Chapter05 Nutrition 2015

Lipids (脂質)

Chapter

5

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Figure 05.CO: Lipids

© Danny E Hooks/ShutterStock, Inc. 2

Importance of lipids in nutrition

• Fat is an essential nutrient (必需營養素).

• Fats contribute greatly to the flavor (香味) and texture (質地) of foods.

• Fats are one of macronutrients (巨量營養素).

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What are lipids?

• The term lipids applies to a broad range of organic molecules that dissolve easily in organic solvents such as alcohol (醇), ether (醚), or acetone (酮), but are much less soluble in water. (廣義定義)

• Hydrophobic = water-fearing = lipophilic = fat-loving (親油、疏水的)

• Hydrophilic = water loving = lipophobic = fat fearing (親水、疏油的) 4

What Are Lipids?

• Hydrophobic (疏水的)/ hydrophilic (親水的)

• Lipophilic (親油的)/ lipophobic(疏油的)

• Classes of lipids (狹義定義) • Triglycerides (三酸甘油酯) • Phospholipids (磷脂質) • Sterols (固醇類)

© PhotoDisc

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Lipids in foods and in the body

• Triglycerides = triacylglycerol (三酸甘油酯)

• Phospholipids (磷脂質) (2% of dietary lipids)

• Sterols (固醇類) (a small percentage of our dietary lipids)

© PhotoDisc

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© PhotoDisc

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三酸甘油酯

© PhotoDisc

Typical structure of phospholipids.

(lecithin)

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卵磷脂磷脂質

疏水區

親水區

© PhotoDisc

Structure of a sterol (固醇), cholesterol (膽固醇), and a cholesteryl ester (膽固醇酯).

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固醇

Cholesterol in the body

• 1/3 from dietary lipids (飲食攝取) • 2/3 from endogenous (體內合成) • An important component of cell

membranes • A precursor in the synthesis of sex

hormones, adrenal (腎上腺) hormones (e.g., cortisol), vitamin D, and bile salts (膽鹽)

© PhotoDisc

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Figure 05.UN01: A generic fatty acid.

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Figure 05.UN02: A generic triglyceride.

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Figure 05.UN03: Fat.

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Figure 05.F01: Fatty acid structure. The basic structure of a fatty acid is a carbon chain with a methyl end (–CH3) and an acid (carboxyl) end (–COOH).

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丁酸

Figure 05.F02: Fatty acid chain lengths. Fatty acids can be classified by their chain length as short-, medium-, or long-chain fatty acids. 15

短鍊

中鍊

長鍊

Fatty Acids Are Key Building Blocks

• Chain Length • 4–24 carbons

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Medium-chain fatty acid (6-10 carbons)中鏈脂肪酸

• Caporic acid (六碳酸, 己酸, 羊油酸) • Caprylic acid (八碳酸, 辛酸, 羊脂酸) • Capric acid (十碳酸, 葵酸, 羊臘酸)

© PhotoDisc

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Figure 05.F03: Fatty acid chain lengths and liquidity (液化). As the chain length of saturated fatty acids increases, they become

more solid at room temperature. 18

Figure 05.F04: Fatty acid nomenclature (命名). The carbons are identified by their locations in the chain.

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Figure 05.F05: Saturated, monounsaturated, and polyunsaturated fatty acids. Hydrogens saturate the carbon chain of a saturated

fatty acid. 20

硬脂酸

油酸

亞麻油酸

飽和脂肪酸

單元不飽和脂肪酸

多元不飽和脂肪酸

無雙鍵

1個雙鍵

2個雙鍵


• Saturation (飽和度)

• Saturated (飽和)

• All single bonds between carbons

• Monounsaturated

• One carbon-carbon double bond

• Polyunsaturated

• More than one carbon-carbon double bond

單元不飽和

多元不飽和

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Fatty Acids

• Straight hydrocarbon chain terminating with a carboxylic acid group

• Fatty acid nomenclature (脂肪酸命名) • Delta (∆) system - length, number/ position of

double bonds (e.g. 18:2 ∆9,12) • Double bonds counted from omega (methyl)

end (e.g. 18:2 ω-6 or 18:2 n-6)

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Fig. 5-2, p. 133

The structure of linoleic acid, showing the two systems for nomenclature.

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亞麻油酸

Figure 05.05: Hydrogens saturate the carbon chain of a saturated fatty acid.

硬脂酸

油酸

亞麻油酸

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Figure 05.UN04: Red meat, onions, and garlic.

© iStockphoto/Thinkstock

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Visible lipids vs. Invisible lipids

Figure 05.F06: Liquid or solid at room temperature? Short-chain and unsaturated fatty acids cannot pack tightly together than long-chain saturated fatty acids. 26


• Geometric (幾何) and Positional (位置) Isomers (異構物) • Cis fatty acid (順式脂肪酸) • Trans fatty acids (反式脂肪酸)

• Hydrogenation (氫化) • Conjugated linoleic acid

(共軛亞麻油酸)

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• Types of fatty acids • Cis (順式) and trans (反式)

• Hydrogenation (氫化) produces trans fatty acids (反式脂肪酸)

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Figure 05.F07: Cis and trans fatty acids. Fatty acids with the bent cis form are more common in food than the trans form.

自然界中順式脂肪酸居多

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• Types of fatty acids • Essential fatty acids (必需脂肪酸)

• Linoleic acid (亞麻油酸) and alpha-linolenic acid (α-次亞麻油酸)

• Can’t be made in the body • Used to make eicosanoids (類二十烯酸)

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Figure 05.F08: Omega-3, omega-6, and omega-9 fatty acids.

α-次亞麻油酸, n-3

亞麻油酸, n-6

油酸, n-9

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Gamma-linolenic acid (GLA) (n-6) (γ -次亞麻油酸)


• Types of fatty acids • Omega-3 • Omega-6 • Omega-9

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• Nonessential (非必需) and Essential (必需) Fatty Acids • Elongation (延長) • Desaturation (去飽和) • Nonessential fatty

acids • Essential fatty acids

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Figure 05.F09: Essential and nonessential fatty acids. Your body makes some types of fatty acids, but others are essential in your diet.

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Figure 05.09: Your body makes some types of fatty acids, but others are essential in your diet.

Vitamin F

必需脂肪酸

非必需脂肪酸

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Alpha-linolenic acid (ALA)

It is named all-cis-9,12,15-octadecatrienoic acid (18:3 (n−3)). The first double bond is located at the third carbon from the n end. Thus, α-linolenic acid is a polyunsaturated n-3 (omega-3) fatty acid. It is an isomer of γ-linolenic acid (GLA), a polyunsaturated n-6 (omega-6) fatty acid. 37

Gamma-linolenic acid (GLA)

γ-Linolenic acid (gamma-linolenic acid or GLA, sometimes called gamoleic acid) is an essential fatty acid (?) found primarily in vegetable oils. However, it can be synthesized from linoleic acid (C18:2 n-6).

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• Building Eicosanoids (類二十烯酸) • Omega-6 fatty acids • Omega-3 fatty acids

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Figure 05.UN05: Think about healthy fats.


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Table 05.T01: Omega-6 to Eicosanoids

non-essential

essential

類二十烯酸

(凝血氧烷, 血栓素) (前列腺素)

(+1 double bond)

(+2 C)

(+1 double bond)

(+2 C)

(+1 double bond)

亞麻油酸

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花生四烯酸(C20:4)

Table 05.T02: Omega-3 to Eicosanoids 42

Triglycerides (三酸甘油酯) • Structure

• Glycerol (甘油) + 3 fatty acids (脂肪酸)

• Functions • Energy source • Energy reserve • Insulation and protection • Carrier of fat-soluble

compounds • Sensory qualities in food

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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.

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Figure 05.UN07: Some foods, like cupcakes, have a high amount of fat.


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Figure 05.F10: Forming a triglyceride. Condensation reactions attach three fatty acids to a glycerol backbone to form a

triglyceride. These reactions release water. 46

Figure 05.F11: Functions of triglycerides. Fat performs a number of essential functions in the body.

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Figure 05.UN08: A generic triglyceride

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Figure 05.UN09: A generic glycerol (甘油)

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Triglycerides in Food

• Sources of omega-3 fatty acids • Soybean (大豆), canola (菜籽油),

walnut (核桃), flaxseed (亞麻籽) oils • Salmon (鮭魚), tuna (鮪魚), mackerel

(鯖魚) • Sources of omega-6 fatty acids

• Vegetable oils (植物油) • Nuts and seeds (堅果及種子)

© Photodisc

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Figure 05.UN06: Fish contains healthy fats. © iStockphoto/Thinkstock

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Table 05.T03: Omega-3 Fatty Acids in Selected Foods Data from U.S. Department of Agriculture, Agricultural Research Service. USDA National Nutrient Database for Standard Reference, Release 25. 2012. www.ars.usda.gov/ba/bhnrc/ndl. Accessed 1/20/13.

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Triglycerides In Foods

• Commercial Processing of Fats • Hydrolysis (水解) • Oxidation (氧化) • Hydrogenation (氫化)

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Figure 05.F12: Fat is a major energy source. When at rest, muscles prefer to use fat for fuel.

© Photodisc 54

Figure 05.F13: Adipose cells store fat. Evolution has selected fat, rather than glycogen, as its primary energy storage medium.

Photo: © Donna Beer Stolz, Ph.D., Center for Biologic Imaging, University of Pittsburgh Medical School.

脂肪細胞可增加體積1000倍

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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.

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Figure 05.F14: Sites for fat storage differ for men and women. Whereas men often store excess fat in their abdomens (腹部) ,

women tend to store it in their hips (臀部). 57

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.

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Figure 05.F15: Fat is a micronutrient carrier. Fat holds more than just energy. It also carries important nutrients, such as fat-soluble vitamins and carotenoids.

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Figure 05.F16: Fat imparts a rich, sensory quality to food.

© Photodisc 60

Figure 05.F17: The diversity of fats. Fats are mixtures of saturated and unsaturated fatty acids. Source: Adapted from Nutrition Today (1996). 61

椰子油奶油

牛油棕櫚油

棉籽油

菜籽油橄欖油

花生油紅花籽油

葵花油玉米油大豆油

飽和油脂

單元不飽和油脂

多元不飽和油脂

Figure 05.UN10: Oils have become so familiar that we often forget they are highly processed, highly refined foods.

© Bragin Alexey/ShutterStock, Inc. 62

Figure 05.FY01_UN01: Look out for fats on the health store shelf.

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Phospholipids (磷脂質) • Structure

• Glycerol + 2 fatty acids + phosphate group

• Functions • Component of cell membranes • Lipid transport as part of

lipoproteins • Emulsifiers (乳化劑)

• Food sources • Egg yolks (蛋黃), liver (肝),

soybeans (黃豆), peanuts (花生) 64

Figure 05.F18: Phospholipid. A phospholipid is soluble in both oil and water. This is a useful property for transporting fatty

substances in the body’s watery fluids. 65

疏水端

親水端

Figure 05.F19: Cell membranes are phospholipid bilayers. These membranes primarily consist of phospholipids.

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Figure 05.UN11: A generic phospholipid.

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Figure 05.F20: Phospholipids and emulsification. Phospholipids form water-soluble packages called micelles that suspend fat-

soluble compounds in watery media.

微膠粒

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Figure 05.UN12: Lecithins (卵磷脂) in food products are used to reduce the separation of ingredients.

© Comstock/Thinkstock

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Figure 05.FY02_UN01: Butter.

© Multiart/ShutterStock, Inc. 70

Figure 05.FY02_UN02: Butter spread.

© Denise Campione/ShutterStock, Inc. 71

Figure 05.FY02_UN03: Margarine spread. 72

Sterols (固醇類): Cholesterol (膽固醇)

• Structure • 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

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Figure 05.F21: Sterols. Sterols are multi-ring structures. Because of its role in heart disease, cholesterol has become the best known sterol.

27 C

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Figure 05.F22: Cholesterol is a precursor of vitamin D and sterol hormones.

雌激素

黃體激素

糖皮質(激)素礦皮質(激)素

雄激素

膽汁

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Table 05.T04: Cholesterol in Selected Foods Data from U.S. Department of Agriculture, Agricultural Research Service. USDA National Nutrient Database for Standard Reference, Release 25. 2012. www.ars.usda.gov/ba/bhnrc/ndl. Accessed 1/20/13.

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Lipid 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 (乳糜微粒) and moved into lymphatic system (淋巴系統)

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Figure 05.F23: Triglyceride digestion. Most triglyceride digestion takes place in the small intestine.

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Figure 05.F24A: Digestion and absorption of triglycerides. Minimal fat digestion takes place in the mouth and stomach.

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Portal vein 肝門靜脈

Lacteal 乳糜管

Figure 05.F24B: Digestion and absorption of triglycerides. Bile salts surround the remaining products of fat digestion.

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Lipid Digestion and Absorption

• Digestibility • Steatorrhea (脂肪瀉)

• Lipid Absorption • Lipoprotein (脂蛋白) • Chylomicron (乳糜微粒)

• Digestion and Absorption of Sterols

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Lipids in the Body

• Lipoproteins (脂蛋白) carry lipids around the body • Chylomicrons (乳糜微粒)

• Delivers dietary lipids from intestines to cells and liver

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Lipids in the Body • Very low density lipoproteins (VLDLs, 極低密度脂蛋白) • Deliver triglycerides to cells

• Intermediate-density lipoproteins (IDLs, 中密度脂蛋白) • Remove some phospholipids

• Low-density lipoproteins (LDLs, 低密度脂蛋白) • Deliver cholesterol to cells

• High-density lipoproteins (HDLs, 高密度脂蛋白) • Pick up cholesterol for removal or recycling

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Figure 05.F25: Lipoprotein sizes and composition. Lipoproteins become less dense as they increase in size. LDL is about double the size of HDL. 84

Figure 05.UN13: Chylomicron (乳糜微粒).

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Figure 05.UN14: VLDL.

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Figure 05.UN15: IDL.

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Figure 05.UN16: LDL. 88

Figure 05.UN17: HDL.

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Figure 05.26: Plaque buildup in a coronary artery.

© W. Ober/Visuals Unlimited

脂肪斑 90

Lipids in the Diet

• Recommended intake • Consume at least two weekly servings of oily fish • Balance of calories from fat and carbohydrate • Reduce sat and trans fat intake • Total fat: 20–35 percent of calories • Need ~2 percent of calories as essential fatty

acids • Improve balance of omega-3 : omega-6 fatty

acids 91

Figure 05.UN18: You can check your lipid levels through a blood test.

© Monkey Business Images/Dreamstime.com 92

Figure 05.UN19: Consume fish, especially oily fish, at least twice a week.

© iStockphoto/Thinkstock 93

Lipids in the Diet

• Essential Fatty Acid Requirements • Omega-6 and Omega-3 Balance • Current Dietary Intakes: about 33% of calories from lipids in USA • Role of Fat Replacers

© Photodisc

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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 one-third of our fat calories.

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Figure 05.F27: Recommended Fat Intake. The Dietary Guidelines for Americans recommends a maximum fat intake of 20 to 35

percent of total calories. 96

Figure 05.F28: Overview of dietary sources of fatty acids. Adapted from Cancer smart. Scientific American. 1998;4(3):9. Photos: (man) © Photodisc; (peanuts) © Photodisc; (french fries) © Kirsta Mackey/ShutterStock, Inc.; (pecans) © Photodisc; (salmon) © John A. Rizzo/Photodisc/Getty Images

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Lipids in the Diet • Fat substitutes

• The Olestra Controversy: Are Fat Replacers Safe? composition • Sucrose + 6 - 8 fatty

acids = ester (酯類) • Indigestible –

provides zero kcals • Reduces absorption

of fat-soluble vitamins 98

Figure 05.F29: Olestra’s structure. The structure of olestra is unlike the structure of a triglyceride.

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Figure 05.FY03_UN01: Regular mayonnaise (蛋黃醬、美乃滋)

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Figure 05.FY03_UN02: Light mayonnaise

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Table 05.T06: Fat Can Markedly Increase Calories in Food Data from U.S. Department of Agriculture, Agricultural Research Service. USDA National Nutrient Database for Standard Reference, Release 25. 2012. www.ars.usda.gov/ba/bhnrc/ndl. Accessed 1/20/13.

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Figure 05.LT01_UN01: The Nutrition Facts panel shown here shows all of the lipid-related information you can find on a food label.

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Lipids and Health

• Obesity • High-fat diets promote weight gain

• Heart disease • High saturated and trans fat intake raises

LDL cholesterol • Cancer

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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 omega-3 fatty acids: eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).

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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.

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Position on Fat Replacers • It is the position of the Academy of Nutrition and

Dietetics 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 2010 Dietary Guidelines for Americans.

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The Women’s Health Initiative (倡議)

• This study sought to explore whether a dietary intervention (膳食介入) 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 intake of specific types of fat.

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NCEP Tips for Healthful Eating Out

• Choose restaurants that have low-fat, low-cholesterol 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 (油炸).

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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.

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Chapter05 Nutrition 2015