Chapter 5 – The Lipids: Fats, Oils, Phospholipids, and Sterols
Introducing the Lipids
Lipid – a family of organic compounds soluble in organic solvents but not in water
95% are triglycerides – made up of three units of fatty acids and one unit of glycerol
other classes are phospholipids (lecithin) and sterols (cholesterol).
Phospholipids are similar to triglycerides but with a phosphorus-containing acid instead of one of
the fatty acids
The Usefulness of Fats
In the body:
Energy stores – fats are the body’s chief form of stored energy
Muscle fuel – fats provide most of the energy to fuel muscular work
Padding – fats protect the internal organs from shock through fat pads inside the body cavity
Insulation – fats insulate against temperature extremes through a fat layer under the skin
Cell membranes – fats form the major material of cell membranes
Raw materials – fats are converted to other compounds, such as hormones, bile, and vitamin D
In foods:
Nutrient – fats provide essential fatty acids
Energy – fats provide a concentrated energy source in foods
Transport – fats carry fat-soluble vitamins A, D, E, and K along with some phytochemicals, and
assist in their absorption
Raw materials – fats provide raw material for making needed products
Sensory appeal – fats contribute to the taste and smell of foods
Appetite – fats stimulate the appetite
Satiety – fats contribute to feelings of fullness
Texture – fats help make food tender
-most body cells can store only limited amounts of fat but some cells are specialized for fat
storage. These fat cells seem able to expand almost indefinitely – the more fat they store, the
larger they grow.
-andipose (fat) tissue secretes hormones that help regulate the appetite and influence other body
functions
-fats pack tightly together without water and can store twice as much energy as carbohydrate
glucose in a small space
A Close Look At Lipids
Triglycerides: Fatty Acids and Glycerol
Fatty acids-organic acids composed of carbon chains of various lengths.
-most fatty acids are found in triglycerides
-fatty acids can differ in chain length or in degree of saturation
depending on which fatty acids are in a triglyceride, a fat will be hard or soft
-shorter chain fatty acids, more unsaturated, are softer and melt more at room temperature
Saturated versus Unsaturated Fats
-saturation refers to whether or not a fatty acid chain is holding all of the hydrogen atoms possible
(having no points of unsaturation)
one point of unsaturation = monounsaturated fatty acid
two or more points = polyunsaturated fatty acid
-the more unsaturated the fatty acid, the more liquid it is at room temperature
Phospholipids
-consists of a molecule of glycerol with two, rather than three, fatty acids attached. In place of
the third is a molecule containing phosphorus
-is soluble in water
-can serve as an emulsifier – a substance that mixes with both fat and water and
permanently disperses the fat in the water
-lecithin and other phospholipids play key roles in the structure of cell membranes by acting as
emulsifiers and helping fats travel back and forth across the lipid-containing membranes of cells
into the watery fluids on both sides
-has health-promoting properties, ability to lower blood cholesterol
Sterols
-large complicated molecules consisting of interconnected rings of carbon atoms with side chains
of carbon, hydrogen, and oxygen
-cholesterol serves as the raw material for making bile (emulsifier) which is important to
digestion
-other sterols include vitamin D, which is made from cholesterol and steroid hormones
--cholesterol forms major part of plaques that narrow the arteries in artherosclerosis, underlying
cause of heart attacks and strokes
Lipids in the Body
Digestion and Absorption of Fats:
-enzyme produced by the tongue plays major role in digesting milk fat in infants
-in stomach fat separates from the watery components and floats as a layer on the top
-little fat digestion takes place in the stomach
-one end of bile molecules attract and hold fat while other is attracted to and holds water.
-by the time fat enters small intestine, gallbladder (stores liver’s output of bile) has contracted and
squirted bile into intestine. Bile mixes fat particles with watery fluid by emulsifying them,
suspending them in fluid until fat-digesting enzymes contributing by the pancreas can split them
into smaller particles for absorption
-when intestinal contents are emulsified, fat-splitting enzymes act on triglycerides to split fatty
acids from their glycerol backgrounds.
-fats then must travel through mucus coating of digestive tract lining to absorptive cells of
intestinal villi. Cells extract the lipids.
-very little fat is excreted by healthy system
-person without a gallbladder will still produce bile from the liver but delivers it continuously to
the small intestine. Initially they must reduce their fat intakes because they can no longer store
bile and release it at mealtimes. Their systems can handle only a small amount of fat at a time
Transport of Fats:
-smaller parts of lipid digestion pass directly into the bloodstream to the liver
-glycerol and shorter-chain fatty acids
-larger lipids must form lipoproteins before they can be released into the lymph
lipoproteins – clusters of lipids associated with protein, which serve as transport vehicles for
lipids in blood and lymph. Major classes are the chylomicrons, the VLDL, the LDL, and the
HDL
-inside the intestinal cells they are re-formed into triglycerides and clustered together with
proteins and phospholipids to form chylomicrons
chylomicrons – clusters formed when lipids from a meal are combined with carrier proteins in the
cells of the intestinal lining. Transport food fats through the watery body fluids to the liver and
other tissues
Key point – small lipids travel in the bloodstream unassisted. Large Lipids are incorporated into
chylomicrons for transport in the lymph and blood. Blood and other body fluids are watery, so
fats need special transport vehicles – the lipoproteins – to carry them in these fluids.
How can I use my stored fat for energy?
-when a body starts to run out of available fuel from food it begins to use stored fat to use for
energy. Fat cells respond by dismantling stored fat molecules and releasing fat components into
the blood. Energy-hungry cells break these components down into smaller fragments. Each
fragment is combined with a fragment derived from glucose and the energy-releasing process
continues, liberating energy, carbon dioxide, and water.
-carbohydrate must be available when fat is broken down. Without it, ketones will build up in the
blood and urine
Dietary Fat, Cholesterol, and Health
High intakes of certain dietary fats are associated with serious diseases
-heart and artery disease is number-one killer of adults in US
-suggested that high-fat diet may incur greater-than-average risk of some cancers
-obesity
Major Lipoproteins:
Very-low-density lipoproteins (VLDL) – carry triglycerides and other lipids made in the liver to
the body cells for their use
Low-density lipoproteins (LDL) – transport cholesterol and other lipids to the tissues. LDL are
made from VLDL after they have donated may of their triglycerides to body cells
High-density lipoproteins (HDL) – lipoproteins that return cholesterol from the tissues to the liver
for dismantling and disposal; contain a large proportion of protein. Critical in the process of
carrying cholesterol away from body cells to the liver for disposal
HDL and LDL play major roles with regard to heart health and are the focus of most
recommendations made for reducing the risk of heart disease
-elevated LDL concentrations in the blood are a sign of high risk of heart attack
-elevated HDL concentrations are associated with a low risk
difference in health lies in the proportions of lipids they contain and tasks they perform
What does food cholesterol have to do with blood cholesterol?
Most saturated fats raise blood cholesterol more than food cholesterol does
-main dietary factors associated with elevated blood cholesterol are high saturated fat and trans
fat intakes
-dietary cholesterol makes smaller but still significant contribution to elevated blood cholesterol
Lowering LDL Cholesterol
-trim the fat, especially saturated and trans fat, from foods
-eliminate as much saturated and trans fats as possible
Essential Polyunsaturated Fatty Acids
Body can use carbohydrate, fat, or protein to synthesize nearly all fatty acids.
-exceptions are linoleic acid and linolenic acid. These must be supplied by diet
-serve as raw materials from which the body makes eicosanoids – biologically active compounds
that regulate body functions, affecting a wide range of diverse body functions: muscle relaxation
and contraction, blood vessel dilation and blood vessel constriction, blood clot formation, blood
lipid regulation, and immune response to injury and infection including fever, inflammation and
pain.
-aspirin relieves fever, inflammation and pain by slowing synthesis of eicosanoids
Deficiencies of Essential Fatty Acids:
-When diet is deficient in all of the polyunsaturated fatty acids symptoms of reproductive failure,
skin abnormalities, and kidney and liver disorders appear
-in infants, growth is retarded and vision is impaired
-body stores some essential fatty acids so extreme deficiency disorders are seldom seen except
when intentionally induced in research or on rare occasions when inadequate diets have been
provided to infants or hospital patients by mistake
hydrogenation – process of adding hydrogen to unsaturated fatty acids to make fat more solid and
resistant to the chemical change of oxidation
-when unsaturated points in oils of food are oxidized the oils become rancid and food tastes “off”
-making fats harder and more stable when heated at high temperatures prevents spoilage
-resulting product is more saturated as they accept hydrogen and the oil hardens
-more spread able, more resistant to damage from oxidation or breakdown from high
temperatures.
Hydrogenated oil has a high smoking point so it is suitable for frying – temperature at which fat
gives off an acrid blue gas
Key point – vegetable oils become more saturated when they are hydrogenated. Hydrogenated
fats resist rancidity better, are firmer textured, and have a higher smoking point than saturated
oils, but they also lose the health benefits of unsaturated oils
Trans fatty acids – not made by the body by occur naturally in tiny amounts mainly in dairy foods
and beef, fatty acids with unusual shapes that can arise when polyunsaturated oils are
hydrogenated
-consuming fatty acids raise LDL and lower HDL by producing inflammation
DRI committee concludes people should consume as little trans fat as possible
Key point – The process of hydrogenation also creates trans fatty acids. Trans fats act somewhat
like saturated fats in the body