Transcript Biochem14_Lipids

Chapter
5
Lipids
Photo © PhotoDisc
Lipids
• Lipids are organic molecules that dissolve
easily in organic solvents such as alcohol,
ether, or acetone.
• Lipids are much less soluble in water.
Lipids
• Lipids are generally hydrophobic and
lipophillic.
• Lipid vary greatly in their level of solubility.
Triglycerides
• Triglycerides are the largest category of lipids.
• Fat cells store triglycerides in adipose tissue.
Triglycerides
• In foods, we call triglycerides fats and oils.
• Fats are usually solid and oils liquid at room
temperature.
Fatty Acids
• Fatty acids are composed of both triglycerides
and phospholipids.
• They are often attached to cholesterol.
Fatty Acids
• Fatty acids determine essential characteristics
of a fat such as whether it is liquid or solid at
room temperature.
• Fatty acids that are not attached to other
compounds are called “free” fatty acids.
• Some free fatty acids have distinct flavors.
Fatty Acids Are Key
Building Blocks
• Chain Length
• 4–24 carbons
Chain Length
• Foods contain fatty acids with chain length
between 4 and 24 carbons.
• The shorter the chain, the more liquid the fatty
acid.
• Shorter fatty acids are more water soluble.
Fatty Acids Are Key
Building Blocks
• Degree of saturation
• Saturated
• All single bonds between carbons
• Monounsaturated
• One carbon-carbon double bond
• Polyunsaturated
• More than one carbon-carbon double bond
Degree of Saturation
• Foods with more unsaturated fatty acids
typically have lower melting points are tend to
be liquid at room temperature.
• Foods with more saturated fatty acids typically
have higher melting points and tend to be
solid at room temperature.
Liquid or Solid?
Fatty Acids Are Key
Building Blocks
• Types of fatty acids
• Cis and trans
• Hydrogenation produces trans fatty acids
• Trans fatty acids raise LDLs
• Essential fatty acids
• Linoleic acid and alpha-linolenic acid
• Can’t be made in the body
• Used to make eicosanoids
Phospholipids
• About 2 percent of dietary lipids are
phospholipids.
• The human body can make those it needs.
Phospholipids
• Phospholipids are unique in that they are
soluble in both fat and water.
• They are found in cell membranes.
• They are found in blood and other body fluids
where they keep fats suspended in the watery
environment.
Phospholipids
• Structure
• Glycerol + 2 fatty acids + phosphate
group
• Functions
• Component of cell membranes
• Lipid transport as part of lipoproteins
• Emulsifiers
• Food sources
• Egg yolks, liver, soybeans, peanuts
Sterols
• A small percentage of dietary lipids are sterols.
• Cholesterol is a primary example.
• Cholesterol is an important component of cell
membranes and a precursor to sex hormones,
adrenal hormones, vitamin D, and bile salts.
Sterols: Cholesterol
• Functions
• Component of cell membranes
• Precursor to other substances
• Sterol hormones
• Vitamin D
• Bile acids
• Synthesis
• Made in the liver
• Food sources
• Found only in animal foods
Nonessential and
Essential Fatty Acids
• The body can make saturated and Omega-9
Fatty Acids, so it is not essential to get them
from the diet.
• The body cannot manufacture omega-6
linoleic and omega-3 alpha-linolenic acids.
They must come from food and are called
essential.
Triglycerides
• Triglycerides are the major lipids in both the
diet and in the body.
• They add flavor and texture to foods.
• They are an important source of energy for the
body.
Triglycerides
• Structure
• Glycerol + 3 fatty acids
• Functions
•
•
•
•
Energy source and reserve
Insulation and protection
Carrier of fat-soluble vitamins
Sensory qualities in food
Triglycerides in Food
• Sources of omega-3 fatty acids
• Soybean, canola, walnut, flaxseed oils
• Salmon, tuna, mackerel
• Sources of omega-6 fatty acids
• Vegetable oils
• Nuts and seeds
Photos © Photodisc
Energy Reserve
• We store excess dietary fat as body fat.
• Fat is stored inside fat cells called adipocytes.
• Adipocytes form body fat tissue called adipose
tissue.
Fat Storage
Digestion and Absorption
• Mouth and stomach
• Minimal digestion of triglycerides
• Small intestine
• Emulsified by phospholipids
• Digested by pancreatic lipase
• Absorbed into intestinal cells
• Formed into chylomicrons and moved into
lymphatic system
Fat Digestion
Lipids in the Body
• Lipoproteins carry lipids around the body
• Chylomicrons
• Delivers dietary lipids from
intestines to cells and liver
Lipids in the Body
• Very low density lipoproteins (VLDL)
• Deliver triglycerides to cells
• Low-density lipoproteins (LDL)
• Deliver cholesterol to cells
• High-density lipoproteins (HDL)
• Pick up cholesterol for removal or recycling
Lipids in the Diet
• Recommended intake
• Reduce saturated and trans fat intake
• Total fat: 20–35% of calories
• Need ~ 2% of calories as essential fatty
acids
• Improve balance of
omega-3: omega-6 fatty
acids
Photos © Photodisc
Lipids in the Diet
• Fat substitutes
• Different types of
composition
• Olestra
• Sucrose + fatty acids
• Indigestible – provides
zero kcals
• Reduces absorption of
fat-soluble vitamins
Lipids and Health
• Obesity
• High-fat diets promote weight gain
• Heart disease
• High saturated and trans fat intake raises LDL
cholesterol
• Cancer
AHA Position on Omega-3
Fatty Acids
• The American Heart
Association
recommends eating
fish (particularly fatty
fish) at least two times
a week. Fish is a good
source of protein and
doesn’t have high
saturated fat that fatty
meat products do.
• Fatty fish such as
mackerel, lake trout,
herring, sardines,
albacore tuna, and
salmon are high in two
kinds of of omega-3
fatty acids:
eicosapentaenoic acid
(EPA) and
docosahexaenoic acid
(DHA).
Function of Triglycerides
• Fat performs a
number of essential
functions in the body.
• Fat is also a major
energy source. When
at rest, muscles
prefer to use fat for
fuel.
A Healthy Balance
• An appropriate balance of calories from fat
and carbohydrates can have multiple health
benefits aside from controlling weight.
• These include cholesterol levels, vitamin
intake, etc.
Fat = Storage
• Evolution has selected fat, rather than glycogen,
as its primary energy storage medium.
• A gram of fat stores more than six times as much
energy as a gram of glycogen.
Micronutrient Carrier
• Fat is a micronutrient
carrier.
• Fat holds more than
just energy.
• It also carries
important nutrients,
such as fat-soluble
vitamins and
carotenoids.
Recommended Fat Intake
• The Dietary Guidelines for
Americans recommends a
fat intake of 20 to 35
percent of total calories.
• Saturated fat should
supply no more than 10
percent of our total
calories, or about onethird of our fat calories.
ADA Position on Fat
Replacers
• It is the position of the American Dietetic Association
that the majority of fat replacers, when used in
moderation by adults, can be safe and useful adjuncts
to lowering the fat content of foods and may play a
role in decreasing total dietary energy and fat intake.
• Moderate use of low-calorie, reduced-fat foods,
combined with low total energy intake, could
potentially promote dietary intake consistent with the
objectives of Healthy People 2010 and the 2005 Dietary
Guidelines for Americans.
The Women’s Health
Initiative
• This study sought to
explore whether a
dietary intervention
that reduces intake of
total fat and increases
intakes of fruits,
vegetables, and grains
will reduce the risk of
cardiovascular disease.
• The results found that
the intervention had no
effect on the risk of
coronary heart disease,
stroke, or overall
cardiovascular disease.
• Since this study was
initiated, scientific
thinking has evolved to
focus on modifying
intakes of specific
types of fat.
NCEP Tips for Healthful
Eating Out
• Choose restaurants that have low-fat, lowcholesterol menu items.
• Don’t be afraid to ask for foods that follow
your eating pattern.
• Select poultry, fish, or meat that is broiled,
grilled, baked, steamed, or poached rather
than fried.
Diet and Cancer
• The evidence linking dietary fat to cancer is
inconclusive. The case looks strong when we
compare cancer rates between countries:
Overall cancer rates are generally higher in
countries with high fat intake, and lower in
countries where people eat less fat. But in
population studies within those countries, the
evidence linking fat to cancer is weaker.