Unit 5 HNSC 1210 - note PDF

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Chapter 5 Lipids Lipids are organic compounds soluble in organic solvents but not in water. The term fat is more familiar then lipids, so we will use this term in the course. Fat is the body’s chief storage place for excess food energy. It enables us to survive during times of famine. Fat cells seem able to expand indefinitely to store excess energy. Fat provides us with a large amount of energy to perform much of the body’s work. Fat tissue, however does more than just store fat. It secretes hormones that help regulate appetite and influences other body functions. Fat serves many other important functions in our body as well: 

pads the vital internal organs, acting as shock absorbers

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insulates the body from temperature extremes is part of cell membranes

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can be converted to other compounds (hormones, bile, Vit D) as needed

Fats in foods serve important functions:

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provides essential nutrients (e.g., essential fatty acids, fat soluble vitamins) provides concentrated source of energy & raw materials for making needed products

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provide tastes, smell and a tender texture to foods

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stimulates appetite and contribute to feelings of fullness

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See Table:“The Usefulness of Fats” Recommendations for Dietary Fat See Table: “Recommendations Concerning Intakes of Fats for Healthy People” DRI: 20 – 35% energy from dietary fat 16 carbons

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Shorter chain fatty acids tend to be softer and melt more readily at room temperature.

b. Degree of Saturation (double bonds, C=C) See Figure: “Three Types of Fatty Acids”

Fatty acids can also be classified by degree of saturation. Saturation refers to the number of hydrogen atoms a fatty acid chain is holding. The more hydrogen atoms attached, the more saturated the fatty acid is.

Where there is no hydrogen attached, this is a point of unsaturation, and a double bond occurs. This occurs especially in the fatty acids of plants and fish. Saturated fatty acid:no points of unsaturation 

tend to be solid at room temp.

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animal fats (e.g. butter, lard), tropical oils (e.g. coconut oil, palm oil)

See Figure “Top Contributors of Saturated Fats to the Canadian Diet” Monounsaturated fatty acid (MUFA): 1 point of unsaturation 

vegetable oils (e.g. canola oil, olive oil)

Polyunsaturated fatty acid (PUFA): 2 or more points of unsaturation 

tends to be liquid oils at room temperature

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vegetable oil and fish oil

See Figure: “Fatty Acid Composition of Common Food Fats”

The degree of saturation affects the temperature at which fat melts; the more unsaturated the fat, the more liquid it tends to be at room temperature. Animal fats are generally the most saturated and the hardest.

Researchers report benefit from replacing saturated and trans fats (which we will discuss shortly) with mono and/or poly unsaturated fats. Saturated fat can increase your ‘bad’ blood cholesterol level (LDL), while trans fat can increase your ‘bad’ blood cholesterol level (LDL) and decrease your ‘good’ blood cholesterol level (HDL), therefore may increase the risk for heart disease. We will take a closer look at cholesterol levels in this unit. Effects of processing on unsaturated fats Margarines A. Hydrogenation See Figure: “Hydrogenation Yields Both Saturated and Trans Fatty Acids”

Hydrogenation is a process where hydrogen is added to unsaturated fatty acids, to make a more solid fat. E.g., turning vegetable oil into

hard margarine.

The hydrogen attaches at the point(s) of unsaturation (the double bond), and the fatty acid becomes saturated (no longer has a double bond). This makes the fatty acid more solid at room temperature and increases the shelf life. Points of unsaturation are more vulnerable to oxidation which is when oxygen mixes with the fat, causing it to go rancid or taste “off”. That is why we store cooking oil in tightly sealed containers. Therefore, the hydrogenated fats are more resistant to oxidation (as it no longer has points of unsaturation). They are more resistant to breakdown from high cooking temperatures and will have a longer shelf life. Hydrogenated fats also have a higher smoking point and are more suitable for frying. When the hydrogen is added, some of the unsaturated fatty acids become more saturated and the oil hardens and becomes easier to spread. Once hydrogenated, the unsaturated fats lose their health benefits. Problems with hydrogenation Hydrogenation affects not only the fatty acids in fats, but the vitamins as well. For example, it decreases the activity of vitamin K in the body. When unsaturated fatty acids are hydrogenated, some change shape instead of becoming saturated. This process creates unusually shaped unsaturated fatty acids that look like saturated fatty acids. These fatty acids are referred to as trans fatty acids. Consuming trans fats poses a risk to heart health and arteries by raising levels of bad cholesterol and lowering levels of good cholesterol in our body. They also produce inflammation. The DRI committee recommends that people consume as little trans-fat as possible. Trans fat lipid: As of dec 2005 all food labels they must provide the gram of trans fat that is contain in a product. Fast food, commercial food, partially hydrogenated soft drinks, baked goods are often high in trans-fat. They may contain a total of 50% in the full fat calculation. Be careful of eating oily fried food because most commercial restaurant tend to use the fake one. See Figure “Major Sources of Trans Fat in North American Diet”

B. Blending different oils to produce a “soft margarine” Another way that we can make margarine to avoid creating trans fatty acids is through a process of blending oils together. This creates a soft, non-hydrogenated margarine (e.g., Becel). This product will not remain solid at room temperature however, as the fats in this product are unsaturated. Classification of Lipids Phospholipids 

Glycerol backbone + 2 fatty acids + phosphorus containing molecule

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Water and fat soluble

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Can serve as emulsifier Examples - Lecithin is a phospholipid found in egg yolk. In mayonnaise, the lecithin

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blends the vinegar with the oil and keeps it from separating. Lecithin has no special health promoting abilities The phosphorus molecule makes the phospholipid soluble in water.

There are lecithin supplements available on the market that claim to lower blood cholesterol values, but these claims are not substantiated. We can make all the lecithin our body needs (i.e., it is not essential).

Emulsifier – mixes with fat and water; allows fat to stay dispersed in water.

Phospholipids also play a key role in the structure of cell membranes. Sterols Sterols are large molecules consisting of interconnecting rings of carbon atoms, with side chains of carbon, hydrogen and oxygen.

Cholesterol is an example of a sterol. Cholesterol is important in the structure of cell membranes, therefore is part of every cell and is necessary for optimal body functioning. What’s good about cholesterol? Cholesterol is necessary to make: 1. 2. 3. 4.

Bile – important for fat digestion Vitamin D Sex hormones, e.g., estrogen, testosterone Stress hormones, e.g., epinephrine

What’s bad about cholesterol? Cholesterol also forms the major part of the plaques that narrow the arteries in atherosclerosis, an underlying cause of heart attacks and strokes. Cholesterol is made by the body so therefore is not essential. Lipid Digestion& Absorption Now that we have discussed the different types of fat we find in foods, we are going to look at what happens to the fat after we eat it. In the stomach In unit 3, we discussed that a minor amount of fat digestion occurs in the mouth (specifically milk fat). Once the fat enters our stomach, the fat separates from the watery chyme and floats on top of it. Because fat does not mix with the watery stomach contents, little fat digestion occurs in the stomach. In the small intestine In the small intestine, bile works to emulsify the fat with watery digestive fluids because one end attracts and holds fats, while the other end is attracted to and held by water. By the time the fat enters the small intestine, the gallbladder has already contracted and squirted the stored bile into the intestine. The bile then emulsifies and suspends the fat particles so that fat-digesting enzymes from the pancreas can split them into smaller particles for absorption. See Figure: “The Action of Bile in Fat Digestion”

If a person has their gall bladder removed, their liver will still produce bile, and the bile will continuously be released into the small intestine in small amounts. As a result, their system can handle only small amounts of fat at a time. See Figure: “The Process of Lipid Digestion and Absorption” Once the intestinal contents are emulsified, fat-splitting enzymes work on triglycerides to free the fatty acids from the glycerol backbone. The free fatty acids, glycerol and monoglycerides (glycerol molecule with 1 fatty acid) cling together in balls surrounded by bile. The bile helps to move the fats across the watery layer of mucus that coats the absorptive lining of the digestive tract. At the lining of the digestive tract: 

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small lipid particles enter directly into the capillary network and travel unassisted to the liver. large lipid molecules are re-formed into triglycerides in the intestinal cells and are packaged together with proteins and phospholipids to form chylomicrons, which can safely travel in the watery blood.

Lipid Transport See Figure: “Lipoproteins” Lipoproteins serve to move lipids in blood and lymph: 1. Chylomicrons: clusters formed when dietary lipids combine with carrier proteins in the cells of the intestinal tract. They transport dietary fat through the water body fluids to the liver and other tissues. 2. Very low density lipoprotein (VLDL): transport triglycerides and other lipids from the liver to various tissues in the body for use. 3. Low density lipoprotein (LDL):transport cholesterol and other lipids from the liver to other tissues (muscle, fat). LDL are made from VLDL after they have donated most of their triglycerides to body cells. 4. High density lipoprotein (HDL): are critical in the process of carrying cholesterol away from body cells to the liver for disposal.

LDL are larger, lighter and richer in cholesterol, while HDL are smaller, denser and packaged with more protein. Lipid Metabolism When the body starts to run out of fuel from food, it turns to body stores 

Fatty acids are used for energy by many organs: typical fuel mix at rest is 50/50 carbohydrate/lipid for liver and muscle

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Any time fat is broken down for energy, carbohydrate must be available Otherwise ketones develop (products of incomplete fat breakdown) and build up in blood and urine

Blood Cholesterol &Cardiovascular Disease Risk (see Chapter 11 - Diet and Health in textbook – review Cardiovascular Disease section) 

High LDL levels increases risk for heart disease.

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High HDL levels are associated with a low risk.

See Table: “Major Risk Factors for Heart Disease” in Chapter 11 Factors that increase the risk of heart disease include: 

High LDL cholesterol

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Low HDL cholesterol

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High blood pressure (hypertension)

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Diabetes

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Obesity

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Physical inactivity

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Smoking

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A diet high in saturated and trans fats, and low in vegetables, fruits and whole grains.

Cholesterol Cholesterol is an important sterol in the structure of brain and nerve cells. It is part of every cell and necessary for the functioning of the body. About 75-80% of our blood cholesterol is synthesized de novo (in the body). It is thought that saturated fat triggers the body to produce cholesterol and trans fat may also play a role.

For most people, a limited amount of cholesterol-containing foods does not pose a risk for increased levels of blood cholesterol, as our body adapts to slow its own cholesterol synthesis when the diet provides greater amounts. Some people however, have a genetic tendency towards high blood cholesterol, and must limit foods that are high in fat (especially saturated and trans fat) and may require medication to reduce the de novo synthesis of cholesterol. Food cholesterol has less of an impact on blood cholesterol levels than most people think. Dietary cholesterol plays a lesser role than saturated and trans fat in cardiovascular disease 

~60% exhibit little increase in blood cholesterol with high cholesterol diet

Dietary cholesterol plays a smaller, but still significant contribution to blood cholesterol values, however many foods that are higher in dietary cholesterol are also nutritious foods (i.e., eggs, meats, cheese, animal derived foods) so we do not want to completely eliminate them from the diet. Therefore moderation, not elimination is the key. Atherosclerosis 

Most common form of heart disease

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Characterized by plaque development along artery walls

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Starts with accumulation of soft fatty streaks, especially at points where the arteries branch.

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Soft fatty streaks gradually enlarge and harden to form plaque

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Damage the artery walls by narrowing the passage through them and making them inelastic.

See Figure: “The Formation of Plaques in Atherosclerosis” in Chapter 11 of the textbook.

Plaques Mounds of lipid materials mixed with smooth muscle cells and calcium.

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Major contributor – diet high in saturated fat. See Figure: "Saturated Fatty Acids and CVD Risk” in the Controversy of Chapter 5.

Atherosclerosis develops through a complex inflammatory response to tissue damage: 

White blood cells (macrophages) are sent to the site of damage

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LDL particles become trapped in blood vessel walls

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Free radicals oxidize the LDL Macrophages engulf oxidized LDL and become foam cells (filled with lipid) – these foam

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cells become a source of oxidation 

Muscle cells in artery walls proliferate to try to heal damage

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Muscle cells combine with foam cells

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Fibrous plaques form (scar tissue)

Problems associated with atherosclerosis Narrowing of arteries can cause an increase in pressure from blood flow. Normally, the arteries expand with each heartbeat to allow the pulses of blood to flow through. However, when the arteries become hardened and narrowed by plaques, they can no longer expand. Therefore, blood pressure increases. Hypertension and atherosclerosis work together to worsen cardiovascular disease. The artery walls are already narrowed and hardened, and are now stressed further by high internal pressure. Since the hardened arteries cannot expand, the heart’s beats raise the blood pressure. Hardened arteries also fail to let blood flow freely through the kidneys, which control blood pressure. The kidneys sense the reduced flow of blood and react as though the blood pressure is too low. Atherosclerosis begins early in life. Many people have well developed plaques by age 30. Diet plays an important role in reducing the risk for plaque development. An atherogenic diet (high in saturated fat, trans fat and cholesterol) increases LDL cholesterol whereas a well chosen diet (low in fat and high in antioxidant nutrients) can lower CVD risk. See Table: “Other Dietary Factors that May Protect Against Cardiovascular Disease” in Chapter 11. Ways to Increase HDL levels and Lower LDL levels Increase HDL: - maintain a healthy body weight

- engage in regular physical activity - quit smoking Lower LDL: - maintain a healthy body weight - increase intake of soluble fibre - reduce intake of saturated and trans fat See Table: “How Much Does Changing the Diet Change LDL Cholesterol” in Chapter 11. See Figure: “Food Fat, Saturated Fat, and Calories” Essential Fatty Acids (EFA) Essential fatty acids are fatty acids that the body cannot make itself, that we must get from foods. They are composed of 2 types. 1. Linoleic acid (e.g., omega-6 fatty acid) 2. Linolenic acid (e.g., omega-3 fatty acid) The EFAs can be used by the body to make eicosanoids.

Eicosanoids 

Biologically active compounds that regulate body functions

Eicosanoids act somewhat like hormones. They can effect muscle relaxation and contraction, blood vessel dilation and constriction, blood clot formation, blood lipid regulation and immune response to injury and infection. EFA’s serve many functions in our body: See Table: “Functions of the Essential Fatty Acids” Research is showing EFA’s benefits to improving cognitive abilities in infants. 1. Omega-6 (n-6) fatty acids Omega-6 fatty acids get their name because of a double carbon bond 6 carbons from the end of its chain. Linoleic acid is an example of an omega-6 fatty acid.

DRI recommendation: 5-10% energy from n-6 fatty acids Sources: vegetable oils, seeds, nuts, whole grain products See Table: “Food Sources of Omega-6 and Omega-3 Fatty Acids” See Figure: “Omega-3 and Omega-6 Fatty Acids Compared” 2. Omega-3 (n-3) fatty acids Omega-3 fatty acids get their name because of a double carbon bond 3 carbons from the end of its chain. Linolenic acid is an example of an omega-3 fatty acid. DRI recommendation: 0.6-1.2% energy from n-3 fatty acids Sources: flaxseed, flaxseed oil, canola oil, fish (especially deep sea fish) See Table: “Food Sources of Omega-6 and Omega-3 Fatty Acids” Research has shown that a higher intake of the omega-3 FA’s can often improve the health of the heart.

There are three main types of omega-3 fatty acids: 1. ALA (alphalinolenic acid) – from plant sources 2. EPA (eicosapentaenoic acid)– from fish sources 3. DHA (docosahexaenoic acid) – from fish sources When given linolenic acid, the body can make other omega-3 FA’s. Two specific omega-3 FA’s are receiving great interest from research – EPA and DHA However the body only makes limited amounts of these omega-3’s. They are found abundantly in the oils of certain fish (deep sea, fatty fish like salmon, char, mackerel, herring, cod, etc). Results from many population studies and clinical trials have supported a recommendation to eat fish. Research first began on these fatty acids when researchers were trying to determine why people in Greenland and Alaska, with high fat diets, had such low rates of heart disease. Research led to the high marine-based diet with an abundance of fish and an abundance of these heart healthy fatty acids. The EPA/DHA content tends to be higher in grilled, baked or broiled fish. Fish used

for frying in a fast food setting and in frozen products (ex. fish sticks), tend to be species with lower DHA/EPA content.

The problems with supplements It is preferred to get your EPA/DHA from food rather than supplements at this point. Many studies are show benefit from the DHA/EPA in foods. Also, fish oil supplements may raise LDL cholesterol levels in some in high intakes of omega-3’s (>3g/day) can increase bleeding time, interfere with wound healing, and suppres function.Supplements also lack the other important nutrients that fish provides (protein and minerals such a selenium). Fish oil supplements may be made from the skin and livers of fish, increasing the risk of contamination from industrial contaminants...