clotting
• Regulate inflammation
• Regulate blood pressure

 Essential functions of cholesterol
• Is part of cell membrane
• Is needed to make
- Vitamin D
- Bile acid
- Sex hormones

How Much Fat Do We Need Each Day?
 Overall intake of fat in the U.S. is higher than it should be
 Acceptable Macronutrient Distribution (AMDR)
recommendation
• 20–35% of daily kilocalories should come from fat
 For heart health
• Consume no more than 10% of total kilocalories from
saturated fat
• Limit trans fats to < 1%

Essential Fatty Acid Recommendations
 Adequate Intake (AI) for the essential fatty acids
• Alpha-linolenic acid
- Men 1.6 grams/d
- Women 1.1 grams/d
• Linoleic acid
- Men 17 grams/d
- Women 12 grams/d
*Currently Americans only consume about 0.1–0.2 grams/d of EPA and DHA

Essential Fatty Acid Recommendations
 AMDR for the essential fatty acids
• 0.6–1.2% of total kilocalories should be alpha-linolenic
acid
• 5–10% of total kilocalories should be linoleic acid
 American Heart Association recommendations
• People diagnosed with heart disease should consume ~1
gram of essential fatty acids per day

Facts About Fats, Oils, and Cholesterol
 Intake of trans fats and saturated fats correlate with
increased risk of
• Cardiovascular disease
• Stroke
• Cancer
 Substituting or replacing trans fats and saturated fats with
MUFA and PUFA can lower risk

Dietary Cholesterol Is Not Essential
 The liver synthesizes cholesterol needed by the body
• Liver synthesizes ~900 mg/d
• Liver decrease synthesis based on dietary intake
 To reduce the risk of cardiovascular disease, dietary
cholesterol should be limited to < 300 mg/d

 Dietary lipids are an important part of a healthy diet,
especially essential fatty acids
 Saturated fat, trans fats, and cholesterol intake should be
limited
 Dietary fat intake per day
• Should range from 20 to 35% of total kilocalories
• 5–10% of kilocalories from linoleic acid
• 0.6–1.2% of kilocalories from alpha linolenic acid
• Limit saturated fat intake to < 10% of total fat intake
• Limit trans fat intake to < 1% of total fat intake
• Limit cholesterol intake to < 300 mg

Best Food Sources of Fat
Sources of unsaturated fats 
Figure 5.20

Food Sources of Omega-3 Fatty Acids
Figure 5.21

 Lean meat and poultry, fish, low-fat or nonfat dairy
products and limited amounts of nuts and cheese
• Best sources of essential fatty acids
• Limit intake of saturated and trans fats
 Commercially prepared baked goods and snack items
• Are high in kilocalories
• Are high in saturated and trans fats
• Should be consumed rarely
 Use vegetable oils in the place of butter

Foods Containing Trans Fat and
Cholesterol?
 Hydrogenated fats are used by many commercial food
producers to
• Provide rich texture
• Increase shelf live
• Decrease incidence of rancidity
 During the hydrogenation process trans fats form
 Some trans fats are naturally occurring

Major Food Sources of Trans Fat for
American Adults
Figure 5.23

Trans Fats
 Trans fat may actually be worse for heart health than
saturated fats
• Raise LDL cholesterol
• Lower HDL cholesterol
 FDA requires trans fat to be listed on food labels
 The food industry is working to find replacements for trans
fats in foods

Food Sources of Cholesterol and Plant Sterols
 Cholesterol comes mainly from animal products
 The cholesterol produced in plant cell walls and oils is so
minimal they are considered cholesterol free
 Phytosterols and stanols
• Lower LDL levels by competing with cholesterol for
absorption
• Are found in soybean oil, many fruits, vegetables,
legumes, sesame seeds, nuts, cereals, and other plant
foods
• Food manufacturers are fortifying foods with them to
help lower cholesterol

 Trans fats are made by heating oil and adding hydrogen gas
to saturate the carbons of the fatty acids
 Trans fats raise LDL cholesterol and lower HDL
cholesterol
 Trans fats are found in many commercially prepared foods
and must be listed on the food label
 Other oils are being tested to replace trans fats in foods
 Cholesterol is found mostly in animal products
 Phytosterols are found in vegetable oils, nuts, legumes,
whole grains, fruits, and vegetables

Fat Substitutes
 Designed to provide the creamy properties of fat for fewer
kilocalories
 Fall into three categories
• Carbohydrate-based: Majority of fat substitutes
• Protein-based: Provide a creamy texture in the mouth
• Fat-based: Give physical attributes of fat for fewer
kilocalories
 Overconsumption of kilocalories from regular, low-fat, or
fat-free products can lead to weight gain

Lipids metabolism introduction

 Fat substitutes provide the properties of fat for fewer
kilocalories and grams of fat
 Fat substitutes can be carbohydrate based, protein based, or
fat based
 Some substitutes such as olestra work by passing
unabsorbed through the GI tract
 Reduced-fat and fat-free foods still contain kilocalories and
should be eaten in limited amounts

Accumulate
in the
arteries
Fat and
Cholesterol
Blood flow
is impeded
Arteries
narrow
• Not
enough
oxygen to
the heart
• CHEST
PAIN
Decrease
oxygen and
nutrients
What Is Heart Disease and What Factors
Increase Risk?
 Cardiovascular disease is the number-one killer of adults (in
the United States)

Emerging risk factors
• C-reactive protein (CRP)
• Homocysteine
• Chlamydia pneumoniae
• Lp(a) protein
• Metabolic syndrome or
Syndrome X

 Heart disease is the leading cause of death in the United
States
 It develops when atherosclerosis causes narrowing of the
coronary arteries and decreased flow of oxygen and
nutrients to the heart
 Elevated LDLs are a major risk factor
 Uncontrollable risk factors include age, gender, family
history, and type 1 diabetes
 Controllable risk factors include type 2 diabetes, high blood
pressure, smoking, physical inactivity, excess weight, low
HDLs, elevated LDLs
 HDLs can be raised by losing weight, regular exercise, and
quitting smoking
 Syndrome X is a group of risk factors

Lowering Blood Cholesterol Levels
 Consume less saturated and trans fats
 Consume less cholesterol
 Make smart, informed food choices
• Avoid or consume processed food in moderation
• Eat at least two servings of fish per week
• Consume more plant foods
• Consume antioxidants and phytochemicals
• Garlic may help lower cholesterol
• Flavonoids may help prevent LDLs from oxidizing
 Exercise
 Alcohol, in moderate amounts, can reduce the risk of heart
disease

Putting It All Together
 For optimal long-term health a diet should include a proper
balance of all nutrients including fat
 There are different types of lipids, some essential and
others not required from foods
 Goal
• Consume mostly unsaturated fats
• Limit amount of saturated and trans fats
 A plant-based diet plentiful in whole grains, fruits, and
vegetables, with some low-fat dairy and lean meats,
poultry, fish, and vegetable oil will be high in fiber and
lower in saturated fats, trans fats, and dietary cholesterol

Lipids metabolism introduction

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Lipids metabolism introduction