5 Lipids and Fats

5.1 Overview

Lipid is a name for a group of organic compounds that are insoluble in water. Dietary fat is one type of lipid that, in nature, is found as a triglyceride in oils and fats. Most foods contain more than one type of triglyceride (polyunsaturated, monounsaturated, or saturated). Butter, for example, is 66 percent saturated and 34 percent unsaturated fat. Other lipids include phospholipids, sterols, and cholesterol.

Digestion of fat occurs in the small intestine. Bile, provided by the liver and gall bladder, emulsifies fat and makes digestion possible. Lipase, an enzyme secreted by the pancreas, breaks a triglyceride into a monoglyceride and two fatty acids, which are passively absorbed in cells lining the small intestine.

A fatty acid consists of a chain of carbon atoms, some of which may be linked with single bonds and some with double. Fatty acids with all single bonds are saturated; those with double bonds are unsaturated. The degree of saturation has health implications. Saturated fatty acids are associated with an increased risk of heart disease, whereas monounsaturated and polyunsaturated fatty acids are associated with a decreased risk.

There are two essential fatty acids that the body is incapable of manufacturing that must be consumed in the diet. They are linoleic acid and alpha-linolenic acid, both of which are found in vegetable oils.

To traverse the bloodstream, triglycerides form lipoproteins. An elevated low-density lipoprotein (LDL) level is associated with an increased risk of heart disease. Conversely, an adequate level of high-density lipoprotein (HDL) is associated with a decreased risk of heart disease. A fat that is a liquid at room temperature is an oil, while one that is solid is a fat. At nine calories per gram, fat makes a substantial contribution to daily caloric intake.

Trans fats are formed when polyunsaturated fatty acids are hydrogenated during the food-manufacturing process. Diets containing trans fats have been associated with adverse health effects. Food manufacturers have worked to remove trans fats from processed foods, but it’s still important to read the nutrition facts label to check trans fat content. The best approach is to reduce overall fat consumption. Although fat is an important nutrient, moderation in consumption is recommended.

Objectives

• Discuss how lipids contribute to good health
• Explain the steps of fat digestion
• Describe fatty acid saturation as it relates to double carbon-to-carbon bonds
• Recognize LDL and HDL and match each to health benefit or risk
• Recommend a healthy diet that includes essential fatty acids

5.2 Triglycerides and Fatty Acids

Overview

Fat and other substances found in the lipid group are important to childhood growth and the maintenance of healthy skin. During digestion, fat in meals provides a carrier for the absorption of vitamins, including vitamins A, D, E, and K. In the body, absorbed fat is an immediate source of energy or can be stored in adipose tissue for later use. The presence of fat insulates the body and promotes temperature regulation by preventing rapid heat loss. Fat supports and cushions organs such as the kidneys.

Key Concepts

• Lipid and fat definitions
• The molecules that form a triglyceride
• Lipid digestion

Understanding Lipids

Although the terms fat and lipid are used interchangeably, they are not the same. The term lipid describes a broad group of organic compounds that are insoluble in water.¹ You may be familiar with the saying that oil is immiscible in water. Fat is a lipid and an example of a nutrient that is insoluble in water. Two others that are important to our bodies are phospholipids and cholesterol.

In nature, fats and oils are commonly found as triglycerides. There are few free fatty acids. Triglycerides are large macromolecules that contain a three-carbon alcohol backbone or glycerol with three fatty acids attached. The attached fatty acids can be different. During digestion, two of the attached fatty acids are cleaved from the glycerol backbone, producing two free fatty acids and leaving behind a monoglyceride.

The Fats in Our Foods

Seldom is a food composed of just one type of fatty acid. It is usually a combination of several. For example, the fatty acid profile of corn oil reveals a combination of 13 percent saturated fatty acids, 25 percent monounsaturated, and 62 percent polyunsaturated.² In practice, however, when classifying a food as a source of fat, we usually refer only to the predominant fatty acid. Corn oil, therefore, is said to be a source of polyunsaturated fats, even though it also contains a modest amount of saturated and monounsaturated fat. Butter, described as a source of saturated fat, is 66 percent saturated fatty acid, 4 percent polyunsaturated, and 30 percent monounsaturated.

Fat Digestion and Absorption

Although some short-chain (fewer than six carbons) and medium-chain (six to ten carbons) fatty acids may be digested in the stomach, digestion primarily takes place in the small intestine. Bile, manufactured by the liver and secreted by the gallbladder, emulsifies fat by breaking it up into small droplets. As a result, enzymatic activity progresses rapidly, and digestion takes place.

Pancreatic lipase, secreted into the small intestine through the common bile duct, breaks a triglyceride into a monoglyceride (a glycerol backbone with one attached fatty acid) and two free fatty acids. Monoglycerides and free fatty acids are then passively absorbed into the cells of the intestinal villi. There they are re-formed as triglycerides and released into the lymphatic system for eventual arrival in the bloodstream.

Saturated Fatty Acids

A saturated fatty acid has all single carbon-to-carbon bonds in the carbon chain. An example of a saturated fatty acid found in beef that has no effect on blood cholesterol levels is stearic acid, which has sixteen carbons and no double carbon-to-carbon bonds.³

There are other saturated fatty acids, however, that are hypercholesterolemic, or associated with elevations in blood cholesterol levels. Examples are lauric acid (twelve carbons), myristic acid (fourteen carbons), and palmitic acid (eighteen carbons). These fatty acids can be found in coconut oil, palm oil, and butterfat, respectively. Other sources of saturated fatty acids include bacon, sour cream, aged cheeses, marbled beef (beef with fat droplets interspersed throughout the muscle), and whole milk.

The DGA, 2020-2025 describes a healthy eating pattern as one that limits saturated fats and trans fats.⁴ It recommends that only 10 percent of daily calories be supplied by saturated fats, citing a connection between diets high in these fats and coronary vascular disease risk. Solid fats are significant sources of saturated fats in the American diet, and reducing intake of these fats is an important health goal.

Table 5.1 lists four saturated fatty acids that are commonly found in our foods. Stearic fatty acid has sixteen carbons in the chain with no double bonds. This configuration is exemplified by the notation C16:0.

Unsaturated Fatty Acids

A monounsaturated fatty acid has one carbon-to-carbon double bond and is usually a liquid at room temperature. Over 70 percent of olive oil, an important component of the Mediterranean diet, is an eighteen-carbon monounsaturated fatty acid called oleic.⁵ A polyunsaturated fatty acid has two or more carbon-to-carbon double bonds. The predominant fatty acid in both corn and soybean oil is polyunsaturated.⁶

Essential Fatty Acids

There are two fatty acids that the body cannot manufacture from other fatty acids. They must be obtained in the diet in the form of a triglyceride. While our bodies can place double bonds between most carbons in the chain of an existing fatty acid to create a different fatty acid, the enzymes needed to place bonds at the omega-3 and omega-6 positions are lacking.

If you do not regularly consume a small amount of alpha-linolenic (omega-3) and linoleic (omega-6) fatty acid, your health will decline, and a deficiency will result.6 Fortunately, these essential fatty acids are found in a variety of plant oils and foods. In fact, it is not at all difficult to eat sufficient amounts for good health. Only one tablespoon of vegetable oil a day provides more than enough.⁷

Table 5.2 lists two essential fatty acids. Notation for an unsaturated fatty acid includes the number of double bonds and the location of the first carbon with a double bond starting from the omega (CH3-) side of the molecule.

Summary

Lipid is a term for organic compounds that do not dissolve in water. A lipid includes both dietary fat, which is composed of triglycerides, and other molecules manufactured by our own bodies, like cholesterol and phospholipids. When we describe a food as a source of saturated or unsaturated fat, we are referring to the type of fat present in the largest amount. In reality, most foods contain a variety of fats in differing proportions. There are two essential fatty acids—linoleic and alpha-linolenic—that we must consume in our foods. Unsaturated fats are recognized as healthier than saturated fats, and the DGA recommends limiting saturated fat to no more than 10 percent of daily total calories.

5.3 Lipoproteins and Cholesterol

Overview

Phospholipids are found throughout the body, especially in brain and cell membranes. Lecithin is an example of one manufactured by our bodies. Eggs, wheat germ, and peanuts are good sources of phospholipids.

Cholesterol is a waxy lipid that is found in almost every cell in the body. Without cholesterol, the body would not be able to function normally. Cholesterol is used by the body to manufacture vitamin D, cortisone, estrogen, and testosterone. It is also a part of bile, skin oils, and the myelin sheaths of nerve cells.

To travel in the bloodstream, triglycerides form lipoproteins. A lipoprotein is a complex of triglyceride, phospholipid, cholesterol, and protein. Lipoproteins range from very low density containing large amounts of triglycerides to high density containing small amounts.

Key Concepts

• Phospholipid characteristics and function
• Cholesterol function and endogenous production
• Lipoprotein types and function

Phospholipids

As with triglycerides, phospholipids also have a glycerol backbone; however, one of the fatty acids is replaced with a compound containing phosphorous. Phospholipids are dipolar; they have both a hydrophobic (water-repelling) and a hydrophilic (water-attracting) end. This characteristic allows phospholipids to act as emulsifiers in a mixture of water and fat.

A mix of two substances that do not blend or mix without the presence of a stabilizing emulsifier is called an emulsion. Examples of emulsions include mayonnaise, vinegar, salad dressings, and butter. Emulsifiers maintain a water-fat mixture by surrounding droplets of fat and suspending them in water.

Phospholipids are very important to digestion, as they form small fat droplets that are accessible to enzymes and absorptive cells. Phospholipids in cell membranes regulate the passage of fat-soluble and water-soluble substances in and out of the cell.

Sterols

Sterols have a different molecular structure than either fats or phospholipids. They are classified as lipids because they do not dissolve in water. An example of a sterol is cholesterol. Our bodies can make cholesterol, so we do not need to consume it in the foods we eat.

Cholesterol

Cholesterol is a necessary substance that has important functions in the body. It contributes to cell membrane structure and fluidity. Cholesterol in our skin helps us make vitamin D when in sunlight. Cholesterol is hydrophobic (doesn’t go into solution in water) and travels in blood plasma as part of macromolecules called lipoproteins.

About 80 percent of the cholesterol found in the body is manufactured by the liver, with only 20 percent provided by the diet. A diet high in fiber (twenty to thirty-five grams per day) reduces blood cholesterol levels through the process of binding bile and removing cholesterol from the body.

Overall, the liver produces around 1,000 mg of cholesterol daily, and most people consume an additional 200-400 mg in the foods they eat. Foods from animal sources such as milk, eggs, cheese, and muscle and organ meats are sources of cholesterol. As only animal-based foods contain cholesterol, no plant foods are a source of this lipid.

The body can make or synthesize its own cholesterol (endogenous) and rate limits or balances endogenous cholesterol with that consumed in the diet (exogenous). Scientists no longer believe that dietary cholesterol increases the risk of cardiovascular disease in healthy people.⁸ In the past, the DGA recommended limiting the daily intake of cholesterol from foods; however, recent research indicated that dietary cholesterol has little to no effect on blood cholesterol, and the DGA 2020-2025 does not include a recommended dietary limit.

Lipoproteins

Lipoproteins in the body include chylomicrons, LDLs, and HDLs. See Table 5.3. Chylomicrons are manufactured from recently absorbed dietary fats and transport triglycerides from the small intestine to body cells. Blood drawn shortly after a meal high in fat appears “milky” due to the presence of chylomicrons, which seldom remain in the bloodstream for any length of time.

Most of the cholesterol found in the bloodstream is LDL. Synthesized by the liver, LDLs are made from chylomicrons and are high in triglycerides and cholesterol. Excessive LDL-cholesterol levels are associated with an increased risk of heart disease. Diets high in saturated fat and cholesterol reduce the liver’s demand for circulating LDLs, resulting in higher blood levels and greater deposition of cholesterol in vessels.

HDL has a higher proportion of protein than any other lipoprotein. HDL is a reverse transport lipoprotein. While chylomicron and LDLs move cholesterol through the bloodstream to body cells, HDLs scavenge cholesterol deposits in the bloodstream and transport them back to the liver. Moderately high HDL-cholesterol levels are associated with a decreased risk of heart disease.

Summary

Phospholipids are dipolar molecules that suspend hydrophobic molecules like triglycerides and cholesterol in the watery environment of the intestine. This makes the digestion of fats possible. Cholesterol is a sterol that stabilizes cell membranes. It is part of a vitamin D precursor in the skin that is activated by sunlight. Most of the cholesterol in the body is endogenous, and production is regulated to balance that consumed in foods. The DGA 2020-2025 does not emphasize limiting dietary cholesterol. Lipoproteins are blood transport molecules that move triglycerides and cholesterol throughout the body. LDLs are associated with the deposition of cholesterol in arteries, whereas HDPs are associated with the removal of cholesterol.

5.4 The Lipids We Eat

Overview

As a common practice, lipids that are in a solid state at room temperature (e.g., vegetable shortening) are called fats, while those in a liquid state (peanut oil, olive oil, or corn oil) are called oils. In our foods, both fats and oils are sources of desirable flavors that increase palatability.

Key Concepts

• Where fats and oils are found
• Using the senses to detect fat rancidity
• Avoiding trans fats and building a healthy diet

Fats and Oils Give Us Calories

The calories in vegetable oils, such as peanut oil, olive oil, or soybean oil, are derived solely from fat. Baked products made with shortenings, such as pastries, donuts, cookies, and cakes, are significant sources of fat as well as carbohydrates, as are most foods that are deep-fat-fried in vegetable oils.

Spreads and salad dressings are high in fat and can represent significant contributions to daily caloric intake. A tablespoon of butter contains 11 grams of fat, and a tablespoon of salad dressing contains about 7 grams. A typical serving of salad dressing in a restaurant is ¼ cup (4 tablespoons) and contains about 28 grams of fat, or 252 calories.

Animal fats are found in all types of meats, milk (unless skim), and dairy products. Animal fats tend to be higher in saturated fatty acids. Saturated fats with a carbon chain length (the number of carbons) of twelve, fourteen, and sixteen have been associated with an increased risk of heart disease, and consuming foods rich in these fatty acids should be limited.⁹

Fat Rancidity

Polyunsaturated fatty acids are susceptible to oxidation or rancidity. We can detect rancidity through our senses as a stale smell and an undesirable taste. As oils age or are exposed to oxygen, light, or heat, large molecules break at double bonds, generating short-chain fatty acids and free radicals. Free radicals attack other fatty acids and break them at a double bond, thereby continuing the process. This process disrupts cell membranes. Rancid foods are unhealthy and should be discarded and not eaten.

Trans-Fatty Acids

Vegetable oils are naturally low in saturated but high in polyunsaturated fatty acids. This characteristic makes them heart healthy when compared to animal fats.

For some uses, however, oil is not as desirable as a more solid fat. For example, to butter your toast, wouldn’t you rather have butter or margarine you can spread than oil you must pour?

Partial hydrogenation makes vegetable oils more solid and therefore more useful. Hydrogenation is a process through which hydrogen atoms are added to a polyunsaturated fatty acid molecule at the point of a carbon-to-carbon double bond. During hydrogenation, some double bonds become single. As a result, polyunsaturated fatty acids become more saturated, and some monounsaturated fatty acids become completely saturated. This process creates a synthetic fat called a trans fat, which the American Heart Association recognizes as unhealthy.9

Fatty acid molecules with fewer double bonds pack more tightly, thereby becoming more solid. Partially hydrogenated oils can be heated to a higher temperature in a deep-fat fryer or formed into a shape, as in margarine. They are more stable and less susceptible to rancidity than their nonhydrogenated counterparts.

Formation of Trans-Fatty Acids

In their natural state, fatty acids found in vegetable oils exist as a cis isomer (an isomer is a molecule that has the same chemical formula but a different geometric shape). The cis isomer gives the molecule a 120-degree bend at a carbon-to-carbon double bond, resulting in a molecule that had kinks or bends. During hydrogenation, some hydrogen atoms at remaining carbon-to-carbon double bonds migrate to opposite sides of the carbon chain, and the bend is lost. The molecule becomes more flattened out and is a trans isomer, which packs more closely together with other trans molecules than its cis counterpart.

As of January 2006, food manufacturers have been required to list trans fat on nutrition facts labels. For more information on this regulation and how to read a label, check the US Food and Drug Administration Fact Sheet on Trans Fat at https://www.fda.gov/food/food-additives-petitions/trans-fat As a result of increased public awareness of trans fat, manufacturers have replaced much of the trans fat in processed foods. Unfortunately, it is often replaced with saturated oils such as coconut and palm.

Health Implications

In the body, trans-fatty acids have more in common with saturated than unsaturated fatty acids. High levels elevate total blood cholesterol and LDL cholesterol while decreasing HDL cholesterol. The FDA describes trans fats as detrimental to health and with no purpose in the diet. In 2015 the FDA described partially hydrogenated oils (PHO) as no longer recognized as safe, and a deadline of January 2020 was set for the removal of PHO from manufactured foods.¹⁰

Best Approach

If a label claims that a food product is saturated fat-free, it must contain fewer than 0.5 grams of trans-fatty acid. Trans-fatty acids are typically found in foods that are fried or that contain margarine or shortening. Some examples of foods that contain trans-fatty acids include snack crackers, donuts, chips, chocolate chip cookies, taco shells, prepared mixes such as muffin mix, and margarine.

Reducing overall dietary fat is the best way to reduce the intake of trans-fatty acids. Reducing reliance on shortening and margarine also is beneficial. Read nutrition facts labels, and when possible, purchase foods that have three grams or less of fat per serving with zero grams of trans-fatty acid.

Fat is an important nutrient for all people. Providing nine calories per gram, fat is the most concentrated source of energy in your diet. Surprisingly, fat has a role in weight reduction and maintenance. By decreasing the rate at which the stomach empties, a meal that includes modest amounts of fat enhances satisfaction and delays the onset of hunger. Fat is necessary for health; however, due to its role in cardiovascular disease and obesity, it should be handled with caution.

Summary

All fats and oils provide calories, but due to their configuration, some are more likely to be unhealthy than others. Saturated fats, particularly those with a carbon chain length of twelve, fourteen, or sixteen, are associated with an increased risk of heart disease and should be limited in meal patterns. Double bonds in unsaturated fats break down, and oils become rancid when exposed to light, heat, or oxygen in the air. Our sense of smell and taste tell us when it’s time to discard a rancid fat. Trans fats are created when oils are processed to make them more useful in food manufacturing. In 2015 the FDA recognized trans fat as detrimental to health and required partially hydrogenated oils, a source of trans fats, removed from manufactured foods by 2020.

References

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7. What’s in Food: The Importance of Omega-3 and Omega-6 Fatty Acids. European Food Information Council. https://www.eufic.org/en/whats-in-food/article/the-importance-of-omega-3-and-omega-6-fatty-acids. Updated March 2019. Accessed January 16, 2021.
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10. Trans Fats. US Food and Drug Administration. https://www.fda.gov/food/food-additives-petitions/trans-fat. Reviewed May 2018. Accessed January 16, 2021.