Chapter 11: Lipids

An Introduction to Lipids

Lipids, a molecular class that includes fats, are important molecules with many roles in the human body. The word “fat” has a negative connotation for many people which leads to the misconception that lipids are dangerous and bad. Lipids are essential for life. When food is scarce, humans can survive due to energy stored as body fat when food was available. This ability allows survival during famine. Normal fat reserves kept our ancestors alive and are healthy and necessary for us!

Lipids are organic compounds composed of carbon, hydrogen, and oxygen. The common characteristic of lipids is limited-to-no solubility in water. When eaten, they yield high energy: 9 kcals per gram. Lipids perform three primary biological functions within the body; they are structural components of cell membranes, energy storage molecules, and important signaling molecules.

The three main types of lipids are triglycerides, phospholipids, and sterols. Triglycerides, which are insoluble in water, make up more than 95 percent of lipids in the diet and are commonly found in vegetable oil, butter, whole milk, cheese, cream cheese, and meats. Triglycerides occur naturally in many foods, including plant-based foods avocados, olives, corn, and nuts. They are listed on the Nutrition Facts panel as “Fat.” In the kitchen, lipids such as butter and lard that are solid at room temperature are often called “fats,” while lipids that are liquid at room temperature are often called “oils.”

Phospholipids make up about 2 percent of dietary lipids. They are water-soluble to some extent and are found in both plants and animals as a major component of membranes, the protective barrier around cells. In fact, phospholipids are synthesized in the body to form cell and organelle membranes. In blood and body fluids, phospholipids form structures in which fat is enclosed and transported throughout the bloodstream.

Sterols are the least common type of lipid. Cholesterol is the most well-known sterol. Though cholesterol has a notorious reputation, the body gets only a small amount of its cholesterol through food—the body produces most of it. Cholesterol is an important component of the cell membrane and is required for the synthesis of some hormones, vitamin D, and bile salts.

Roles of Lipids in the Body

1. Storing Energy

Excess energy from digested food is incorporated into adipose tissue, or fatty tissue. Most of the energy required by the human body is provided by carbohydrates and lipids. As discussed in the Carbohydrates chapter, glucose is stored in the body as glycogen. While glycogen provides a ready source of energy, lipids are the best energy reserve. Glycogen takes up space because it is bulky with water. Lipids pack together tightly without water and store more energy in less space. One gram of fat contains nine kcals of energy, compared to four kcals for a gram of carbohydrate or protein. Fat-derived energy powers the muscles; muscle cells can use lipids efficiently to produce energy.

Most body cells store fat in limited supplies, but adipose cells are specialized for the purpose; they can expand almost indefinitely to store triglycerides. A certain amount of fat stored in adipose tissue is normal and healthy; too much causes stress on the body and can be detrimental to health. Another risk involving lipid comes from excess lipid in the bloodstream. When particles carrying digested lipids accumulate, lipids can adhere to walls of arteries, leading to cardiovascular disease. Thus, while lipids are needed for survival and good health, large quantities in storage or in the wrong places can be harmful.

2. Regulating and Signaling

Individuals with low body fat stores may have poor temperature control, fatigue easily, and even develop pressure sores on their skin. Triglycerides are important for a healthy immune system and help the body produce and regulate hormones. Adipose tissue secretes the hormone leptin, which regulates hunger and satiety. In the reproductive system, low levels of fatty acids may lead to cessation of menstruating and infertility. Omega-3 and omega-6 essential fatty acids help regulate cholesterol and blood clotting and control inflammation in the joints, tissues, and bloodstream. Fats are important in nerve transmission, memory, and tissue structure. The brain is composed of almost 60% fat, much of which comes from “essential” lipids, the types that must be eaten. They help form nerve cell membranes, insulate neurons, and facilitate the signaling of electrical impulses.

3. Insulating and Protecting

Some body fat is visceral fat, which is adipose tissue surrounding organs. Vital organs such as the heart, kidneys, and liver are protected by visceral fat. You may be most familiar with subcutaneous fat, or fat underneath the skin. This blanket layer of tissue insulates the body and helps maintain the internal climate. Fat pads on hands and buttocks and prevent friction, as these areas frequently contact hard surfaces. It also gives the body extra padding for engaging in physically demanding activities.

4. Aiding Digestion and Increasing Bioavailability

Dietary fats carry lipid-soluble micronutrients, allowing their absorption in the intestines. This improved absorption is also known as increased bioavailability. Fat-soluble vitamins A, D, E, and K are common in foods containing fat, but are also concentrated in green leafy vegetables, carrots, and broccoli. These vitamins are best absorbed when combined with foods containing fat. Fats also increase the bioavailability of phytochemicals, plant compounds such as lycopene (found in tomatoes) and beta-carotene (found in carrots) associated with health. As a result, eating tomatoes with olive oil or salad dressing helps lycopene absorption.

Removing the fat from food may decrease the fat-soluble vitamin content. When grain and dairy are processed, essential nutrients can be lost. Manufacturers may replace these nutrients (the process is called enrichment) but added nutrients may not be as bioavailable or useful as nutrients in whole foods.

Roles of Lipids in Food

Energy Source

Fat-rich foods naturally have a high caloric density and are a convenient source of energy. Remember, 1 gram of lipid provides 9 kilocalories of energy, compared to 4 kilocalories found in 1 gram of carbohydrate or protein. When energy needs are high, the body welcomes the high-caloric density of fat. Recommendations for percent of kcals from lipids are higher for infants and children than for adults. An infant or child on a low-fat diet may grow poorly and suffer developmental problems. Individuals with high energy needs and those recuperating from illness may benefit from higher lipid intake. When the body has used its stored carbohydrates (which can occur as soon as twenty minutes of exercise), it initiates fat usage. Excess fat storage, however, is not healthy.

Effect on Eating Habits

Fat contains dissolved compounds that contribute to mouth-watering flavors. Fat also adds texture to food. Baked foods are supple and moist. Frying foods locks in flavor and lessens cooking time.

Fat also contributes to satiety, or the sensation of fullness. Lipid in the digestive tract slows the movement of food and chyme, promoting the sense of fullness. The sensation of fullness may lag behind the speed of eating, which is one reason we are told to slow down and eat slowly!

Types of Lipids

Lipids serve key roles and perform specific functions within the body. As we discuss the types of lipids (triglycerides, phospholipids, and sterols), we will compare their structures and functions and examine their impact on human health.

Fatty Acids

Fatty acids are composed of a carbon chain with a carboxylic acid (−COOH) group on one end and a methyl group (−CH3) on the other end. Fatty acids can differ from one another in three important ways—carbon chain length, degree of saturation, and shape of bonds in the chain. These characteristics determine how the fatty acid is handled in the body as well as if a compound is solid or liquid at room temperature.

It’s All in the Chain

Dietary fatty acids have chain lengths between four and twenty-four carbons; most contain an even number of carbon atoms. Shorter carbon chains lead to lower melting point, which means the fat is more likely to be liquid.

Fatty Acid Types in the Body

The fatty-acid profile of the diet affects the lipid profile of the body and may affect body weight, composition, and metabolism. Saturated fatty acid consumption is linked to higher weight. Alternatively, diets rich in unsaturated fatty acids from fish oil reduce weight gain, at least in mice.^[1]

Degrees of Saturation

Fatty acids are chains of carbon atoms and their attached hydrogen atoms. “Saturation” refers to how many hydrogen atoms are attached to the carbons in the chain. If all carbon atoms in a fatty acid chain are connected by single bonds, each internal carbon may also bond to two hydrogens, and the end carbon can bond to three hydrogens. If this maximum of hydrogens is attached, the fatty acid is called “saturated.”

If the carbon chain includes double bonds between any two of the carbons (C=C), it is called an unsaturated fatty acid. The double bond means that each carbon in the double bond can bind one less hydrogen, so the fatty acid is not filled or saturated by hydrogen. A fatty acid with one double bond is a monounsaturated fatty acid. Olive oil is a rich source of monounsaturated fatty acids, with approximately 75 percent of its fatty acids being monounsaturated. Oleic acid, an eighteen-carbon unsaturated fatty acid, is a major component. Monounsaturated fats help regulate blood cholesterol levels and reduce the risk for heart disease and stroke.

A polyunsaturated fatty acid (PUFA) has two or more sets of carbon connected by double bonds. Vegetable oils such as soybean oil have high amounts of polyunsaturated fatty acids. Unsaturated fats are considered “healthier” fats; experts recommend using unsaturated oils for cooking whenever possible. Flaxseed oil is rich in alpha-linolenic acid, an omega-3 unsaturated fatty acid, and is a thin liquid at room temperature. Avocados are also rich in unsaturated fats. Most vegetable and fish oils contain high quantities of polyunsaturated fats.

Foods with a high percentage of saturated fatty acids tend to be solid at room temperature. Examples of these are fats found in chocolate (stearic acid, an eighteen-carbon saturated fatty acid is a primary component), butter, and meat. Interestingly, some tropical oils are liquid at room temperature yet high in saturated fat. Palm oil (often used in food processing) is highly saturated and has shown to raise blood cholesterol. Reading food labels carefully enables consumers to choose the fats they buy and consume.

Cis or Trans Fatty Acids?

A double bond in a fatty acid carbon chain can have two different shapes: hydrogen atoms bonded to the same side of the carbon chain are in the cis configuration. If on opposite sides of the bond, the hydrogens are in the trans configuration. The fatty acids containing them are called cis or trans fatty acids. When the hydrogen atoms are on the same side in a cis fatty acid, the carbon chain has a bent structure. Most naturally-occurring unsaturated fatty acids have the cis configuration and are easily incorporated into the human body.

Trans fatty acids have hydrogen atoms attached on opposite sides of the carbon chain at the position of the double bond between carbons. The result is a carbon chain that is straight, like an unsaturated fatty acid, but with a stiff double bond somewhere along the chain. This results in a compound that is not natural to the human body and is associated with an increased risk for heart disease. Trans fatty acids are not common in natural foods; most that human have eaten were produced in factories by a process called hydrogenation. Hydrogenation converts vegetable oils into semisolid fats (shortening, margarine) for use as a substitute for butter or lard. It is no longer legal to use trans fats in the manufacture of foods sold in the US. [2]

Some naturally-occurring trans fats occur in ruminants (cows, sheep, and goats), resulting in trans fatty acids in meat, milk, and dairy products. Intake of small amounts of these types does not have the health risks associated with the now-banned artificial trans fats.

References

1. Mori T, Kondo H. Dietary fish oil upregulates intestinal lipid metabolism and reduces body weight gain in C57BL/6J mice. J Nutr. 2007;137(12):2629-34. https://pubmed.ncbi.nlm.nih.gov/18029475/
2. “Fats and Cholesterol” The Nutrition Source. Harvard School of Public Health. https://nutritionsource.hsph.harvard.edu/what-should-you-eat/fats-and-cholesterol/. Accessed February 23, 2025.

Dietary Fatty Acids

Fatty acids are components of all cells. The body can synthesize most types of fatty acids it needs; these are nonessential fatty acids. There are two types of fatty acids that the human body cannot make as it does not have all the necessary enzymes. These essential fatty acids must be obtained from food.

The two types, omega-3 and omega-6 fatty acids, are named by the location of the first double bond between carbons in the carbon chain, counting from the methyl, or omega, end of the chain. Omega-3 and omega-6 fatty acids are precursors to important signaling molecules called eicosanoids, which help regulate blood flow and blood pressure, smooth muscle contraction, immune response, inflammation, sleep/wake cycles and other important functions.

Eicosanoids derived from omega-6 fatty acids are associated with increases in blood pressure, immune response, and inflammation. Eicosanoids derived from omega-3 fatty acids lower these responses and are associated with heart health. Both types are necessary for healthy balance between these functions in the body; a dietary balance between the two is best for optimal health. Omega-3 essential fatty acids are also important for brain development and health.

 

The essential omega-6 fatty acid is linoleic acid (LA). It is present in foods that are common in the western diet, including meat, grain, and vegetable oils. Deficiency of linoleic acid is rare. On the other hand, the omega-3 fatty acids, alpha-linolenic acid (ALA) and its derivatives docosahexaenoic acid (DHA) and eicosapentanoic acid (EPA) are found in foods that are less common in the western diet: fish, seeds, and nuts. Consuming equal amounts of these two types is a healthy goal, which means that most people who eat a western diet should consider increasing their fish and nut consumption and lowering intake of meat and grains.

Triglycerides

Triglycerides, the main form of lipid found in the body and in the diet, are composed of three fatty acids connected by a glycerol. On its own, glycerol is a thick, smooth, syrupy compound used in the food industry. In triglycerides, a glycerol molecule is the backbone; the fatty acid chains connected to it can vary in size and shape, leading to the wide range of fats in food.

Phospholipids

Like triglycerides, phospholipids have a glycerol backbone. Phospholipids have only two fatty-acid chains, however, with the third position on the glycerol backbone taken by a phosphate group coupled to a nitrogen-containing group. The fatty acid chains are hydrophobic, which means they do not dissolve in water. The phosphate group, however, is hydrophilic, which means it will dissolve in water. Molecules with regions with each of these characteristics are called amphipathic or amphiphilic. This unique structure makes phospholipids water soluble.

The amphiphilic nature of phospholipids allows their function as components of cell membranes. The phospholipids form a double layer in cell membranes, thus effectively protecting the inside of the cell from the outside environment while at the same time allowing for transport of fat and water through the membrane.

Phospholipids are emulsifiers that can keep oil and water mixed. Without emulsifiers, oil and water will form two layers, like we see in vinaigrette dressing. When emulsifiers are added, the oil will form tiny droplets in the water, leading to a creamy appearance, as in mayonnaise or ranch dressing. Lecithin (phosphatidylcholine), found in egg yolk, honey, and mustard, is a common food emulsifier. Mayonnaise demonstrates lecithin’s ability to blend vinegar and oil to create a stable, spreadable condiment. Food emulsifiers enhance the appearance, texture, and stability of many processed foods.

 

Amphipathic molecules are important for blood, nerve, and liver health. The human body can make the phospholipids and other molecules, such as lecithin, that it needs, so they are not essential nutrients.

 

Sterols

Sterols are different in structure from triglycerides and phospholipids. Most sterols are complex molecules of interlinking rings of carbon atoms, with side chains of carbon, hydrogen, and oxygen. Cholesterol is a well-known sterol because it is found in plaque that narrows the arteries in atherosclerosis. For many years, dietary cholesterol was believed to be a major risk for heart disease, though this story is changing as knowledge increases; other types of fat, specifically trans and saturated fat, are now known to have a larger role in heart disease.

Cholesterol in healthy amounts has important roles in the body. It is important in cell membrane structure and in the brain. Cholesterol is used to make vitamin D, glucocorticoids, and the sex hormones progesterone, testosterone, and estrogens. Plants also make sterols which resemble cholesterol. Eating plant sterols may lower the absorption of cholesterol in the small intestine, which may lead to lower cholesterol levels in the bloodstream. Cholesterol is made in the liver, which means it is non-essential.

Digestion and Absorption of Lipids

Lipids are large and most are not water-soluble. They must be broken into small components for absorption and transport through body. Our digestive enzymes are water-based as is blood; how does the body break down hydrophobic lipids and transport them through the bloodstream for the cells to use?

Mouth to the Stomach

Saliva in the mouth contains amphipathic molecules as well as lipase, an enzyme which digests triglycerides and phospholipids. Chewing helps break apart food so emulsifiers and lipase can start the work of digestion.

In the stomach, gastric lipase also breaks triglycerides into diglycerides and fatty acids. Two to four hours after eating a meal, roughly 30 percent of the triglycerides have been clipped into diglycerides and fatty acids. The stomach’s churning and contractions help to disperse the fat molecules, while the diglycerides become additional emulsifiers. However, amid all this activity, little fat digestion occurs in the stomach.

Into the Bloodstream

As stomach contents enter the small intestine, the digestive system must break large fat drops in the chyme into tiny droplets so enzymes can reach in and digest the lipids. The solution to this hurdle is bile. Bile contains bile salts, lecithin, and other amphipathic molecules derived from cholesterol. As physical processes in the small intestine break up fat droplets, the droplets are coated with the amphipathic emulsifiers to keep them apart. Smaller and smaller droplets have a greater surface area, which makes more lipids accessible to the digestive enzymes. This process is call emulsification.

Once the stomach contents have been emulsified, fat-cleaving enzymes such as pancreatic lipase clip fatty acids of triglycerides and diglycerides from their glycerol backbones. Hydrophobic molecules in the chyme clump together in temporary structures called micelles. Micelles have a fatty acid core with a water-soluble exterior. This structure allows fat to pass through the watery layer of mucus coating the absorptive lining of the digestive tract. As micelles travel though the intestine in chyme, they contact the microvilli of the lining. Fat components are released and travel into the cells lining the digestive tract.

Just as lipids require special handling in the digestive tract to move within a water-based environment, they require similar handling to travel in the bloodstream. Inside intestinal cells, monoglycerides and fatty acids reassemble into triglycerides. Triglycerides, cholesterol, and phospholipids form into special structures inside the intestine cells. These structures, lipoproteins, have an inner core of the lipids from food (mostly triglycerides and cholesterol esters, but also phospholipids and other fat-soluble compounds from food, such as fat-soluble vitamins). The outer envelope of a lipoprotein is made of phospholipids with their hydrophilic ends pointing outward and their hydrophobic ends pointing inward. The outer envelope also contains proteins and cholesterol.

The specific type of lipoprotein made in small intestine cells is the chylomicron (Figure 11.11), a large lipoprotein that leaves the small intestine cells and enters the lymphatic system. It is carried through lymph vessels to the bloodstream via the subclavian vein in the neck. Chylomicrons transport food fats through the body’s water-based environment to the liver and other body tissues.

Cholesterol is poorly absorbed compared to phospholipids and triglycerides. Cholesterol absorption is aided by an increase in dietary fat and hindered by high fiber content, which is why high intake of fiber is recommended to decrease blood cholesterol. Fruits, vegetables, and oats can bind bile salts and cholesterol, preventing absorption and carrying them out of the colon. If fats are not absorbed properly, the feces will contain high amounts of fat. Persistent fat malabsorption is called steatorrhea, which can result from diseases that affect absorption, such as Crohn’s disease and cystic fibrosis.

Storing and Using Body Fat

Before prepackaged food, fitness centers, and weight-loss programs, our ancestors had different priorities. Their concern was finding enough to eat, not for losing last ten pounds to fit into a bathing suit for vacation. Humans evolved the ability to reserve fuel for a rainy day.

Short-term storage of energy as glycogen in the liver and muscles was described in the Carbohydrates chapter. When the muscles and liver reach capacity for glycogen storage, the excess is converted into triglycerides and stored as fat.

 

When fat is eaten, it travels through the bloodstream in chylomicrons. Tissues can remove the lipids they need. Muscles remove lipids to turn into energy. Certain tissues, such as such as breasts, layers under the skin and in the abdomen, thighs, and buttocks have a special affinity for circulating lipids, because they have fat storehouses called adipose tissue. Capillary walls contain an enzyme called lipoprotein-lipase that clips the triglycerides in lipoproteins into fatty acids and glycerol, enabling these smaller molecules to enter adipose cells. Once inside, fatty acids and glycerol are reassembled into triglycerides and stored.

When a person’s energy expenditure exceeds the fuel available from a recent meal and stored glycogen is exhausted, adipose tissue dismantles triglycerides and dispenses glycerol and fatty acids into the blood. Energy-hungry cells break them down again; the fragments go through chemical reactions that yield energy, carbon dioxide, and water.

Understanding Blood Lipids

You may have heard of LDL and HDL with respect to heart health. Low-density lipoprotein (LDL) and high-density lipoprotein (HDL) are lipoproteins that transport lipids in the bloodstream. Lipoproteins are characterized by size, density, and composition. HDL particles are denser and smaller than LDL. Both are important in maintaining health.

Major Lipoproteins

Recall that chylomicrons transport fats throughout the watery environment within the body. Within about ten hours of eating a meal, circulating chylomicrons have released triglycerides, leaving remnants that are rich in cholesterol and protein. Remnants return to the liver, where the components are repackaged into different lipoproteins (Figure 11.15):

Very low-density lipoproteins (VLDL) are made in the liver from chylomicron remnants. They contain triglycerides, cholesterol, and other lipids and travel through the circulatory system. This gives cells another opportunity to obtain fatty acids by lipoprotein lipase action. As triglycerides are removed, VLDLs increase in density and become intermediate-density lipoproteins.

Intermediate-density lipoproteins (IDL) contain less triglycerides, so they are richer in cholesterol and denser than VLDLs. Some IDLs return to the liver; some are transformed in the bloodstream into low-density lipoprotein.

Low-density lipoproteins (LDL) are rich in cholesterol and deliver cholesterol safely to body cells that need it. Cells have surface receptors that recognize a protein molecule on the outside of the LDL particle. Circulating LDLs in the bloodstream bind to these LDL receptors and are taken into the cell. Once inside the cell, LDLs are taken apart, their cholesterol released, and all components of the LDL made available for use by the cell. This important process is necessary for cells to obtain cholesterol for their membranes and other uses. Liver cells also have LDL receptors to remove LDLs from the bloodstream and regulate blood cholesterol levels. LDL is nicknamed “bad cholesterol” because excess LDLs in the bloodstream can lead to heart disease. A deficiency of the LDL receptors and high amounts of circulating LDLs are two risk factors. High intake of trans fat is a risk, which led to the banning of trans fat in foods in the US. High levels of saturated fats are believed to block LDL receptors as well, increasing the risk of heart disease.

High-density lipoproteins (HDL) are lipoproteins made by the liver for the purpose of removing cholesterol from the bloodstream and returning it to the liver before it can adhere to the walls of arteries. HDLs are dense in protein and low in cholesterol content. Because of their cholesterol-scavenging role, HDLs are commonly called “good cholesterol.”

Blood Lipid Recommendations

Blood lipid analysis is a clue to the health of a person’s heart and circulatory system. While it is frequently called a “cholesterol test,” blood lipid analysis does not measure free cholesterol. As with all medical tests, the exact molecules measured changes slightly as more information is gained and may vary from lab to lab.

In general, blood lipid analysis measures triglycerides, LDL, and HDL in the blood after fasting for 9 to 12 hours. This fasting time allows lipids from the last meal to be processed by the body. Concentrations are given in milligrams/deciliter (mg/dL). Current guidelines recommend testing every five years for anyone over age twenty. If there is family history of high cholesterol, healthcare providers may suggest a test before age twenty.

According to the National Institutes of Health (NIH), desirable blood lipid concentrations are^[1]:

• Total cholesterol: under 200 mg/dL
• LDL (“bad”) cholesterol: under 100 mg/dL
• HDL (“good”) cholesterol: at least 60 mg/dL
• Triglycerides: under 150 mg/dL

A high ratio of HDL/LDL is associated with lower risk of heart disease. In short, elevated LDL indicates an increased risk of heart disease, while elevated HDL indicates a reduced risk.

References

1. National Heart, Lung, and Blood Institute. Cholesterol and Your Heart: What You Need to Know. NIH Publication No. 22-HL-8191. September 2022. https://www.nhlbi.nih.gov/resources/cholesterol-your-heart-what-you-need-know-fact-sheet

Recommendations for Lipid Consumption

Both the amount and the type of lipids in the diet affect health.

Types of Lipids

Regular consumption of omega-3 fatty acids such as alpha-linolenic acid is recommended because they promote lower total cholesterol and lower triglycerides.^[1] Polyunsaturated fatty acids are associated with lower LDL and higher HDL, both of which are helpful for heart health. Studies indicate that saturated and trans fatty acids increase LDL. Trans fatty acids are especially dangerous because they also decrease HDL levels.

Steps toward healthy blood lipid profiles include limiting saturated fats and trans fats. Saturated fats are common in meat, whole-fat dairy products, and tropical oils. Artificial trans fats are no longer allowed in foods sold in the US. Suggestions for healthy eating include:

• Increase soluble fiber to reduce cholesterol absorption. Try eating more oatmeal, oat bran, kidney beans, apples, pears, citrus fruits, barley, and prunes.
• Add fatty fish, which have high levels of omega-3 fatty acids that to reduce inflammation and LDL levels. Consume mackerel, lake trout, herring, sardines, tuna, salmon, and halibut. Grilling or baking is the best to avoid saturated fats that could be added from frying oil.
• Walnuts, almonds, peanuts, hazelnuts, pecans, some pine nuts, and pistachios contain unsaturated fatty acids that aid in lowering LDL. Make sure the nuts are raw and unsalted. Avoid sugary or salty nuts. One ounce each day is a good amount.
• Olive oil, which contains antioxidants and monounsaturated fat, may lower LDL while leaving HDL intact. Two tablespoons per day in place of saturated fats may contribute to heart health without adding extra calories. Extra virgin olive oil promises a greater effect, as the oil is minimally processed and contains more antioxidants.

Amounts of Lipids:

• The acceptable macronutrient distribution range (AMDR) for lipid for adults is 20-35% of total kcals.
• Less than 10% of total kcals should come from saturated fat. Consider lean and low-fat versions of meat, poultry, milk, and milk products.
• For children between four and eighteen years, between 25 – 35% percent of daily kcals should be from fat.
• For all age groups, most fats should come from polyunsaturated and monounsaturated fats such as fish, nuts, and vegetable oils. The AHA guidelines also recommend consuming fish, especially oily fish, at least twice per week.^[2]

Types of Fat in Food

Saturated fat. Found in animal products, dairy products, palm and coconut oils, and cocoa butter. Popular foods such as cheese, pizza, cookies, chips, desserts, chicken, burgers, sausages, and hot dogs may be high in saturated fat. Limit to less than 10 percent of your overall dietary fat consumption.

Monounsaturated fat. Found in plant oils such as olive oil. Other sources are nuts (almonds, cashews, pecans, peanuts, walnuts), avocados, sesame oil, high oleic safflower oil, sunflower oil, and canola oil.

Polyunsaturated fat. Found in fish and in plant-based foods and oils. Common sources are nuts (walnuts, hazel nuts, pecans, almonds, and peanuts), soybean oil, corn oil, safflower oil, flaxseed oil, canola oil, and fish (trout, herring, and salmon).

Alpha-linolenic acid (ALA) (an essential omega-3 fatty acid). Good sources include canola oil, flaxseed oil, soybean oil, olive oil, nuts, seeds, whole grains, legumes, and green leafy vegetables.

DHA and EPA (omega-3 fatty acids). Sources include cod liver oil, tuna, herring, mackerel, salmon, and trout. The body can make these two omega-3 fats, (eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)) from ALA. It is more efficient to eat them directly, however, as the body converts ALA to EPA and DHA inefficiently.

Linoleic acid (LA) (an essential omega-6 fatty acid). Sources include eggs, poultry, most vegetable oils, wheat germ oil, whole grains, baked goods, and cereals. Omega-6 fatty acids are also present in flaxseeds, sunflower seeds, sesame seeds.

Cholesterol. How much cholesterol is made in the liver is affected by how much is eaten. A person with a cholesterol-rich diet will manufacture less cholesterol. A vegan who eats no cholesterol (which is only made by animals), will make plenty of cholesterol in their liver. Limiting cholesterol intake is no longer believed to be the most important factor in maintaining healthy blood lipids.

The Omega-3:Omega-6 Balance

Omega-3 and omega-6 fatty acids are precursors of important signaling molecules in humans. The signals from the two types have opposite effects on functions such as blood pressure, inflammation, blood clotting, and cell proliferation. It is therefore important that the diet includes plenty of each type, and in approximately equal amounts. However, the standard western diet includes a huge imbalance, with omega-6 fats eaten in up to 20-fold excess over omega-3 fats. Researchers believe that this imbalance increases inflammation and blood pressure, which elevates the risks for allergies, arthritis, asthma, coronary heart disease, diabetes, cancers, autoimmunity, and neurodegenerative diseases, all of which are associated with inflammation in the body.

Omega-6 fatty acids are plentiful in a western diet rich in grain-fed meat and grain. Foods rich in omega-3s are less common; most people do not eat large amounts of grass-fed animal products, seeds, fish. and nuts. Vegetable oils from grains rich in omega-6 fats are common in fast-food preparations, snack foods, cookies, and crackers. Adjusting the balance of the types of fats in the diet may have a greater effect than severely limiting fat intake, an idea that was widely believed from the 1950s through the 2000s but is now beginning to be replaced.

References

1. Fish Oil. Mayo Clinic. https://www.mayoclinic.org/drugs-supplements-fish-oil/art-20364810. Updated August 10, 2023. Accessed February 23, 2025.
2. Fish and Omega-3 Fatty Acids. American Heart Association. https://www.heart.org/en/healthy-living/healthy-eating/eat-smart/fats/fish-and-omega-3-fatty-acids. Updated August 23, 2024. Accessed February 23, 2025.

Lipids and the Food Industry

Fatty acids are susceptible to attack by oxygen molecules at their points of unsaturation. The oxidation of fatty acids makes oils rancid and gives the food prepared with it an unappetizing taste. Polyunsaturated fats, with multiple unsaturated bonds, are the most susceptible to spoilage. They should be stored in air-tight containers, preferably in the dark and in the cold.

Because saturated fats are more stable, for about 100 years, food manufacturers artificially saturated vegetable oils (including inexpensive oils from seeds that were otherwise considered waste, such as cottonseeds). This manufacturing process created solid fats such as shortening and margarine, which were inexpensive, stable, and easy to cook with. Unfortunately, the process also led to the presence of dangerous trans fats in these foods. Now illegal in the US, common trans-fat-containing products are associated with adverse health effects on several generations of people.

Trans fatty acids occur in small amounts in nature, in meat and milk from ruminants including cows and goats. These naturally-occurring trans fats are not believed to have the same health risks as artificial ones.

Lipids and Disease

Because heart disease, cancer, and stroke are the three leading causes of death in the United States, it is critical to address dietary and lifestyle choices to decrease risk factors for these diseases. According to the US Department of Health and Human Services (HHS), the following risk factors are controllable: high blood pressure, high cholesterol, cigarette smoking, diabetes, poor diet, physical inactivity, being overweight, and obesity.

Excessive fat consumption and consumption of the unhealthier fats is associated with an increased risk for disease. High levels of blood lipids, hyperlipidemia, is associated with an increased risk of:

• Cardiovascular disease. Many types are related to the process of atherosclerosis. Over time the arteries thicken and harden with plaque buildup, causing restricted or no blood flow to areas of the body.
• Heart attack. A heart attack occurs when blood flow to a section of the heart is cut off due to a blood clot. Dietary and lifestyle changes can lower the risk of a recurrence.
• Ischemic stroke. The most common type of stroke in the United States, ischemic stroke, occurs when a blood vessel in the brain or leading to the brain becomes blocked, usually from a blood clot. If part of the brain suffers lack of blood flow and/or oxygen for three minutes or longer, brain cells will start to die.
• Congestive heart failure. The heart does not pump blood as well as it should, and it is not meeting the body’s demand for blood and oxygen.
• Arrhythmia or abnormal rhythm of the heart. The heart may beat above one hundred beats per minute (tachycardia) or below sixty beats per minute (bradycardia), or the beats are not regular. The heart may not be able to pump enough volume of blood to meet the body’s needs.
• Heart valve problems. Stenosis occurs when heart valves do not open far enough to allow proper blood flow. When the heart valves do not close tightly and blood leaks between chambers, it is called regurgitation. When valves bulge back into the upper chamber, this condition is called mitral valve prolapse.
• Obesity. Obesity is the excessive accumulation of body fat. Obesity is the fastest growing cause of death in America. Between 1980 and 2005, the number of adolescents who are overweight tripled and the prevalence of the disease among younger children doubled^[1]. Obesity is a risk factor for developing diabetes and heart disease.

What You Can Do

To help combat obesity, recommended changes include:

• Reducing the type and amount of carbohydrates and sugar consumed daily.
• Eating fewer calories.
• Limiting saturated fats and choose mono- and poly-unsaturated fats.
• Choosing whole-grain and high-fiber foods. At least half of daily grain intake should originate from whole grains. The Adequate Intake value for fiber is 14 grams per 1,000 kilocalories.
• Being active. Exercise on a regular basis. Increasing your energy expenditure by just twenty minutes of physical activity three times per week will improve your overall health.

References

1. Childhood Obesity. US Department of Health and Human Services. https://aspe.hhs.gov/reports/aspe-childhood-obesity-white-paper Published April 30, 2005.  Accessed February 23, 2025.

Suggestions for Choosing Fats

Lipids are part of a healthy diet. Eating a moderate amount of lipid and choosing healthier lipids are both important.

Lipids provide 9 kcals/g. The recommended percentage of lipids (AMDR) in the diet is 20 – 35% of total kcals. A diet of 2000 kcals in a day has 400 to 700 kcals from lipids, which is 44 – 78 grams.

Limiting saturated fat intake to 10% of total kcals means a person on an “average” 2000 kcal diet would eat less than 200 kcals of saturated fat, which is 22 grams.

We are not used to thinking of food this way, but practice reading food labels or using a meal-tracking program or app will lead to an appreciation for how much lipid is present in various foods.

When selecting prepared foods, be aware of how it is cooked. Frying and deep-frying may add more fat. Watch for high-fat sauces and dressings. Higher-fat meats tend to have fat marbled throughout. Lean cuts of red meat and “white meat” from chicken and turkey are lower in saturated fat.

Low fat does not equal healthy. Common replacements for fat in fat-free foods are starch and sugar. These refined carbohydrates are pleasant to eat and may not lead to satiety in the way that fat does, which could lead to over-eating.

Consume omega-3 fats each day. Include fish, walnuts, chia seeds, ground flaxseeds, flaxseed oil, or soybean oil in your diet every day.

Limit saturated fats in home preparation of meals. Try canola or olive oil spreads or sprays instead of butter. Avoid using high-fat meat gravies, cheese, and cream sauces. Grill, bake, stir-fry, roast, or bake your foods instead of frying in solid fats. Marinate foods to be grilled in fruit juices and herbs. If you like salad dressing, experiment with making your own using healthier oils such as cold-pressed olive oil, flaxseed oil, or sesame oil. Saturated fats such as butter and coconut oil are solid at room temperature. Monounsaturated and polyunsaturated fats such as olive oil and canola oil are liquid at room temperature.

Try a new oil. Some examples of flavorful oils are sesame oil and peanut oil. Replace less flavorful cheeses with small amounts of strongly flavored cheeses such as Romano, parmesan, and asiago.

Educate yourself about the environmental impact of your choices. Beef fats such as butter and milk products have a higher environmental cost than some plant-based products. However, tropical oils such as coconut and palm oil are associated with habitat destruction and other environmental issues.

Awareness of the roles of lipids and the need to control lipid intake will facilitate your ability to make choices beneficial to personal health as well as the global ecosystem. Our food choices impact systems small and large.

Attributions

Adapted by Pattie S. Green, Ph.D. and Jonathan E. Pottle, Ph.D. at Tacoma Community College from Human Nutrition by University of Hawai‘i at Mānoa Food Science and Human which is licensed under a Creative Commons Attribution 4.0 International License. Specifically, the following parts were adapted here:

• The Functions of Lipids in the Body
• The Role of Lipids in Food
• How Lipids Work
• Nonessential and Essential Fatty Acids
• Digestion and Absorption of Lipids
• Tools for Change
• Lipids and the Food Industry
• Lipids and Disease