Chapter 5 THE Lipids PDF

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Chapter 5 THE Lipids...

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CHAPTER 5 THE LIPIDS

Objectives •

Identify the four major categories of dietary lipids.

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Describe the structural differences between saturated, monounsaturated, and polyunsaturated fats.

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Name a food source rich in monounsaturated fat, polyunsaturated fat, and saturated fat.

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Identify dietary sources of cholesterol, though this is not a learning objective identified in the book.

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Name two roles of phospholipids in the body.

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Describe the process of lipid digestion and explain how emulsification assists in the process.

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List the four major lipoproteins and describe their functions in the transport of lipids.

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Identify the two essential fatty acids, their primary structural difference, and food sources of each in the diet.

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Describe sources of saturated fat in the U.S. diet.

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Describe the AMDR for lipids and how to use a Nutrition Facts Panel to evaluate a food’s fat content.

Lipids Diverse class of macronutrients •

There are four different classes of lipids with very different molecular shapes and functions

What they have in common is that all are oily

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Oily is also referred to as nonpolar or hydrophobic (meaning water-fearing).

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Oily things do not dissolve in water

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All lipids are oily because they are mostly composed of lots of carbon (C) and hydrogen (H) ▪ They also contain a little oxygen (O) Dietary Lipids

Represent the highest energy source: 9 kcal/g About 33% of our calorie intake typically comes from dietary lipids ▪ The AMDR for lipids is 20-35% of total calories ▪ We should eat mostly unsaturated fats, few saturated fats, and no trans fats Which of these sources of dietary fat contain the fewest calories per tablespoon? 1. Butter 2. Canola oil 3. Coconut oil 4. Margarine (stick) 5. Margarine (spread) 6. Olive oil 7. All contain the same calories

Lipids have important roles in the body 1. Lipids are the primary source of the body’s energy reserves (stored in adipose tissue) 2. The adipose tissue also cushions, protects, and insulates the body’s organs 3. Some types of lipids (the phospholipids) are the main component of all cell membranes Lipids have important roles in food 1. Fats in the diet contribute to the sensation of feeling full •

Fats stay in the stomach longer

•

Fats are absorbed over a longer period of time

2. Fats increase the flavor and palatability of foods 3. Fats contribute to the texture and aroma of foods Fat May have a Bad Name, But Dietary Lipids are Important for Our Health!

Good lipids are brain food: dietary lipids can decrease our risk of developing dementia! Chemical Terminology Review Chemical bonds, parts of molecules, or whole molecules may be described as polar or nonpolar. Here is what these terms mean: •

Nonpolar: ▪ Is due to the presence of (lots of) nonpolar C-H bonds ▪ Nonpolar can also be called oily, hydrophobic, or water-fearing

•

Polar: ▪ Often involves charged groups of atoms, such as a charged phosphate group ▪ Polar can also be called watery, hydrophilic, or water-lovong

4 Classes of Lipids and their Structures 1. Fatty acids •

Long, linear hydrocarbon chains (long chains of carbon atoms bound to hydrogen atoms) that can exist by themselves (NOT bound to anything)

•

These chains are said to have an omega end and carboxylic acid end (more about this later)

2. Triglycerides •

Composed of one glycerol molecule bound to three fatty acid molecules

3. Sterols •

Large molecules composed of 4 carbon-based rings (hexagons) that appear fused together

•

Sterols are the only type of lipid that does NOT contain any fatty acids

4. Phospholipids •

Composed of a polar “head” molecule (composed of a glycerol molecule with a charged phosphate group) bound to two nonpolar fatty acid “tails”

The four primary classes of lipids are diverse in both structure and function

Fatty Acids are long hydrocarbon chains with “omega” and “alpha” ends

Just FYI: there are two ways to “write” fatty acid hydrocarbon chain structures: “longhand” and “shorthand” “Longhand” shows every last C and H atom in the chain:

“Shorthand” just shows the bonds (lines) between the carbon atoms; it does not show the actual C and H atoms that make up the backbone:

Fatty Acids Vary in Three Ways 1. The number of carbons in the hydrocarbon chain •

For example, a C:18 fatty acid is 18 carbons long, while a C:22 fatty acid is 22 carbons long

• 2. The degree of saturation of the carbons in he hydrocarbon chain •

Saturation refers to the number of H atoms attached to the carbons

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A fatty acid is said to be saturated when maximum number of hydrogens are attached to the carbons

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This is only possible when there are no double bonds between any two carbons

▪ Go back to the longhand figure in the prior slide and note how one fatty acid is saturated with hydrogens, but the other is not! ▪ 3. Where in the hydrocarbon chain the unsaturation occurs •

The site of unsaturation will be marked by a C=C double bond Fatty Acids Have Three Possible Degrees of Saturation

Fatty acids may be: 1. Saturated 2. Monounsaturated 3. Polyunsaturated Saturated Fatty Acids

Note how saturated fatty acids are straight because they have no C=C double bonds Monounsaturated Fatty Acids

Note how monounsaturated fatty acids are “kinked” in one spot because they have one C=C double bond Polyunsaturated Fatty Acids

Note how monounsaturated fatty acids are “kinked” in multiple spot because they have multiple C=C double bonds The degree of saturation of fatty acids in foods makes a difference The more unsaturated the fatty acid, the more it “kinks” The more it “kinks” the more space it takes up •

It cannot be tightly packed together

The less tightly it can pack, the less solid and more liquid it becomes

•

Animal fats tend to have saturated fatty acids that are solid at room temperature, while plant fats tend to have fatty acids that are liquid at room temperature ▪ Solid fats can get stuck in arteries, causing atherosclerosis (clogged arteries) ▪ The longer the saturated fat, the worse it is for our health

Which of these dietary fat sources contains the most monounsaturated fat? 1. Butter 2. Canola oil 3. Coconut oil 4. Margarine (stick) 5. Margarine (spread) 6. Olive oil 7. All the same

Unsaturated fatty acids may also be “Cis” or “Trans”, depending on where the remaining hydrogen atoms are relative to each other

Trans fatty acids increase risk of heart disease more than any other type of fat (including saturated animal fat) As little as 1% of total calories from trans fat can increase risk of heart disease and stroke •

This is because trans fats are infamous for raising “bad” cholesterol and lowering “good” cholesterol!

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Small amounts of trans fats are found naturally in full-fat dairy and beef

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Historically, industry likes to partially hydrogenate liquid vegetable oils to make them more stable and more solid, so e.g. your margerine will not make your toast all soggy and rancid •

Partial hydrogenation causes the formation of trans fats

•

The FDA banned trans fats in 2018

In unsaturated fatty acids, the site of the first unsaturation (the first C=C bond) is always measured from its omega end

Because the first C=C bond in this fatty acid occurs at the third carbon from the omega end, this is called an “omega-3” fatty acid In unsaturated fatty acids, the site of the first unsaturation (the first C=C bond) is always measured from its omega end

Because the first C=C bond in this fatty acid occurs at the sixth carbon from the omega end, this is called an “omega-6” fatty acid Only Two Fatty Acids are Considered Essential Essential means our bodies cannot make them, so we must eat them (i.e. they are required in our diet) Both essential fatty acids are C:18 chains of 18 carbons in length, but they differ in the site of the first unsaturation (C=C double bond): a. Omega-6 fatty acids b. Omega-3 fatty acids

Essential Omega-6 Fatty Acids The “mother” of all omega-6 fatty acids is Linoleic Acid Linoleic Acid is found in plant foods only: •

Vegetable oils like corn oil

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Walnuts

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Sunflower and pumpkin seeds Essential Omega-6 Fatty Acids

Once inside our bodies, Linoleic Acid is efficiently converted into a longer omega-6 fatty acid called Arachidonic Acid The body the converts the Arachidonic Acid into hormone-like Omega-6 Eicosanoids The Omega-6 Eicosanoids have important functions in the body in that they promote blood clotting and inflammation responses Essential Omega-3 Fatty Acids The “mother” of all omega-3 fatty acids is Linolenic Acid Linolenic Acid is also found in plant foods only: •

flaxseeds and chia seeds

•

walnuts

Once inside our bodies, Linolenic Acid is supposed to be converted into a longer omega-3 fatty acids called DHA (Docosaenoic Acid) and EPA (Eicosapentaenoic Acid) •

Unfortunately the body has a hard time doing this

So instead of eating the plant version of omega-6 fatty acids (the linolenic acid), we can also choose to eat animal foods from animals whose bodies have already done the conversion of Linolenic Acid into DHA and EPA

Good sources of EPA and DHA include cold-water fish No matter how we got the DHA and EPA (made from plant Linolenic Acid inside our bodies, or by eating them as pat of cold water fish), the body then converts the DHA and EPA into the hormone-like Omega-3 Eicosanoids The Omega-6 Eicosanoids have important functions in the body in that they have anti-blood clotting and anti-inflammatory properties •

So they can work together with the omega-6 eicosanoids, which promote blood clotting and inflammation, to fully control blood clotting and inflammation

Essential Omega-6 and Omega-3 Fatty Acids Must Be Converted by the Body into Hormone-Like Eicosanoids (e.g. Prostaglandins)

We must eat the proper ratio of omega-3 and omega-6 fatty acids

Unfortunately, most of the processed foods we eat have corn oil, which is rich in omega-6, not omega-3 On top of that we have already seen how our bodies do a relatively poor job of converting whatever little omega-3 fatty acid linolenic acid we do eat into the DHA and the EPA we need to make the necessary omega-3 eicosanoids The Standard American Diet is Unbalanced Although both kinds of unsaturated fats are good for us, the RATIO of omega-6 to omega-3 is what is really important for good health The ideal Omega-6: Omega-3 ratio is 4 or 5:1 The Standard American Diet usually is at least 10:1 •

Americans consume only ⅓ of essential fats in Omega-3 form

•

The problem is that we eat too much Omega-6, and not enough Omega-3 Triglycerides are also commonly called “fats” or “oils”

Each fat or oil (triglyceride) molecule is composed of a three-carbon glycerol molecule with three fatty acids attached •

This is why they are called TRIglycerides

Triglycerides make up 95% of lipids in our food and 99% of the stored fat in our bodies

Different Foods will have fats (triglycerides) with fatty acids of different degrees of saturation

Common Food Sources of Saturated Fats Foods of animal origin

E.g. solid butter, fat on meat Tropical oils, like coconut oil

Common Food Sources of Monounsaturated Fats •

Olive oil

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Avocados

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Some nuts like peanuts and almonds

Common Food Sources of Polyunsaturated Fats •

Corn oil

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Soy oil

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Walnuts

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Wheat germ

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Sunflower seeds Common Food Sources of Trans Fats

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Junk food

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Processed food

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Fried foods

Beware that just because trans fats have now been banned by the FDA as of 2018, foods labeled “trans fat free” may still contain “less than 0.5 grams” of trans fat per serving. So if you eat multiple foods with 0.49 grams each you are still eating considerable amounts of trans fat! Keep in mind no food is completely saturated or unsaturated (all are a mix)

Some foods in the US diet are higher in saturated fats than others

Use Food Labels to Choose Healthy Fats

How do we use the food labels to determine the amounts and types of fats in the foods we eat?

Sterols are complex lipids They look very different from the other three kinds of lipids because they do not have any fatty acids Instead, sterols have four interconnected carbon rings (hexagons) attached to an oxygen and a hydrocarbon side chain Cholesterol is a sterol produced by the liver

The presence of cholesterol in cell membranes is a distinguishing characteristic between plant and animal cells Cholesterol is only found in animal foods It is NOT an essential nutrient as the liver produces sufficient amounts to meet the body’s needs Of course cholesterol IS present in lots of foods:

Which of these sources of dietary fat contain cholesterol? 1. Butter 2. Canola oil 3. Coconut oil

4. Margarine (stick) 5. Margarine (spread) 6. Olive oil 7. They all contain cholesterol. Cholesterol has varied functions in the body Our bodies do need cholesterol, we just don’t have to eat it to get it Cholesterol is a critical component of animal cell membranes! •

The cholesterol floats around the phospholipids that make up the animal cell membrane to regulate its fluidity

•

Obviously, cholesterol is NOT present in cell membranes of plants

It serves as an important precursor (raw material) for the synthesis of bile acids, vitamin D, and steroid hormones However, dietary cholesterol does NOT provide calories BTW plants also have sterols But NOT cholesterol, which is found only in animals! In plants instead we find sterols and stanols Dietary plant sterols and stanols reduce the intestinal absorption of dietary cholesterol found in animal products •

So plant sterols and stanols are good for us Phospholipids make up plant and animal cell membranes

Phospholipids are composed of a three-carbon glycerol molecule with two fatty acids and a phosphate group on the third carbon Unlike other lipids, the unique structural arrangement allows phospholipids to suspend in water

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They will spontaneously orient themselves into a phospholipids “bilayer” that makes up the cell membrane

The unique nonpolar-chain-with-a-polar-head shape of phospholipids means they can do other things too They also function as a component of lipoproteins that transport lipids in the blood Lecithin is a phospholipid found in the body as well as in food products •

Can be found in egg yolks, liver, and some plant foods

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NOT an essential nutrient

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Functions as an emulsifier (breaks up lipids; keeps them from clumping together), which keeps water and lipids from separating Organs Involved in Lipid Digestion and Absorption

salivary Glands stomach liver • gallbladder

produces bile

•

stores bile

•

release enzymes

pancreas

small intestine •

adds bile, enzymes and emulsifies lipids

Most lipid digestion occurs in the small intestine There is a limited amount of lipid digestion in mouth and stomach by lipases Lipid digestion in the small intestine is facilitated by: •

Bile acids

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Emulsification

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Lipases

The process of emulsification aids digestion by allowing fats to mix with water Fats clump in watery solutions like blood, unless they have been emulsified Bile acids make emulsification possible Emulsified lipids facilitate the action of lipases that help break down the chemical structure of the lipid molecules during digestion Bile acids are made from cholesterol

Lipids are not soluble in water and thus require a carrier for transport around the watery body Remember: all lipids are oily (a.k.a. nonpolar or hydrophobic)! Dietary lipids must be incorporated into transport particles called lipopoteins for absorption These lipoproteins carrying the lipids are absorbed initially into lymphatic system which delivers to bloodstream

There are 4 different kinds of lipoproteins a) Chylomicrons b) Very low–density lipoproteins (VLDLs) c) Low-density lipoproteins (LDLs) d) High-density lipoproteins (HDLs) Lipoprotein Classification The 4 types of lipoprotein differ in: •

Size

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Density

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What type of lipid they like to carry

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Where they come from and where they are going

All 4 types of lipoproteins however are protein-studded “bags” made up of a layer of phospholipids The 4 different Lipoproteins vary in size, density, contents and function

Lipoproteins Circulate Lipids Throughout the Body

4 Transporters of Lipids 1. Chylomicrons •

lipoproteins made in intestinal cells or enterocytes

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transport fat (triglycerides) from the intestine to the body

2. Very low-density lipoprotein (VLDL) •

made in the liver

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transports fat (triglycerides) from the liver to the body

3. Low-density lipoprotein (LDL) •

made in the liver

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transports cholesterol from the liver to the cells of the body

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"bad" cholesterol

4. High-density lipoprotein (HDL) •

collects fat and cholesterol from the body to transport back to the liver (“reverse cholesterol transporter”)

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"good” cholesterol

Current Fat Intake and Recommendations Institutes of Medicine recommends: •

AMDR total fat between 20% to 35% of total calories

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Most fat should come from unsaturated sources

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Eat as little saturated fat (...