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Lecture

Chapter 6    Lipids
On the syllabus this is listed for Wk 11.

 You already know quite a bit about lipids:
*  The Dietary Guidelines For Americans tell us to decrease total fat, especially saturated
    fat, trans fats, and cholesterol.
*  The Dietary Goals for Americans tell us to limit our fat intake to between 20-35%
    of our total kcals.
*  The Healthy Eating Pyramid tells us to include plant oils as part of a healthy
    diet.
*  1 gram of fat = 9 kcals
*  Fat in foods: salad oils, butter, margarine, mayonnaise contain  90%+ fat; walnuts,
    bacon, avocado, bologna contain ~80% fat; cheddar cheese, peanut butter contain
    ~75% fat; steak, hamburger, Big Mac sandwich, fried chicken, French fries contain
    ~70% fat; chocolate bar, whole milk, doughnut, ice cream contain ~60% fat . . . you
    get the picture.
*  Fats provide taste, texture, and aroma in foods.

Health and Nutrition Tip of the Day:  Americans eat approximately 100 acres of pizza each day, or about 350 slices per second.  Although pepperoni  is America's favorite topping, other popular pizza toppings like mushrooms, green peppers, and onions are lower in fat.  Keep your pizza lower in fat by asking for vegetarian toppings and low-fat cheese.

The Lipid family includes:
A.  Triglycerides
B.  Phospholipids
C.  Sterols
What do these lipids have in common?  They do not readily dissolve in water, but do dissolve in organic solvents such as chloroform, benzene, and ether.

A.  Triglycerides (TG)  are commonly known as fats and oils. TGs make-up about 95% of
     the fat in our body and in foods. TGs have a backbone of glycerol (see pg. 191), and
     3 fatty acids (FA). FAs are chains of carbons linked together and
     surrounded by hydrogens.
There are 3 types of FAs:
1.  saturated FAs: these have the maximum number of hydrogens bonded to carbons;
    there are no double bonds (see pg. 191, Fig. 6-3).
    The saturated FAs are implicated in heart disease. We find saturated FAs in animal
    products and tropical oils (See Table 6-1 on pg. 195). These are usually solid at room
    temperature.
2.  Unsaturated FAs: these are missing hydrogens and have double bonds
    a.  monounsaturated FA (MUFA): is missing 2 hydrogens and has one double bond
         Some examples of MUFAs: olive oil, canola, oil, peanut oil (See Fig.6-4 on pg.
         192). MUFAs seem to be protective against heart disease. These are usually liquid
         at room temperture.
          * Fig. 6-6 on pg. 192 show a trans fat structure.
    b.   polyunsaturated FA (PUFA): is missing more than 2 hydrogens and has 2 or more
         double bonds. (see fig. 6-5 on pg. 192)
         Omega-3 and omega-6 FAs belong to the PUFA group.
         PUFAs are neutral in term of heart disease. (see Fig. 6-9 on pg. 193). These are
         liquid at room temperture.

B.  Phospholipids : the best known is lecithin. It has a backbone of glycerol, 2 FAs, and a third site that contains phosphate. The FAs make phospholipids soluble in fats; the phosphate group in the choline allow them to dissolve in water. Phospholipids are used as emulsifiers (See pg. 200, Fig. 6-13).

C.  Sterols: have a multiple ring structure (see fig. 6-15 on pg. 202). Cholesterol is a sterol. Plants and animals have sterols; but only animal products contain cholesterol-meats, eggs, fish, poultry, dairy products.
Bile acids, the sex hormones, cortisol, vitamin D are examples of sterols.

Hydrogenation: a chemical process where hydrogens are added to MUFAs or PUFAs to reduce the number of double bonds, making the fats more saturated (solid) and more resistant to oxidation (protecting against rancidity). Hydrogenation produces trans-fatty acids. (See Fig. 6-6, pg. 192)

Trans-Fatty Acids: fatty acids with an unusual configuration around the double bond. (See Fig. 6-6, pg. 192). These have health effects similar to those of saturated FAs.

Lipid Digestion
In the small intestine, churning and bile from the gallbladder help break fats from the diet into small droplets.  This allows pancreatic lipase to access these fats for digestion.  The products of fat digestion, primarily fatty acids and monoglycerides, combine with bile to form micelles, which facilitate the absorption of these materials into the cells of the small intestine.  Medium-chain triglycerides do not require bile for digestion and absorption.  Medium-chain-length fatty acids are water-soluble and so can be absorbed and transported via the bloodstream.

See Fig. 6-17 on pg. 206 for a step by step account of TG digestion in the GI tract.
See Fig. 6-18 on pg. 207 A look at the absorption of triglycerides.

Lipid Absorption
See Fig. 6-18 on pg. 207, Absorption of Lipids.  Glycerol, short and medium chain FAs are absorbed directly into the bloodstream. Monoglycerides and long chain FAs form complexes called micelles.  These allow solubility in the watery digestive fulids. Once inside the intestinal cells they reassemble into new TGs.
TGs arrive in the small intestine only to be broken down for absorption; then once absorbed, the fragments are recombined to form TGs again...

Once re-formed, the TGs, plus cholesterol and phospholipis are bundled into transport vehicles called chylomicrons. These chylomicrons travel into the lymphatic system until they enter the bloodstream at the thoracic duct. The blood then carries these lipids to the rest of the body. 

Lipid Transport
As a group, the transport vehicles for fats are called lipoproteins. The proteins are water-soluble, so can carry fats which are fat-soluble through the bloodstream which is a watery medium. (Fats and water don't mix, right? so the lipoprotein is a package that solves that problem).
The body makes 4 kinds of lipoproteins-chylomicrons, VLDL (very-low-density lipoproteins), LDL (low-density lipoproteins), and HDL (high-density lipoproteins). Each type contains different kinds and amounts of proteins and lipids: the more lipids, the lower the density and the more proteins, the higher the density.  (See fig. 6-19, on pg. 209)

The blood cholesterol linked to heart disease is LDL cholesterol. HDL also carry cholesterol, but elevated HDL represent cholesterol returning from the rest of the body to the liver for breakdown and excretion.
High LDL cholesterol is associated with a high risk of heart attack, where high HDL cholesterol seems to have a protective effect (See Table 6-3 on pg. 208)

In body fluids, water-insoluble lipids are transported as lipoproteins.  Long-chain-length triglycerides, cholesterol, and phospholipids absorbed from the intestine are packaged with protein to form chylomicrons which enter the lymphatic system before entering the blood.  The triglycerides in chylomicrons are broken down by lipoprotein lipase, an enzyme on the surface of cells lining the blood vessels.  The fatty acids released are taken up by surrounding cells.  The chylomicron remnants that remain are returned to the liver.

Very-low-density lipoproteins (VLDLs) are lipoproteins synthesized by the liver.  With the help of lipoprotein lipase, they deliver triglycerides to body cells.  Once the triglycerides have been removed, intermediate-density lipoproteins (IDLs) are transformed into low-density lipoproteins (LDLs).  LDs deliver cholesterol to tissues by binding to LDL receptors on the cell surface.  High levels of LDL are associated with an increased risk of cardiovascular disease.  High-density lipoproteins (HDLs) are made by the liver and small intestine.  They help remove cholesterol from cells for disposal and protect against cardiovascular disease.  High blood HDL cholesterol protects against heart disease.

After eating, chylomicrons and VLDLs deliver triglycerides to cells for energy or storage.  during fasting, triglycerides stored in adipose cells are broken down by the enzyme hormone sensitive lipase and the fatty acids and glycerol are released into the blood.

Functions of Triglycerides (see pg. 198-199).

Essential FAs
There are 2 FA the body can't make: omega-6 FA (linoleic acid) and omega-3 FA (linolenic acid). They must be supplied by the diet.

We generally have no problem getting enough omega-6 FAs. Omega-3 FA we need to make an effort to get. Why?  They're necessary for normal growth and development, they play a role in the prevention and treatment of heart disease, hypertension, arthritis, and cancer.

The essential FAs are used to maintain the structural parts of cell membranes and to make substances known as eicosaniods.  Eicosanoids are biologically active compounds that regulate blood pressure, blood clotting, immune response, etc. To make eicosanioids in sufficient amounts, the body needs the essential FAs.

Health Effects and Recommended Intakes of Lipids
A high fat diet raises the risk of heart disease, some types of cancer, hypertension, diabetes, and obesity.   The same recommendation can help with all of these health problems: eat less fat.

See 'Medical Perspective' pg. 214-217.

The risk of heart disease is increased by diabetes, high blood pressure, obesity, and high blood cholesterol levels.  Risk is also affected by age, gender, genetics, and lifestyle factors such as diet.  Diets high in saturated fat, trans fatty acids, and cholesterol increase the risk of heart disease primarily by increasing blood cholesterol levels.  Diets high in energy promote obesity.  Diets high in omega-6 or omega-3 polyunsaturated fatty acids, monounsaturated fatty acids, and plant foods containing fiber, antioxidants, and phytochemicals reduce the risk of heart disease by affecting cholesterol levels and other risk factors.  Total dietary and lifestyle pattern is more important than any individual dietary factor in reducing heart disease risk.

* Blood lipid profile
* Cardiovascular disease
* Risks from saturated fats
* Risks from trans-fats
* Benefits from MUFAs
* Benefits from Omega-3 PUFAs
* Cancer
* Obesity
* Don't overdo fat restriction

The types of fats used in processing depend on the desired characteristic. Partially hydrogenated vegetable oils and tropical oils are used to improve shelf life and increase the melting point.  the trans and saturated fatty acids in these products may increase health risks.

Recommended Intakes of Fat

A minimum of 3 to 6 g of linoleic acid is recommended to prevent essential fatty acid deficiency.  To reduce chronic disease risk it is recommended that the diet contain a moderate amount of total fat (20-35% of energy) that is ~7% saturated fat, ~10%polyunsaturated fat, and ~10%monounsaturated fat; and dietary cholesterol no more than 300 mg per day.

Reducing fat intake requires decreasing intake of obvious sources of fat such as butter and oils, as well as baked goods, fast foods, and processed convenience foods that contain hidden fats.  Saturated fat and trans fat intake can be reduced by limiting animal fats, margarine, and partially hydrogenated shortenings.  To reduce health risks, the total dietary pattern, including consumption of grains, fruits, and vegetables, is as important as a moderate fat intake.

*  Reduce total fat intake
*  Reduce saturated and trans-fat intake
*  Reduce cholesterol intake
*  Balance omega-3 and omega-6 intakes
*  Select lean meats and nonfat milk
*  Eat plenty of vegetables, fruits, and grains
*  Use fats and oils sparingly
*  Look for invisible fat
*  Choose wisely
*  Read food labels

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Study Guide

The following study guide is for your use. You will not submit any part of it to the instructor.

1.  What are the members of the lipid family? Which lipids predominate in foods and in the
     body?
2. Define fatty acid. Upon what does the degree of saturation depend? Define the
    following fatty acids: saturated, monounsaturated, and polyunsaturated. What is the
    predominant fatty acid found in olive oil? What is the predominant fatty acid found in
    corn oil?
3. What is the chemical composition of a triglyceride?
4. What chemical property of fatty acid influences the firmness of fats at room
   temperature? Generally speaking, which fatty acids are more liquid at room
   temperature, and what is the source of these fatty acids? Which fatty acids are harder
   at room temperature, and what is the source of these fatty acids? Name two vegetable
   oils that are highly saturated.
5. What happens to fat when exposed to oxygen? Which fatty acids spoil most easily due
   to oxidation? What is hydrogenation? What are two advantages and one disadvantage
   of hydrogenation? What is a trans-fatty acid?
6. What compound is the best known phospholipid? What is the basic chemical structure
   of lecithin? Why are phospholipids useful in the food industry? What are the richest
   food sources of lecithin? Why is lecithin not considered an "essential" nutrient?
7. What is the most famous sterol? What is the structure of a sterol? What foods contain
   sterols? What foods contain cholesterol? What other compounds in the body are made
   with cholesterol as the starting material? What is cholesterol not considered an
   "essential" nutrient? What organ manufactures cholesterol?
8. What enzymes in the mouth play a small role in the digestion of triglyceride? Where are
    triglycerides digested? What is the function of bile? Where is bile stored in the body?
    What hormone signals the gallbladder to release bile into the GI Tract? What is
    emulsification? What are the two destinations of bile?
9. What are micelles? What is the group of compounds used as transport vehicles for
    absorbed fats? Describe the basic composition and function of the following
    lipoproteins: chylomicron, VLDL, LDL, HDL.
10. What are the functions of triglycerides in the body? What are the functions of
     triglycerides in foods?
11. What are the two essential fatty acids that the body cannot make? What are
     eicosanoids? What is the primary member of the omega-6 fatty acid family? What
     other omega-6 ftty acid can be made from linoleic acid? What foods provide omega-6
     fatty acids in the diet? What is the primary member of the omega-3 fatty acid family?
     What two other omega-3 fatty acids can be made form linolenic acid? What are good
     food sources of omega-3 fatty acids? What are good sources of EPA
     (eicosapentaenoic acid) and DHA (docosahexaenoic acid) in the diet? Briefly explain
     the effects of fatty acid deficiencies?
12. What is lipoprotein lipase? Briefly describe how body fat is made from dietary CHO,
     protein, and fat. How much of the body's ongoing energy needs during rest are
     provided by fat? What is hormone-sensitive lipase? What part of fat (triglyceride) can
     be used to provide glucose? What are ketones?

Fill in the blanks to follow lipids through the GI tract, through digestion, and through metabolism.
1. Some lipid digestion begins in the _____ by an enzyme known as ______.
2. Fat in the small intestine triggers the release of the hormone _____ which signals the
    gallbladder to release _____.
3. Complete digestion of lipids occurs in the _____.
4. Fats must be  _____ by bile from the _____ prior to digestion.
5. Spherical complexes known as ____ form to carry digested fats into small intestinal
   cells.
6. Fats are absorbed in the ____ of the small intestinal cells.
7. Fats are transported by ____ to the rest of the body.
8. Lipoproteins known as ____ are produced in the small intestine to transport dietary fat
    to cells.
9.  The two lipoproteins produced in the liver are ____ and ____.
10. The lipoprotein that is produced from VLDL is ____.
11. Lipoproteins consist of ____, ____, ____, and ____.
12. The least dense lipoprotein is a  ____.
13. The lipoprotein that is predominantly protein and the most dense is ____.
14. The lipoprotein that is predominantly cholesterol is ____.
15. The enzyme on cells that attract triglycerides into cells is known as ____.
16. _____ is the enzyme in cells that hydrolyzes triglycerides, releasing fatty acids and
    glycerol into the blood.

Use these answers to fill in the blanks.
chylomicrons
hormone-sensitive lipase
mouth; lingual lipase
VLDL; HDL
lipoprotein lipase
CCK; bile
LDL
HDL
small intestine
protein, triglycerides, phospholipids, sterols
emulsified; gallbladder
chylomicron
micelles
lipoproteins
microvilli
LDL

1. What is the purpose of a blood lipid profile?
2. Name the dietary lipid that raises blood cholesterol even more dramatically than dietary
   cholesterol does. What type of fat generally raises LDL? What are the risks associated
   with trans-fatty acid intakes? What strategy may be more effective in preventing heart
    disease even more than reducing total fat intake?
3. How do intakes of monounsaturated fatty acids impact LDL levels in the blood? What
   are the benefits associated with omega-3 polyunsaturated fatty acid intakes?
4. Explain the effect of dietary fat on the development of the following: cancers (in
   general), breast cancer, prostate cancer, and obesity.
5. What are the daily amounts recommended by the Committee on Diet and Health for the
   following: total fat intake, saturated fat intake, and dietary cholesterol intake? What is
   the percent of total fat intake for the average person in the United States? What is
   the percent of total kcals that comes from saturated fat for the average person in the
   USA? What is the average dietary cholesterol intake for the average person in the
   USA?
6.  How can most people obtain the right balance between omega-3 and
    omega-6 fatty acids?


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