1. Jamie Pope, Steven Nizielski, and Alison McCook
NUTRITION
for a Changing World
FIRST EDITION
Chapter 6
The Lipids
The Skinny on Fat
© 2016 by W. H. Freeman and Company & Scientific American

2. Chapter 6
Objectives
Identify the four major categories of dietary lipids
Describe the structural differences between saturated, monounsaturated, and polyunsaturated fats
Name a food source rich in monounsaturated fat, polyunsaturated fat, and saturated fat
Identify dietary sources of cholesterol
Name two roles of phospholipids in the body
Describe the process of lipid digestion and explain how emulsification assists in the process
List the four major lipoproteins and describe their functions in the transport of lipids
Describe how to use a Nutrition Facts Panel to evaluate a food’s fat content

3. Does a High-Fat Diet Always Cause Heart Disease?
How did native Greenlanders with a diet high in fat show the world that not all lipids are created equal?
In 1970 a young doctor, Jorn Dyerberg, visited native Greenlanders, known as the Inuit. Living in one of the coldest places on earth, the Inuit eat a high-fat diet rich in seal blubber and fatty fish; yet unlike people in Denmark and the United States, they have much less heart disease. He found that even through their diet’s were high in fat, they did not have a corresponding high blood cholesterol. So we know that different lipids affect health differently.

4. What is a lipid? There are four major categories of lipids
Lipids are generally insoluble in water and dissolve in organic solids.
Fatty Acids
Major source of energy
Triglycerides
Primarily lipid in our diet
Phospholipids
Primary lipid in cell membranes
Cholesterol
Primary dietary sterol

5. Component of triglycerides and phospholipids
Vary in chain length and number of carbons
Vary in amount of hydrogens attached to each carbon, hydrogenation, or double bonds
Saturated
Unsaturated
Monounsaturated
Polyunsaturated

6. Types of Fatty Acids Saturated fatty acid Fully hydrogenated
Food sources
Animal origin
Tropical oils
Hydrogenated atoms fill every possible bond of the carbon atoms. See how they stack tightly and lay compactly. This makes them more solid.
Food sources include animal fats since animals can make saturated fatty acids. Such food sources include pork, beef, chicken, egg yolk, milk, dairy, lard, and beef tallow. Some vegetable sources are also highly saturated. If you have ever seen palm or coconut oil in the bottle, you will notice they are also solid at room temperature.
Saturated fatty acids are thought to raise blood cholesterol, which we will discuss more in the next chapter on heart disease.

7. Types of Fatty Acids Monounsaturated fatty acids
Less hydrogen atoms and one double bond or point of unsaturation
Food sources
Olives, avocados, and some nuts, like peanuts and almonds
You can see that monounsaturated fatty acids are missing some hydrogens, causing a kink in the straight fatty acid chain and not allowing the fatty acids to stack tightly. This allows for the fat to be more liquid at room temperature than a more saturated fat. Monounsaturated fatty acids are rich in olives, olive and canola oils, and avocados, as well as many nuts, like almonds and peanuts. These fatty acids raise blood cholesterol less. We will talk more about them when we discuss heart disease in the next chapter.

8. Types of Fatty Acids Polyunsaturated fatty acids
More than one point of unsaturation or more than one double bond
Food sources
Most vegetable oils
As you can see, there are many bends in polyunsaturated fatty acids and the structures do not pack tightly; therefore these fats are usually very liquid at room temperature. Unfortunately because of the many double bonds, they are also less stable and more prone to rancidity and spoiling compared to other fats. Polyunsaturated fatty acids are rich in vegetables oils such as sesame, sunflower, soy, and corn oils.
These fatty acids don’t raise blood cholesterol but tend to become rancid, or spoil, the fastest since the double bonds make them less stable to free radicals or oxidation.

9. Major Lipid Category Triglycerides
A three-carbon glycerol molecule with three fatty acids attached
The types of fatty acids that attach to triglycerides give them their characteristics in food. If they are mostly saturated, then they will be a solid fat at room temperature, such as butter. If they are mostly unsaturated, they will be liquid, like vegetable oils. Since triglycerides have three fatty acids attached to each glycerol molecule, they will have a mixture of saturated and unsaturated fatty acids, but the majority will give the fat it’s properties. Triglycerides are a primary form of lipid in food and a primary form of fat stored in the body.

10. Triglycerides have a variety of fatty acids attached to each glycerol molecule. The percentage of fatty acids in the saturated, monounsaturated, or polyunsaturated form are shown above. This shows you good sources of food for the different types of fats in different foods.

11. Major Lipid Category Phospholipid
A three-carbon glycerol molecule with two fatty acids and a phosphate group on the third carbon
Explain the properties of phospholipids and how the phosphate group is water soluble, giving it a “head group” that likes water and the fatty acid tail that attracts lipids, allowing for it’s unique function in cell membranes and lipoproteins.

12. Major Lipid Category Sterol
Complex lipid with four carbon rings and a hydrocarbon side chain
Cholesterol is produced by the liver in amounts to meet our body’s needs
The most common sterol is cholesterol. It is a component of cell membranes and a precursor for the synthesis of bile acids, vitamin D, and steroid hormones, such as estrogen and testosterone.

13. Major Lipid Category Dietary Sources of Sterols
Consume cholesterol in the diet from animal foods

14. Lipid Digestion and Absorption
Mouth
Stomach
Small Intestine
Emulsification
Little digestion of lipids occurs in the mouth and stomach; most occurs in the small intestine. There is some lingual lipase added to food in the mouth and some gastric lipase in gastric juices added to food in the stomach, starting lipid digestion. If we bypass the mouth and stomach, we can still digest and absorb fat, as there is lipase in the intestine from the pancreas as well. In the small intestine we are able to add bile that was produced in the liver from cholesterol and heme and stored in the gall bladder to be released when signaled. This then mixes with lipids in the small intestine and emulsifies. Now it is mixed and can be acted on by lipase released with other digestive enzymes from the pancreas into the small intestine.

15. Lipid Digestion and Absorption
Small intestine
Bile
Pancreatic lipase
Bile is made in the liver from cholesterol and heme and then stored in the gallbladder to be released when stimulated by food entering. The bile coats the lipid, emulsifying it and allowing it to mix and be acted on by pancreatic lipase. Once the fatty acids are cleaved from the glycerol molecule by lipase, the enzyme that digests triglyceride, they can be absorbed into the enterocytes, cells of the intestine.

16. Bile is made in the liver from cholesterol and heme and then stored in the gallbladder to be released when stimulated by food entering. The bile coats the lipid, emulsifying it and allowing it to mix and be acted on by pancreatic lipase. Once the fatty acids are cleaved from the glycerol molecule by lipase, the enzyme that digests triglyceride, they can be absorbed into the enterocytes, cells of the intestine.

17. Bile is made in the liver from cholesterol and heme and then stored in the gallbladder to be released when stimulated by food entering. The bile coats the lipid, emulsifying it and allowing it to mix and be acted on by pancreatic lipase. Once the fatty acids are cleaved from the glycerol molecule by lipase, the enzyme that digests triglyceride, they can be absorbed into the enterocytes, cells of the intestine.

18. Transport of Lipids Lipoproteins
Classified by their density and function
Lipids are not water soluble, so they will separate out if put directly into the blood. Therefore they must be packaged into “trucks” to transport or carry them around to the tissues. These trucks are called lipoproteins. Proteins and phospholipids are on the outside, and triglycerides and sterols are on the inside.

19. Transport of Lipids Chylomicrons Very low–density lipoprotein (VLDL)
Lipoproteins made in intestinal cells or enterocytes
Transport fat from the intestine to the body
Very low–density lipoprotein (VLDL)
Made in the liver
Transports fat from the liver to the body
The primary lipids in our diets are triglyceride chylomicrons. They primarily transport triglycerides into the body to tissues to use for energy and storage.
Fat is very light; hence it floats in water. Chylomicrons are the least-dense lipoproteins, as they contain mostly fats/triglycerides.

20. Transport of Lipids Low-density lipoprotein (LDL)
Made in the liver
Transports cholesterol from the liver to the cells of the body
High-density lipoprotein (HDL)
Collects fat and cholesterol from the body to transport back to the liver
“Reverse cholesterol transporter”
Low-density lipoproteins (LDL) are made in the liver. The liver makes cholesterol and triglycerides. Repackage very low-density lipoproteins.
Low-density lipoproteins are the primary transporters of cholesterol to the cells that need cholesterol. They are commonly referred to as blood cholesterol. Cholesterol is very important; it is part of all cell membranes and the precursor to many hormones in the body. We can make all the cholesterol our body needs in our liver, so it is not essential to consume cholesterol in our diets. Too much cholesterol is associated with an increased risk of heart disease, as LDL is believed to deliver excess cholesterol into the arteries, causing increased blockage when elevated, which we will talk more about in the next chapter. But for this reason LDL cholesterol is sometimes called the “lousy” or bad cholesterol. A diet high in saturated fat and total fat is thought to raise someone’s LDL cholesterol.

22. Essential Fatty Acids Omega-6 fatty acids Linoleic acid (C18)
Linolenic acid (C18)
Although our body can make many lipids, including cholesterol, saturated fat, and omega-9 fatty acids, it cannot make unsaturated fatty acids with the first double bond from the methyl end at the third or sixth carbon (omega-3 or omega-6 fatty acids). Therefore linoleic acid and linolenic acid are fatty acids that our bodies cannot make, and they are required in our diet.

23. Essential Fatty Acids
Getting a proper balance of omega-6 and omega-3 fatty acids is important
Omega-3 fatty acids become EPA and DHA
Findings from Greenland
The US diet is low in omega-3 fatty acids. The Inuits, with their high intake of fish, have high levels of EPA and DHA, which helps protect them against heart disease. Research has shown that eating the right amount of omega-3 fatty acids may reduce blood clotting, inflammation, and blood pressure and help eyesight and cognition. This helps explain why too much fat is never a good thing but too little of specific types of fats can be bad for our health. Our findings from the research in Greenland show the benefits of omega-3 fatty acids.

24. The US diet is low in omega-3 fatty acids
The US diet is low in omega-3 fatty acids. The Inuits, with their high intake of fish, have high levels of EPA and DHA, which helps protect them against heart disease. Research has shown that eating the right amount of omega-3 fatty acids may reduce blood clotting, inflammation, and blood pressure and help eyesight and cognition. This helps explain why too much fat is never a good thing but too little of specific types of fats can be bad for our health. Our findings from the research in Greenland show the benefits of omega-3 fatty acids.

25. Trans Fats Hydrogenation Partial hydrogenation
Adding hydrogen to an unsaturated fat makes it saturated (full of hydrogen)
Takes a liquid oil to a solid at room temperature
Partial hydrogenation
Adding hydrogen to an unsaturated fat can alter the position of the double bonds or trans fatty acids
No one likes to put liquid on their toast because it makes it soggy. Liquid oils, because they are unsaturated, are less stable and go rancid more quickly. Therefore industry often hydrogenates oil to make it more stable and more solid; take oil made into margarine or shortening, for example. However this process takes the hydrogens and repositions them into new bonds, some of which will be trans double bonds. This creates trans fats, which are double bonds with hydrogens on opposite sides. With trans fats there is an increased risk of raising blood LDL cholesterol levels. Research shows that trans fats behave like saturated fats in the body and raise LDL cholesterol, increasing a person’s risk for heart disease.

26. Trans Fats
No one likes to put liquid on their toast because it makes it soggy. Liquid oils, because they are unsaturated, are less stable and go rancid more quickly. Therefore industry often hydrogenates oil to make it more stable and more solid; take oil made into margarine or shortening, for example. However this process takes the hydrogens and repositions them into new bonds, some of which will be trans double bonds. This creates trans fats, which are double bonds with hydrogens on opposite sides. With trans fats there is an increased risk of raising blood LDL cholesterol levels. Research shows that trans fats behave like saturated fats in the body and raise LDL cholesterol, increasing a person’s risk for heart disease.

27. Current Fat Intake and Recommendations
Institutes of Medicine recommends
Total fat should be less than 20% to 35%
Most fat should come from unsaturated sources
Eat as little saturated fat, trans fat, and cholesterol as possible
Safe to substitute fats with carbohydrates, protein, and healthier oils
Other fat substitutes
Olean (or Olestra)
Many products are used as fat substitutes, such as gums and pectins, in fat-free products. These are all considered safe and present in other foods. Olean, or Olestra, is a sugar molecule with fatty acids attached, which the body cannot digest or absorb. Therefore it passes straight through the GI tract without being absorbed. This allows Olean to give the properties to food for taste and texture without adding fat to the body. However it can also decrease the absorption of fat-soluble vitamins since they tend to stay near fat molecules. The FDA requires labeling to include: “Olestra may cause abdominal cramping and loose stools. Olestra inhibits the absorption of some vitamins and other nutrients.”

28. Current Fat Intake and Recommendations
How do we determine the amounts and types of fats in the foods we eat?
Read food labels

29. Current Fat Intake and Recommendations

30. Current Fat Intake and Recommendations

31. Current Fat Intake and Recommendations

32. Summary
Lipids are composed of carbon, hydrogen, and a small amount of oxygen, and they are insoluble in water
The four classes of lipids are fatty acids, triglycerides, phospholipids, and sterols
Fatty acids are the primary components of triglycerides and phospholipids
Fatty acids are either saturated or unsaturated, depending upon the number of hydrogen atoms filling their carbon bonds

33. Summary (Cont’d)
Saturated fatty acids are fully hydrogenated and found in animal foods and tropical oils. Monounsaturated fatty acids have one double bond and are rich in foods like olives, nuts, and avocados. Polyunsaturated fats contain more than one double bond and are rich in most vegetable oils
Triglycerides are the primary source of lipid in our diet, the storage form of lipid in our body, and provide a concentrated energy source and essential fatty acids
Essential fatty acids include linoleic (an omega-6 fatty acid) and linolenic (an omega-3 fatty acid) acids, which are required for the synthesis and production of eicosanoids
Cholesterol is a sterol synthesized in the liver and consumed in the diet through foods of animal origin

34. Summary (Cont’d)
Lipid digestion occurs primarily in the small intestine, facilitated by emulsification by bile acids to form micelles and the action of pancreatic lipase
The transport of lipids in the body occurs by protein-rich lipoproteins, including chylomicrons, very low–density lipoproteins, low-density lipoproteins, and high-density lipoproteins
Recommended fat intake is 20% to 35% of the total calories
Recommendations to limit the intake of saturated and trans fat intake are given based on their ability to raise blood cholesterol and heart disease risk
Getting the right ratio of essential fatty acids is beneficial for health