Lipids are a family of organic compounds mostly insoluble in water.
They yield high energy and are composed mainly of carbon, hydrogen, and oxygen.
Three primary biological functions:
Structural components of cell membranes.
Energy storehouses.
Signaling molecules.
Three main types:
Triacylglycerols (95% of lipids in the body).
Phospholipids (2% of lipids in the body).
Sterols (tiny percent, mostly manufactured by the body).
Functions of Lipids in the Body
Adipose tissue: Fatty tissue that stores excess energy from digested food.
Fat gram contains more than double the energy of a gram of carbohydrate.
Excess fat accumulation can lead to health issues.
Cholesterol:
Component of cell membranes.
Required for synthesis of sex hormones, vitamin D, and bile salts.
Cardiovascular disease: Disease of the heart or blood vessels.
Triacylglycerols (Triglycerides):
Most common class of lipids.
Main form of fat in diet and body.
Made of three fatty acids and one glycerol molecule.
Control body’s internal climate.
Help regulate hormones.
Generally accepted range for body fat:
Males: 10% to 20%.
Females: 20% to 32%.
Bioavailability: Range for good health as body fat percentage.
Phytochemicals: Nonessential plant compounds with beneficial impact on health.
Role of Lipids in Food
High-fat foods contain more calories than high-protein or high-carbohydrate foods.
High-fat foods are a convenient source of energy.
Individuals with high energy needs: Infants, growing children, athletes, people with physically demanding jobs, and those recuperating from illness.
Fats contribute to satiety (sensation of fullness).
5.1: Key Takeaways
Lipids include triacylglycerols, phospholipids, and sterols.
Triacylglycerols are the most common lipid and form most body fat.
Excess energy from food is stored as adipose tissue.
Fats maintain body temperature, cushion organs, regulate hormones, transmit nerve impulses, store memory, and aid in absorption of fat-soluble nutrients.
Lipids transport fat-soluble nutrients and phytochemicals and promote bioavailability.
Fat is a convenient energy source for those with high energy requirements.
Fat provides more than double the energy per gram than protein or carbohydrates, enhances food flavor, and promotes satiety.
5.2: How Lipids Work
Learning Objectives
Explain how the structure of each lipid impacts its function as it relates to health and disease.
Compare and contrast the structure of the following types of fats: triacylglycerols, fatty acids (monounsaturated, polyunsaturated, omega-3, and omega-6), phospholipids, and sterols.
Explain how the fatty acids you consume relate to the fatty-acid composition of your body fat.
Fatty Acids
Differ in three important ways:
Essentiality.
Carbon-chain length.
Degree of saturation.
Fatty acids: An organic compound with a carboxylic acid (-COOH) group at one end and a methyl group (-CH3) at the other.
Saturated fatty acid: Contains the maximum number of hydrogen atoms with no points of unsaturation.
Unsaturated fatty acid: Does not contain the maximum number of hydrogen atoms with points of unsaturation.
Polyunsaturated fatty acid: Contains two or more points of unsaturation.
Fatty Acids (continued)
Nonessential fatty acids: Can be synthesized by the body.
Essential fatty acids: Cannot be synthesized by the body and must be obtained through diet.
Omega-3.
Omega-6.
Eicosanoids: Compounds derived from polyunsaturated fatty acids that control several body functions.
Docosahexaenoic acid (DHA): An omega-3 fatty acid important for brain growth and development in infants.
Fatty Acids (continued, 1)
Excellent sources of omega-3:
Fish.
Flaxseed oil.
Hemp.
Walnuts.
Leafy vegetables.
Excellent sources of omega-6:
Vegetable oils.
Tofu.
Phospholipids
Have a glycerol backbone.
Are diglycerides (two fatty-acid molecules attached to the glycerol backbone).
Third fatty acid chain has a phosphate group coupled with a nitrogen-containing group.
Water soluble.
Amphiphilic: A compound with both water-loving and fat-loving properties.
Emulsifiers: Compounds that allow two immiscible liquids to combine so that no separation occurs.
Sterols
Have a very different structure from triacylglycerols and phospholipids.
Most do not contain fatty acids.
Complex molecules that contain interlinking rings of carbon atoms, with side chains of carbon, hydrogen, and oxygen attached.
Best-known sterol is cholesterol.
Vital substance in the body that poses a concern only when there is an excess accumulation of it in the blood.
5.2: Key Takeaways
Three fatty acids combine with one glycerol to make a triacylglycerol, the body’s storage form of fat.
The carbon chain in fatty acids can be saturated, monounsaturated, or polyunsaturated.
The structure and length of a fatty acid determines whether it is solid or liquid at room temperature.
Essential fatty acids cannot be synthesized by the body and must be included in the diet. Omega-3 and omega-6 fatty acids have opposing functions and must be consumed in proper balance to promote health.
Fatty acids in food influence the composition of fatty acids in the body.
Phospholipids are a large component of cell membranes because of their amphiphilic nature. They are ideal emulsifiers as they keep oil and water mixed.
Sterols contain a multiring structure. Cholesterol serves important body functions such as vitamin D and hormone synthesis and is important in cell membrane and brain structure.
5.3: Digestion and Absorption of Lipids
Learning Objectives
Summarize the steps in lipid digestion and absorption.
Explain how lipids are used for energy and stored in the body.
From the Mouth to the Stomach
Digestion begins in the mouth as lipids encounter saliva.
Chewing and emulsifiers enable digestive enzymes.
Lipase: Enzyme responsible for the breakdown of triacylglycerols and phospholipids.
In the stomach:
Gastric lipase starts to break down triacylglycerols into diglycerides and fatty acids.
Churning and contractions disperse fat molecules.
Going to the Bloodstream
Bile: Substance secreted by the liver that aids in the absorption and digestion of fats.
Contains bile salts, lecithin, and substances derived from cholesterol.
Acts as an emulsifier.
Monoglycerides: Product of lipid digestion, glycerol molecule with one fatty acid attached.
Micelles: Aggregate of phospholipids and other fats and fat-soluble vitamins that cross through the brush border into the enterocytes.
Going to the Bloodstream (continued)
Lipoprotein: Protein that contains a lipid that transports fat through blood and lymph.
Inner core primarily made up of triacylglycerols and cholesterol ethers.
Chylomicron: Formed when lipids are combined with carrier proteins in the cells of the intestinal lining.
Vehicle of transport for fats throughout the watery environment of the body to the liver and other tissues.
5.3: Key Takeaways
In the stomach, fat is separated from other food substances. In the small intestines, bile emulsifies fats while enzymes digest them. The intestinal cells absorb the fats.
Once in the intestinal cell, long-chain fatty acids form a large lipoprotein structure called a chylomicron that transports fats through the lymph system.
Short and medium fatty chains can be absorbed directly into the bloodstream from the intestinal microvillus because they are smaller and water soluble.
Cholesterol absorption is hindered by foods high in fiber.
When energy supplies are low the body utilizes its stored fat reserves for energy.
5.4: Understanding Blood Cholesterol
Learning Objectives
Compare and contrast the roles of LDLs and HDLs in your body.
Explain the purpose of a blood lipid profile and identify healthy ranges.
Major Lipoproteins
VLDLs: Very low-density lipoproteins are made in the liver from remnants of chylomicrons.
Transport triacylglycerols, cholesterol, and fat-soluble vitamins from the liver to various tissues in the body.
IDLs: Intermediate-density lipoproteins transport triacylglycerol, cholesterol, and fat-soluble vitamins in the bloodstream and are a little under half triacylglycerol in composition.
LDLs: Low-density lipoproteins are commonly known as the “bad cholesterol.”
HDLs: High-density lipoproteins are also known as “good cholesterol.”
Major Lipoproteins (continued)
LDL/HDL composition:
LDL is approximately 25% protein and 75% cholesterol and other fats.
HDL is 50% protein and 50% cholesterol and other fats.
LDL/HDL function:
LDLs carry cholesterol into cells for usage but can also deposit it into the walls of blood vessels.
HDLs scavenge excess cholesterol and deliver it back to the liver.
LDL/HDL and inflammation:
LDLs may contribute to heart disease.
HDL reduces the amount of LDL in the bloodstream.
Major Lipoproteins (continued, 1)
LDL/HDL warnings:
High LDL values warn of increased risk for heart disease.
High HDL values indicate reduced risk for heart disease.
Oxidized LDL:
Oxidation: The loss of at least one electron when two or more substances interact.
Can speed up the process of plaque formation in the arteries.
Promote atherosclerosis.
Significantly increases risk for heart attack or stroke.
Blood Cholesterol Recommendations
The desired range for healthy total blood cholesterol is under 200 mg/dL.
To maintain healthy levels, consume the following:
Fatty fish.
Walnuts, almonds, peanuts, hazelnuts, pecans, pine nuts, pistachios (raw and unsalted).
Desired values used to measure an overall lipid profile:
Total Cholesterol
Less than 200 mg/dL is optimal, 200–239 mg/dL is borderline high, and above 240 mg/dL is high.
LDL
Less than 100 mg/dL is optimal, 100–129 mg/dL is near or above optimal, 130–159 mg/dL is borderline high, >160 mg/dL is high.
HDL
<40 mg/dL is low and puts you at risk, >60 mg/dL is optimal.
Triacylglycerols
10–150 mg/dL is optimal, 150–199 mg/dL is borderline high, >200 mg/dL is high.
5.4: Key Takeaways
Some of the major lipoproteins are VLDL, IDL, LDL, and HDL.
VLDL delivers triacylglycerols and other lipids to the body’s tissues and slowly becomes IDL. The liver uses IDL to create LDL, the main transporter of cholesterol.
LDL, or “bad” cholesterol, has low protein composition and high cholesterol content. High levels of LDL have been shown to increase the risk for heart disease. However, particle size matters, and the small, dense LDL are more damaging.
HDL, or “good’’ cholesterol, has a larger proportion of protein and a small cholesterol composition. HDL scavenges excess cholesterol and returns it to the liver for reuse or disposal. A high level of HDL reduces the risk for heart disease.
It is important to maintain a healthy lipid profile with values as recommended by the National Institutes of Health (NIH) in order to minimize the risk of heart disease. Consuming omega-3 fatty acids can help maintain a healthy blood lipid profile.
5.5: Balancing Your Diet with Lipids
Learning Objectives
Describe the current recommended intake levels for lipids.
Recognize sources of saturated and unsaturated fats, and essential fatty acids.
Discuss the functions of essential fatty acids, such as omega-3, and where to find them in foods.
Recommended Fat Intake
Acceptable macronutrient distribution range for adult fat consumption:
Fat calories should be limited to 20% to 35% of total calories.
Consume fewer than 10% of calories from saturated fats.
Think lean and low-fat when selecting meat, poultry, milk, and milk products.
Child and adolescent fat consumption (for children over four):
Between 25% and 35% of caloric intake should be from fat.
Most fats should come from polyunsaturated and monounsaturated fats.
Identifying Sources of Fat
Monounsaturated fat
Found in plant oils.
Common sources are nuts, and nut products, avocados.
Polyunsaturated fat
Found mainly in plant-based foods, oils, and fish.
Saturated fat
Found in animal products, dairy products, pal and coconut oils, and cocoa butter.
Identifying Sources of Fat (continued)
Omega-3 fatty acids (linolenic acid)
Good sources are canola oil, flaxseed oil, olive oil, nuts, seeds, whole grains, legumes, and green leafy vegetables.
DHA and EPA
Good sources are cod liver oil and fish.
Omega-6 fatty acids (linoleic acid)
Eggs, poultry, most vegetable oils, wheat germ oil, whole grains, baked goods, and cereals.
Nuts and seeds.
Omega-3 and Omega-6 Fatty Acids
Omega-3 Fatty Acids
Linolenic acid: An omega-3 fatty acid that is essential for human health.
Eicosapentaenoic acid (EPA): An omega-3 fatty acid made from linolenic acid, found in fish.
Docosahexaenoic acid (DHA): An omega-3 fatty acid that is especially important for brain growth and development in infants.
Omega-6 Fatty Acids
Linoleic acid: An omega-6 fatty acid that is essential for human health.
Arachidonic acid (ARA): An omega-6 fatty acid that is a precursor to the synthesis of eicosanoids.
Omega-3 and Omega-6 Fatty Acids (continued)
Omega-6 fatty acids are essential but can be harmful when they are out of balance with omega-3 fatty acids.
Elevate the risks for allergies, arthritis, asthma, coronary heart disease, diabetes, and many types of cancer, autoimmunity, and neurodegenerative diseases.
Guide to making sense of dietary fat:
When grocery shopping, read and decipher food labels carefully.
Choose unsaturated fats—fatty fish, walnuts, flaxseeds, and olive and canola oil instead of butter and highly marbled red meat.
Choose a plant-based diet.
Limit saturated fat intake.
5.5: Key Takeaways
The recommended fat intake for adults is 20% to 35% of your total caloric intake. Saturated fat must be less than 10% of your total caloric intake, and lowering this to 7% can further reduce the risks for heart disease.
Nuts, seeds, whole grains, legumes, and vegetable oil such as canola, olive, and flaxseed are excellent sources of monounsaturated and polyunsaturated fats.
The polyunsaturated fatty acids linolenic and linoleic acids are used by the body to make substances that carry out many vital functions in the body.
The omega-3 fatty-acid family includes linolenic acid, EPA, and DHA. The omega-6 fatty-acid family includes linoleic acid and ARA. DHA and ARA play crucial roles in brain and eye development. EPA and DHA found in fatty fish play important roles in inflammation reduction and disease prevention.
Many people consume too many omega-6 fatty acids and not enough omega-3 fatty acids. Omega-3 fats and omega-6 fats are precursors to hormones that have opposing properties. A proper balance between both must be obtained to avoid health problems.
5.6: Lipids and Disease
Learning Objective
Describe how saturated fat and cholesterol intake can impact health.
Watch Out for Saturated Fat and Cholesterol
Average American female consumes 242 milligrams of dietary cholesterol per day, and for males the figure is slightly higher—about 348 milligrams.
Improper dietary fat consumption can lead to severe health problems:
Cardiovascular disease
Heart attack
Ischemic stroke
Congestive heart failure
Arrhythmia
Heart valve problems
Watch Out for Saturated Fat and Cholesterol (continued)
What You Can Do
Limit saturated fats
Replace current dietary fats with an increased intake of monounsaturated fats
Choose whole-grain and high fiber foods
Do not be sedentary—get more exercise on a regular basis
5.6: Key Takeaways
Food cholesterol does not affect blood cholesterol as much as some people think. The main cause of unfavorable blood cholesterol values is an overconsumption of saturated fats.
An increased intake of lipids is associated with heart disease, obesity, cardiovascular disease, and other problems.
Making dietary choices that limit the intake of saturated fat to the recommended levels, replacing saturated fat with monounsaturated and polyunsaturated fats, increasing physical activity, and cessation of smoking can reduce the risk of developing heart disease and other ailments.