Chapter 5 NUTR

Chapter 5

The Lipids: Triglycerides,
Phospholipids, and Sterols

© 2009 Cengage - Wadsworth

The Chemist’s View of Fatty
Acids and Triglycerides
• The class of nutrients known as lipids includes
triglycerides (fats and oils), phospholipids, and
sterols. Most are triglycerides with glycerol
backbones and three fatty acids attached.
• Fatty acids vary in carbon chain lengths,
degree of unsaturation, and number of double
bonds.
• Saturation affects the physical characteristics
of the fat and its storage properties.
• Trans-fatty acids, which are altered, have the
same negative health effects as saturated
fatty acids.


• Fatty Acids
The Length of the Carbon Chain
• Long-chain fatty acids are found primarily
in meat, fish, and vegetable oils.
• Medium- and short-chain fatty acids are
found in dairy products.
The Degree of Unsaturation
• Saturated fatty acids carry the maximum
possible number of hydrogen atoms.
• When most of the fatty acids are
saturated it is called a saturated fat.


• Fatty Acids
 Unsaturated fatty acids lack hydrogen atoms and have at
least one double bond.
 The double bond is considered the point of unsaturation.
 Monounsaturated fatty acids lack two hydrogen atoms and
have one double bond.
 When most of the fatty acids are monounsaturated it is
called a monounsaturated fat.
 Polyunsaturated fatty acids (PUFA) lack four or more
hydrogen atoms and have at least two or more double
bonds.
 When most of the fatty acids are polyunsaturated it is
called a polyunsaturated fat.
• Linoleic acid is an essential fatty acid.
• Linolenic acid is an essential fatty acid.


Stearic acid, an 18-carbon saturated fatty acid

Oleic acid, an 18-carbon monounsaturated fatty acid

Linoleic acid, an 18-carbon polyunsaturated fatty acid

• Fatty Acids
The Location of Double Bonds
• The omega number refers to the position
of the first double bond.
• An omega-3 fatty acid has the location of
the double bond in the third position. An
example is linolenic acid.
• An omega-6 fatty acid has the location of
the double bond in the sixth position. An
example is linoleic acid.


• Triglycerides are lipids with three
fatty acids attached to a glycerol.


• Degree of Unsaturation Revisited
Firmness
• Saturated fats are solid at room
temperature.
• Polyunsaturated fats are liquid at room
temperature.
• Shorter fatty acid chains are softer at
room temperature than longer chains.


Stability
• Saturated fat is more resistant to
oxidation.
• Monounsaturated fat is slightly less
susceptible to spoilage.
• Polyunsaturated fat spoils most readily.
• Protection from rancidity
– Sealed in airtight containers away from light
– Add antioxidants
– Hydrogenation


 Hydrogenation
• Protects against oxidation therefore prolonging
shelf life
• Alters texture
 Trans-Fatty Acids
• Changed from cis to trans configuration and act
like saturated fats in the body
• Heart disease connection is being researched.
• Conjugated linoleic acid is a naturally occurring
trans fat that may be beneficial to health.


The Chemist’s View of
Phospholipids and Sterols
• Phospholipids and sterols have
unique chemical structures that allow
them to have unique roles in the
body.
• Sterols have a multiple-ring
structure.


• Phospholipids
Phospholipids in Foods
• Phospholipids contain glycerol, two fatty
acids and a phosphate group with a
molecule of choline.
• Used as emulsifiers in food industry
• A well-known phospholipid is lecithin.
• Food sources of lecithin include eggs,
liver, soybeans, wheat germ and
peanuts.


• Phospholipids
Roles of Phospholipids
• Enable transport of lipids across cell
membranes
• Emulsifiers
• Lechithin is made by the liver.
Supplements increase energy intake and
can cause GI symptoms.


• Sterols – A well-known sterol is
cholesterol.
Sterols in Foods
• Found in plant and animal foods
• Cholesterol is found in animal foods only
—meat, eggs, fish, poultry and dairy
products (exogenous).


• Sterols
Roles of Sterols
• Starting material for bile acids, sex
hormones, adrenal hormones and vitamin
D
• Structural component of cell membranes
• Liver produces 800-1500 mg cholesterol
per day (endogenous).
• Atherosclerosis is a disease that causes
heart attacks. It occurs when cholesterol
forms deposits in the artery wall.


Digestion, Absorption, and
Transport of Lipids
• Special arrangements are made in the
digestion of lipids.
• This is due to the hydrophobic nature of
lipids.
• Lipids tend to separate from the watery
fluids of digestion.
• Digestive enzymes are hydrophilic, or
water loving.
• Bile from the liver emulsifies lipids.
• Enzymes are then able to break down
lipids to monoglycerides and fatty acids.


Transport of Lipids
• Lipid Digestion
In the mouth, the salivary glands
release lingual lipase.
Muscle contractions in the stomach
disperse fat into smaller droplets. Fat
is exposed to gastric lipase enzyme.


Transport of Lipids
• Lipid Digestion
In the Small Intestine
• Cholecystokinin (CCK) signals the
gallbladder to release bile.
• Pancreatic and intestinal enzymes
hydrolyze lipids to monoglycerides and
fatty acids.
• Phospholipids are hydrolyzed.
• Sterols are absorbed as is.


Transport of Lipids
 Bile’s Routes
• Enterohepatic circulation – reabsorbed and
recycled
• Soluble fibers are effective in trapping some bile
and excreting it from the body through the large
intestine.


Transport of Lipids
• Lipid Absorption
 Glycerol and short- and medium-chain fatty
acids diffuse and are absorbed directly into
the bloodstream.
 Monoglycerides and long-chain fatty acids
form micelles, are absorbed, and are
reformed into new triglycerides.
 With protein they are transported by
chylomicrons.


Transport of Lipids
• Lipid transport is made possible by a
group of vehicles known as
lipoproteins.
Chylomicrons
• Largest of the lipoproteins
• Least dense
• Get smaller as triglyceride portion is
removed by the cells


Transport of Lipids
• Lipid Transport
VLDL (Very-Low-Density Lipoproteins)
• Composed primarily of triglycerides
• Made by the liver
• Transport lipids to the tissues
• Get smaller and more dense as
triglyceride portion is removed


Transport of Lipids
• Lipid Transport
LDL (Low-Density Lipoproteins)
• Composed primarily of cholesterol
• Transport lipids to the tissues
HDL (High-Density Lipoproteins)
• Composed primarily of protein
• Transport cholesterol from the cells to
the liver


Transport of Lipids
• Lipid Transport
Health Implications
• High LDL is associated with higher risk of
heart attack and is known as “bad”
cholesterol.
• High HDL seems to have a protective
effect and is known as “good” cholesterol.


Transport of Lipids
• Lipid Transport
Health Implications
• Factors that lower LDL and raise HDL
– Weight control
– Replace saturated fat with monounsaturated
fat and polyunsaturated fat in the diet
– Soluble fibers
– Phytochemicals
– Moderate alcohol consumption
– Physical activity
• Genes influence lipoprotein activity.


Lipids in the Body
• The triglycerides have important roles in the
body.
• Essential fatty acids also play important roles.
• The body can store unlimited amounts of fat
when fat is consumed in excess.
• The liver can also convert excess carbohydrate
and protein to fat.
• Fat needs carbohydrate to break down
efficiently.
• Inefficient breakdown of fat forms ketone
bodies.


Lipids in the Body

• Roles of Triglycerides
Energy reserve when stored
Use carbohydrate and protein
efficiently
Insulation
Shock protection


Lipids in the Body
• Essential Fatty Acids – cannot be made by
the body.
 Linoleic Acid and the Omega-6 Family
• Can make arachidonic acid, which is a
conditionally essential fatty acid
• Supplied by vegetable oils and meats
 Linolenic Acid and the Omega-3 Family
• Must be supplied by food
• Can make EPA (eicosapentaenoic acid) and DHA
(docosahexaenoic acid), important for eyes, brain
and heart


Lipids in the Body
• Essential Fatty Acids
 Eicosanoids
• Made from arachidonic acid and EPA
• Like hormones but have different effects on
different cells
• Include prostaglandins, thromboxanes, and
leukotrienes
 Fatty Acid Deficiencies
• Are rare in U.S. and Canada
• Occur in infants and children with fat-free or low-
fat diets


Lipids in the Body

• A Preview of Lipid Metabolism
Storing Fat as Fat
• Provides twice the energy as
carbohydrate and protein
• Adipose issue readily stores fat.
• Lipoprotein lipase (LPL) hydrolyzes
triglycerides as they pass and directs the
parts into the cells to be used for energy
or storage.


Lipids in the Body
• A Preview of Lipid Metabolism
Using Fat for Energy
• Hormone-sensitive lipase inside the
adipose cells hydrolyzes triglycerides
when needed for energy.
• Fasting metabolizes fat, but fat requires
carbohydrate and protein for complete
breakdown.
• Ketone bodies can be made from fat
fragments.


Health Effects and
Recommended Intakes of Lipids
• High intakes of saturated fat and
trans fat and high blood LDL
cholesterol are related to increased
risk for heart disease.
• Omega-3 fatty acids in the diet
appear to have a protective effect.


Health Effects and

• Health Effects of Lipids
Blood lipid profile
• Reveals concentrations of lipids in the
blood
• Desirable levels
Total cholesterol < 200 mg/dL
LDL cholesterol < 100 mg/dL
HDL cholesterol ≥ 60 mg/dL
Triglycerides < 150 mg/dL


Health Effects and
 Heart Disease
• Elevated blood cholesterol is a risk factor for
cardiovascular disease.
• Cholesterol accumulates in the arteries, restricts
blood flow and raises blood pressure.
• Saturated fat in the diet raises blood cholesterol.
 Risks from Saturated Fats
• Saturated fat in the diet raises LDL cholesterol,
which increases risk of heart disease.
• Food sources include whole milk, cream, butter,
cheese, high-fat cuts of beef and pork, and
coconut, palm and palm kernel oils.


Health Effects and
 Risks from Trans Fats
• Trans-fatty acids in the diet increase LDL
cholesterol and decrease HDL cholesterol.
• Food sources include deep-fried foods using
vegetable shortening, cakes, cookies, doughnuts,
pastry, crackers, snack chips, margarine,
imitation cheese, and meat and dairy products.
• Debate over butter versus margarine
 Risks from Cholesterol
• Dietary cholesterol has less effect on blood
cholesterol than saturated fat and trans fat.
• Food sources of cholesterol include egg yolks,
milk products, meat, poultry and shellfish.


Health Effects and
 Benefits from Monounsaturated Fats and
Polyunsaturated Fats
• Replacing saturated fat and trans fat with
monounsaturated fat and polyunsaturated fat is
the most effective dietary strategy in preventing
heart disease.
• Food sources of monounsaturated fat include
olive, canola and peanut oil and avocados.
• Food sources of polyunsaturated fat include
vegetable oils (safflower, sesame, soy, corn and
sunflower), nuts and seeds.


Health Effects and
Benefits from Omega-3 Fats
• Beneficial effects in reducing risk of heart
disease and stroke
• Food sources include vegetable oils
(canola, soybean and flaxseed), walnuts
and flaxseeds, and fatty fish (mackerel,
salmon, and sardines).
– Need to avoid fish with high levels of mercury


Health Effects and
Balance Omega-6 and Omega-3
Intakes
• Eat more fish (2 3-oz. portions per week)
and less meat.
• Bake, broil or grill the fish.
• Functional foods are being developed.
• Supplements are not the answer.


Health Effects and
 Cancer
• Dietary fat has an association with risks for some
types of cancer, but it is not as strong as the link
to heart disease.
• Fat does not initiate cancer development but may
be a promoter once cancer has developed.
• Some types of cancer have a stronger
relationship to fat intake. Saturated fat from meat
is implicated.
 Obesity can be a consequence of high-fat,
high-kcalorie diets in excess of energy
needs.


Health Effects and
• Recommended Intakes of Fat
 The DRI and the 2005 Dietary Guidelines
recommend fat at 20-35% of energy intake
(400-700 kcalories of a 2,000-kcalorie diet).
 FDA recommends 10% of energy intake
from saturated, 30% of energy intake total
fat
 2005 Dietary Guidelines suggest choosing a
diet low in saturated fat and cholesterol and
moderate in total fat.


Health Effects and
• Recommended Intakes of Fat
Daily Values
• 65 g fat based on 30% of 2000-kcal diet
• 20 g saturated fat based on 10% of
2000-kcal diet
• 300 mg cholesterol
USDA Food Guide considers saturated
fats discretionary kcalories.
Too little fat can be detrimental to
health.


Health Effects and
• Guidelines to Groceries
 Very lean and lean options of meats and
meat alternates should be chosen.
 Choose fat-free and low-fat milks and milk
products.
 Choose a wide variety of vegetables, fruits,
and whole grains.
 Avoid invisible fat from high-fat cheese and
baked and fried foods.
 Choose wisely from many available food
products.


Health Effects and
Fat Replacers
• Ingredients derived from carbohydrate,
protein or fat
• Replace fat in foods
• Artificial fats offer sensory and cooking
qualities but no kcalories.
• Research on olestra supports its safety
but it deceases the absorption of fat-
soluble vitamins and may cause digestive
distress in some consumers.


Health Effects and
Read Food Labels
• Provide information on fat grams and %
Daily Values
• % Daily Values are not the same as % of
kcalories from fat.


High-Fat Foods--
Friend or Foe?


Guidelines for Fat Intake
• General guidelines for reducing total
fat but extremely low-fat diets have
problems
• Specific guidelines for reducing
saturated fat, trans fat, and
cholesterol
• Specific guidelines for increasing
monounsaturated and
polyunsaturated fat, especially the
omega fats


High-Fat Foods and Heart
Health
• Cook with Olive Oil
 Many health benefits
 Still provides 9 kcal/gram
• Nibble on Nuts
 Many health benefits
 Still a high-kcalorie food
• Feast on Fish
 Rich in long-chain omega-3 polyunsaturated
fatty acids
 Mercury caution for pregnant and lactating
women and young children


High-Fat Foods and Heart
Disease
• Limit Fatty Meats, Milk Products, and
Tropical Oils
Read food labels to determine those
lowest in saturated fat.
Choose leaner and lower-fat animal
foods.
• Limit Hydrogenated Foods
Contain trans fats.
Watch convenience foods.


The Mediterranean Diet

• Links with good health but may not
be the only factor
• Focus on whole grains, potatoes, and
pasta, vegetables and legumes, low
fat cheeses, yogurt, fruits, some fish,
other seafood, poultry, a few eggs
and little meat


Conclusions
• Saturated fat and trans fat are poor
for heart health.
• Unsaturated fat in moderation is
good for heart health.
• Food is a mixture of good and bad
fat.
• Manufacturers are complying with
consumer requests.
• Try Mediterranean eating habits.


Chapter 5 NUTR

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Chapter 5 NUTR