Introduction of Lipids

Short Important Questions

1. What are the consequences of excess fat consumption? (1 Mark, 2016)
2. Enlist any three essential fatty acids. (1 Mark, 2018)
3. List the functions of cholesterol. (1 Mark, 2019)
4. What is essential fatty acid? Give examples. (1 Mark, 2022)
5. What is Cholesterol? (1 Mark, 2023)
6. What are PUFA? Give two examples. (1 Mark, 2026)

Long Important Questions

1. Differentiate between saturated and unsaturated fats. List down the functions of fat. (10 Marks, 2017)
2. Differentiate between saturated and unsaturated fats. (5 Marks, 2019)
3. What is cholesterol? List down the sources and functions of cholesterol. (10 Marks, 2021)
4. What is cholesterol? Explain sources of cholesterol and its role in the human body. (5 Marks, 2025)

Introduction to Lipids

Lipids are one of the three major macronutrients required by the human body. They are a diverse group of organic compounds composed mainly of carbon, hydrogen, and oxygen, although they contain much less oxygen than carbohydrates. Lipids include fats, oils, waxes, phospholipids, and sterols. They are insoluble in water but dissolve readily in organic solvents such as ether, chloroform, and benzene.

Lipids are the most concentrated source of energy, providing approximately 9 kcal of energy per gram, which is more than twice the energy supplied by carbohydrates or proteins. In addition to supplying energy, lipids help maintain body temperature by providing insulation, protect vital organs from physical injury, form an essential part of cell membranes, and aid in the absorption of fat-soluble vitamins (A, D, E, and K). They also contribute to normal growth, hormone production, and overall health.

Simple Definition: Lipids are organic compounds, including fats and oils, that are insoluble in water but soluble in organic solvents. Detailed Definition: Lipids are a group of organic compounds composed mainly of carbon, hydrogen, and oxygen. They are insoluble in water, soluble in organic solvents, and serve as a concentrated source of energy while performing important structural and physiological functions in the body.

Characteristics of Lipids

• Composed mainly of carbon, hydrogen, and oxygen.
• Insoluble in water but soluble in organic solvents.
• Include fats, oils, waxes, phospholipids, and sterols.
• Provide approximately 9 kcal of energy per gram.
• Stored in adipose (fat) tissue as an energy reserve.
• Essential for normal growth, development, and body functions.

Common Sources of Lipids

Lipids are obtained from both plant and animal sources.

Plant Sources	Animal Sources
Vegetable oils Nuts Seeds Avocados Coconut	Butter Ghee Cheese Milk and dairy products Meat Fish Egg yolk

Essential and Non-essential Fatty Acids

Fatty acids are the basic building blocks of fats (lipids). They are long chains of carbon atoms with hydrogen atoms attached and a carboxyl (-COOH) group at one end. Fatty acids combine with glycerol to form triglycerides, which are the main form of fat found in foods and stored in the body.

Based on the body’s ability to synthesize them, fatty acids are classified into essential fatty acids and non-essential fatty acids.

A. Essential Fatty Acids (EFAs)

Essential fatty acids are fatty acids that cannot be synthesized by the human body in sufficient amounts and must be obtained through the diet.

Essential fatty acids are polyunsaturated fatty acids that the human body cannot produce in adequate quantities. Therefore, they must be obtained from food to support normal growth, brain function, cell structure, and overall health.

Examples of Essential Fatty Acids

• Linoleic acid (Omega-6)
• Alpha-linolenic acid (Omega-3)
• Arachidonic acid* (Essential mainly during infancy or when it cannot be synthesized adequately.)

Note: In humans, linoleic acid and alpha-linolenic acid are the truly essential fatty acids. Arachidonic acid is often included because it becomes essential under certain conditions.

Sources of Essential Fatty Acids

• Sunflower oil
• Soybean oil
• Flaxseed (linseed)
• Walnuts
• Chia seeds
• Fatty fish (salmon, sardines, mackerel)
• Fish oil

Functions of Essential Fatty Acids

• Support normal growth and development.
• Maintain healthy skin and hair.
• Promote normal brain and nervous system function.
• Help form and maintain healthy cell membranes.
• Aid in the production of hormone-like substances (eicosanoids).
• Support heart health and immune function.

Non-essential Fatty Acids

Non-essential fatty acids are fatty acids that can be synthesized by the human body from other nutrients, so they do not need to be obtained from the diet.

Examples of Non-essential Fatty Acids

• Palmitic acid
• Stearic acid
• Oleic acid

Functions of Non-essential Fatty Acids

• Provide energy.
• Store excess energy in adipose tissue.
• Form structural components of cell membranes.
• Support normal metabolic processes.

Difference Between Essential and Non-essential Fatty Acids

Essential Fatty Acids (EFAs)	Non-essential Fatty Acids (NEFAs)
Cannot be synthesized by the body in sufficient amounts.	Can be synthesized by the body from carbohydrates, proteins, or other fatty acids.
Must be obtained through the diet.	Do not need to be obtained from the diet.
Mainly belong to the polyunsaturated fatty acid (PUFA) group.	May be saturated, monounsaturated, or some polyunsaturated fatty acids synthesized by the body.
Essential for normal growth, brain development, and cell membrane function.	Mainly serve as a source of energy and structural components of cells.
Deficiency may cause poor growth, dry skin, impaired wound healing, and weakened immunity.	Deficiency is rare because the body can produce them.
Found mainly in vegetable oils, nuts, seeds, and fatty fish.	Found in both plant and animal foods and also produced within the body.
Examples: Linoleic acid (Omega-6), Alpha-linolenic acid (Omega-3), Arachidonic acid* (conditionally essential).	Examples: Palmitic acid, Stearic acid, Oleic acid.

Saturated and Unsaturated Fatty Acids

Fatty acids are classified according to the presence or absence of double bonds between carbon atoms in their hydrocarbon chain. Based on this, they are divided into saturated fatty acids and unsaturated fatty acids. Both types are important in the diet, but they differ in their chemical structure, physical properties, food sources, and effects on health.

Saturated Fatty Acids (SFAs)

Saturated fatty acids are fatty acids that contain only single bonds between carbon atoms.

Saturated fatty acids are fatty acids in which all carbon atoms are connected by single bonds, meaning they contain the maximum possible number of hydrogen atoms. They are usually solid at room temperature.

Characteristics of Saturated Fatty Acids

• Contain only single carbon-carbon (C–C) bonds.
• Fully saturated with hydrogen atoms.
• Usually solid at room temperature.
• More chemically stable and less likely to become rancid.
• Excessive intake may increase LDL (bad) cholesterol and the risk of cardiovascular diseases.

Sources of Saturated Fatty Acids

Animal Sources	Plant Sources
Butter Ghee Cheese Whole milk Fatty meat Lard	Coconut oil Palm oil

Examples of Saturated Fatty Acids

• Palmitic acid
• Stearic acid
• Myristic acid

Unsaturated Fatty Acids (UFAs)

Unsaturated fatty acids are fatty acids that contain one or more double bonds between carbon atoms.

Unsaturated fatty acids are fatty acids containing one or more carbon-carbon double bonds. They are generally liquid at room temperature and are considered healthier than saturated fats when consumed in appropriate amounts.

Characteristics of Unsaturated Fatty Acids

• Contain one or more double bonds (C=C).
• Not completely saturated with hydrogen atoms.
• Usually liquid at room temperature.
• Help reduce LDL (bad) cholesterol and support heart health.
• More prone to oxidation and rancidity.

Types of Unsaturated Fatty Acids

1. Monounsaturated Fatty Acids (MUFA)
   It contain one double bond in the carbon chain. It’s main sources are olive oil, canola oil, avocados, peanuts, almonds, etc.
   For example: Oleic acid and Palmitoleic acid
2. Polyunsaturated Fatty Acids (PUFA)
   It contain two or more double bonds in the carbon chain. It’s main sources are sunflower oil, soybean oil, flaxseed, walnuts and fatty fish.
   For example: Linoleic acid (Omega-6), Alpha-linolenic acid (Omega-3), Arachidonic acid

Difference Between Saturated and Unsaturated Fatty Acids

Saturated Fatty Acids	Unsaturated Fatty Acids
Contain only single carbon-carbon bonds.	Contain one or more carbon-carbon double bonds.
Fully saturated with hydrogen atoms.	Not fully saturated with hydrogen atoms.
Usually solid at room temperature.	Usually liquid at room temperature.
Mainly found in animal fats and tropical oils.	Mainly found in vegetable oils, nuts, seeds, and fish.
More stable and less prone to oxidation.	Less stable and more prone to oxidation.
Excess intake may increase LDL cholesterol and heart disease risk.	Generally lowers LDL cholesterol and supports heart health.
Examples: Butter, ghee, coconut oil, palmitic acid.	Examples: Olive oil, sunflower oil, oleic acid, linoleic acid.

Trans Fats

Trans fats are a type of unsaturated fatty acid in which the hydrogen atoms are arranged in a trans configuration around the carbon-carbon double bond. Most trans fats are produced artificially through the process of partial hydrogenation, in which hydrogen is added to liquid vegetable oils to make them more solid and increase their shelf life. Small amounts of trans fats also occur naturally in the meat and milk of ruminant animals such as cows and sheep.

Trans fats improve the texture, stability, and shelf life of processed foods. However, excessive consumption has harmful effects on health, particularly on the cardiovascular system. For this reason, health organizations recommend limiting or avoiding trans fat intake.

Simple Definition: Trans fats are unsaturated fats in which the hydrogen atoms are arranged in a trans configuration around the double bond. Definition: Trans fats are a type of unsaturated fatty acid, mainly formed during the partial hydrogenation of vegetable oils, in which the hydrogen atoms are positioned on opposite sides of the double bond. They are associated with an increased risk of cardiovascular disease.

Sources of Trans Fats

Artificial Sources	Natural Sources
Margarine Vanaspati (hydrogenated vegetable oil) Shortening Biscuits and cookies Cakes and pastries Doughnuts Fried fast foods Packaged snack foods	Small amounts are naturally found in: Beef Lamb Butter Cheese Whole milk

Health Effects of Trans Fats
Excessive intake of trans fats can:

1. Increase LDL (bad) cholesterol.
2. Decrease HDL (good) cholesterol.
3. Increase the risk of cardiovascular diseases.
4. Increase the risk of stroke.
5. Promote inflammation in the body.
6. Increase the risk of type 2 diabetes and obesity.

How to Reduce Trans Fat Intake

1. Choose fresh and minimally processed foods.
2. Limit consumption of fried and fast foods.
3. Avoid foods containing partially hydrogenated oils.
4. Use healthier oils such as olive, sunflower, or canola oil.
5. Read food labels before purchasing packaged foods.

Functions of Lipids (Fats)

Lipids perform several essential functions in the human body. Besides serving as the most concentrated source of energy, they play important roles in growth, protection, metabolism, and the maintenance of overall health.

1. Provide Energy
   The primary function of lipids is to supply energy. Lipids are the most concentrated source of dietary energy, providing approximately 9 kcal of energy per gram, which is more than twice the energy provided by carbohydrates or proteins.
2. Store Energy
   Excess energy obtained from food is stored as fat in adipose (fat) tissue. These fat reserves serve as an energy source during fasting, illness, or prolonged physical activity.
3. Insulate the Body
   Fat deposited beneath the skin acts as an insulating layer, helping maintain normal body temperature by reducing heat loss.
4. Protect Vital Organs
   Fat cushions and protects delicate organs such as the heart, kidneys, liver, and eyes from physical injury and mechanical shock.
5. Aid in the Absorption of Fat-Soluble Vitamins
   Lipids are essential for the absorption and transportation of fat-soluble vitamins A, D, E, and K in the intestine.
6. Form Cell Membranes
   Lipids, especially phospholipids and cholesterol, are major structural components of cell membranes and help maintain their integrity and flexibility.
7. Supply Essential Fatty Acids
   Lipids provide essential fatty acids such as linoleic acid (Omega-6) and alpha-linolenic acid (Omega-3), which are necessary for normal growth, brain development, healthy skin, and immune function.
8. Support Hormone Production
   Lipids, particularly cholesterol, are required for the synthesis of steroid hormones such as estrogen, progesterone, testosterone, and cortisol. They also contribute to the production of vitamin D and bile acids.
9. Improve the Taste and Texture of Foods
   Fats enhance the flavour, aroma, texture, and mouthfeel of foods, making meals more palatable and satisfying.
10. Promote Satiety and reduce hunger
    Lipids slow gastric emptying and digestion, helping people feel full for a longer period after eating and reducing hunger.

Cholesterol

Cholesterol is a waxy, fat-like substance (sterol) found in the cells of all animals. It is an essential component of cell membranes and is required for the synthesis of steroid hormones, vitamin D, and bile acids. Although cholesterol is vital for normal body functions, excessive levels of cholesterol in the blood, particularly low-density lipoprotein (LDL) cholesterol, increase the risk of cardiovascular diseases.

The human body produces most of the cholesterol it needs, mainly in the liver, while the remaining amount is obtained from foods of animal origin.

Simple Definition: Cholesterol is a waxy, fat-like sterol found in animal cells that is essential for the formation of cell membranes, hormones, vitamin D, and bile acids. Definition: Cholesterol is a waxy, fat-like sterol synthesized mainly by the liver and also obtained from animal foods. It is an essential component of cell membranes and serves as a precursor for the synthesis of steroid hormones, vitamin D, and bile acids.

Sources of Cholesterol

Cholesterol is found only in foods of animal origin.

Animal Sources

• Egg yolk
• Liver and other organ meats
• Red meat
• Poultry
• Fish and seafood
• Butter
• Cheese
• Whole milk and cream
• Ghee

Note: Plant foods do not contain cholesterol. However, they contain phytosterols, which may help reduce the absorption of cholesterol in the intestine.

Functions of Cholesterol
Cholesterol performs several important functions in the human body:

1. Forms Cell Membranes
   Cholesterol is an important structural component of cell membranes, helping maintain their strength, stability, and flexibility.
2. Produces Steroid Hormones
   It serves as the precursor for steroid hormones such as:
   • Estrogen
   • Progesterone
   • Testosterone
   • Cortisol
   • Aldosterone
3. Helps Produce Vitamin D
   Cholesterol is converted into vitamin D in the skin when exposed to sunlight.
4. Forms Bile Acids
   The liver converts cholesterol into bile acids, which help digest and absorb dietary fats and fat-soluble vitamins.
5. Supports Normal Brain and Nerve Function
   Cholesterol is an important component of nerve cells and contributes to the normal functioning of the brain and nervous system.
6. Supports Growth and Tissue Repair
   Cholesterol is required for the formation of new cells and the repair of damaged tissues.

Effects of Excess Cholesterol
Excess cholesterol, especially LDL (bad cholesterol), can:

• Accumulate in the walls of arteries.
• Form plaques (atherosclerosis).
• Reduce blood flow.
• Increase the risk of heart attack and stroke.

Types of Cholesterol

1. LDL (Low-Density Lipoprotein)
   • Known as “bad cholesterol.”
   • Carries cholesterol from the liver to body tissues.
   • High levels increase the risk of heart disease.
2. HDL (High-Density Lipoprotein)
   • Known as “good cholesterol.”
   • Carries excess cholesterol back to the liver for removal.
   • Helps protect against cardiovascular disease.