Fats, Oils, and Other Lipids: Structure, Function, and Nutrition

Fats, Oils, and Other Lipids

Overview and Objectives

This chapter explores the classification, structure, digestion, absorption, and functions of fats and other lipids in the human body. It also covers dietary recommendations, food sources, fat substitutes, and the relationship between lipids and heart disease.

• Describe the three classifications of lipids and explain the differences in the structure of triglycerides, phospholipids, and cholesterol.

• Explain how fat is digested, absorbed, and transported in the body.

• Identify the functions of fat in the body.

• Define dietary recommendations for total fat, essential fatty acids, saturated fat, cholesterol, and trans fat.

• Identify major food sources of different types of fats.

• Compare different fat substitutes used in food products.

• Describe the development of atherosclerosis and its role in heart disease risk.

• Explain dietary changes to maintain healthy blood cholesterol and reduce heart disease risk.

Classification and Structure of Lipids

What Are Fats and Why Do You Need Them?

Lipids are a category of compounds containing carbon, hydrogen, and oxygen that are hydrophobic (insoluble in water). Fat is the common name for one type of lipid, known as a triglyceride.

• Functions in Foods:

  • Give flaky texture to baked goods

  • Make meats tender

  • Provide flavor and aromas

  • Contribute to satiety

Fats and other lipids perform important functions in the body:

• Energy storage

• Insulation

• Transport of proteins in blood

• Cell membrane structure

There are three main types of lipids found in foods and in your body:

• Triglycerides (fats)

• Phospholipids

• Sterols

The basic unit of triglycerides and phospholipids is the fatty acid.

Structure of Fatty Acids

Fatty acids are chains of carbon and hydrogen atoms with a carboxyl group (COOH) at one end. There are over 20 different fatty acids, which can vary by:

1. Length of chain

2. Whether carbons have double or single bonds between them

3. Total number of double bonds

Figure 5.1 Structure of a Fatty Acid:

• Carboxyl (acid) group at one end

• Chain of carbon and hydrogen atoms

Types of Fatty Acids

Fatty acids are classified based on their saturation:

• Saturated fatty acids: All carbons bonded to hydrogen; solid at room temperature (e.g., stearic acid)

• Monounsaturated fatty acids (MUFAs): One double bond; liquid at room temperature (e.g., oleic acid in olive oil)

• Polyunsaturated fatty acids (PUFAs): More than one double bond; liquid at room temperature (e.g., linoleic acid, alpha-linolenic acid)

Triglycerides

Triglyceride: Three fatty acids connected to a glycerol backbone. This is the most common lipid found in foods and the body.

• Saturated fats contain mostly saturated fatty acids

• Unsaturated fats contain mostly unsaturated fatty acids

• Oils are fats that are liquid at room temperature

Triglyceride Structure:

• Glycerol backbone

• Three fatty acid chains

Phospholipids

Phospholipids have a glycerol backbone, two fatty acids, and a phosphorus-containing group. The phosphorus-containing head is hydrophilic (attracts water), while the fatty acid tail is hydrophobic (repels water).

• Major component of cell membranes (phospholipid bilayer)

• Lecithin is a major phospholipid in cell membranes and acts as an emulsifier in foods (e.g., salad dressings)

Sterols

Sterols are composed mainly of four connecting rings of carbon and hydrogen. The most well-known sterol is cholesterol.

• Important role in cell membrane structure

• Precursor for vitamin D, bile acids, and sex hormones

• Not required in the diet since the body synthesizes all needed cholesterol

Digestion, Absorption, and Transport of Fats

Fat Digestion and Absorption

Fat digestion begins in the mouth with chewing and lingual lipase, continues in the stomach with gastric lipase, and is completed in the small intestine.

• Bile acids emulsify fat, breaking fat globules into smaller pieces

• Pancreatic lipase breaks down fats into two fatty acids and a monoglyceride

• Lecithin in bile helps package monoglycerides and fatty acids into micelles for absorption

• Short-chain fatty acids enter the bloodstream directly

• Long-chain fatty acids enter the lymphatic system and require transport carriers

Lipoproteins and Fat Transport

Lipoproteins are complexes that transport fat through the lymph and blood.

• Chylomicrons: Carry digested fat through lymph into bloodstream

• Very low-density lipoproteins (VLDL): Deliver fat made in the liver to cells

• Low-density lipoproteins (LDL): Deposit cholesterol on artery walls ("bad" cholesterol)

• High-density lipoproteins (HDL): Remove cholesterol from the body and deliver it to the liver for excretion ("good" cholesterol)

Functions of Fat in the Body

Physiological Roles

• Energy-dense source of fuel: 9 calories per gram

• Glucagon stimulates release of fat for energy when blood glucose declines

• Needed for absorption of fat-soluble vitamins (A, D, E, K) and carotenoids

• Insulates the body to maintain temperature

• Cushions bones, organs, and nerves

Essential Fatty Acids

Linoleic acid (omega-6) and alpha-linolenic acid (omega-3) are essential fatty acids. They help maintain healthy skin cells, nerve function, and cell membranes, and are precursors to eicosanoids (hormone-like substances involved in inflammation, blood clotting, and blood pressure).

• Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are heart-healthy omega-3 fatty acids found in fatty fish (e.g., salmon, herring, sardines)

Cholesterol Functions

• Part of cell membranes

• Precursor for vitamin D, bile acids, and sex hormones

• Dietary cholesterol does not determine blood cholesterol; the body synthesizes all it needs

Dietary Recommendations for Fats

Recommended Intake

• AMDR (Acceptable Macronutrient Distribution Range): 20–35% of total daily calories should come from fat

• Fat provides more than twice the calories per gram compared to carbohydrates or protein

• For heart health, consume less than 10% (ideally less than 7%) of calories from saturated fats

Essential Fatty Acids

• 5–10% of total calories should come from linoleic acid

• Alpha-linolenic acid should make up 0.6–1.2% of total calories

Saturated Fat and Trans Fat

• Minimize intake of saturated fat and trans fat

• Excess saturated fat raises LDL cholesterol

• Trans fats are created by hydrogenation, increase shelf life, and are worse for heart health than saturated fat

• Trans fats raise LDL and lower HDL cholesterol

Cholesterol

• The body makes all the cholesterol it needs

• Healthy individuals over age 2 should limit dietary cholesterol to less than 300 mg daily

• Dietary cholesterol has less impact on blood cholesterol than saturated fat

Food Sources of Fats

Major Food Sources

• Unsaturated fats (MUFAs and PUFAs) are abundant in vegetable oils, soybeans, walnuts, peanut butter, flaxseeds, and wheat germ

• Vegetable oils, nuts, and flaxseeds are good sources of essential fatty acids

• Saturated and trans fats are found in animal products, processed foods, and some baked goods

Fat Substitutes

Types and Uses

Fat substitutes are designed to provide the creamy properties of fat with fewer calories. They can be carbohydrate-, protein-, or fat-based, with most being carbohydrate-based using plant polysaccharides.

Type	Calories per Gram	Properties	How Used
Carbohydrate-based	4	Thickener, water retention	Baked goods, dairy products, sauces
Protein-based	4	Gel-like texture	Processed meats, dairy, dressings
Fat-based	9	Modified fats	Snack foods, baked goods
Additional info: Some fat substitutes are engineered to pass through the digestive tract without being absorbed, reducing calorie intake.

Heart Disease and Lipids

Atherosclerosis

Atherosclerosis is the narrowing of arteries due to buildup of plaque (cholesterol-laden foam cells, platelets, and other substances). It increases the risk of blood clots, heart attack, and stroke.

• Begins with injury to artery lining, contributed by high blood pressure, high cholesterol, and smoking

Risk Factors for Heart Disease

Factors You Cannot Control	Factors You Can Control
Age and sex	Type 2 diabetes mellitus
Family history of heart disease	High blood pressure
Type 1 diabetes mellitus	Smoking
	Physical inactivity
	Excess weight
	Low HDL cholesterol
	High LDL cholesterol

Other potential risk factors include high levels of homocysteine, lipoprotein(a), C-reactive protein, and apolipoprotein B.

Metabolic syndrome is a group of risk factors, including insulin resistance, that increase the risk of heart disease.

Maintaining Healthy Blood Cholesterol

• Minimize saturated fats, trans fats, and cholesterol in diet

• Include fish in weekly choices

• Eat plenty of plant foods, especially those high in soluble fiber

• Select foods rich in antioxidants and phytochemicals

• Exercise regularly and manage weight

• Moderate use of alcohol may reduce risk (not recommended for everyone)

Plant Sterols and Antioxidants

• Plant sterols compete with cholesterol for absorption, helping lower LDL cholesterol

• Sources: soybean oil, fruits, vegetables, legumes, sesame seeds, nuts, cereals

• Antioxidants (e.g., flavonoids) may help reduce LDL cholesterol and inhibit blood clot formation

Dietary Strategies to Lower Saturated Fat

• Replace high-fat dairy with low-fat or nonfat options

• Use jelly instead of butter on toast

• Choose reduced-fat cheese

• Opt for low-fat frozen yogurt instead of ice cream

Mediterranean Diet

The traditional Mediterranean diet is associated with lower risk of heart disease and cancer. It emphasizes:

• Plant-based foods: whole grains, fruits, vegetables, legumes, nuts

• Olive oil, low-fat dairy, water

• Occasional fish, poultry, eggs, meat, sweets, wine

• Active lifestyle, family meals, and community support

Adopting a Mediterranean-style diet can reduce heart disease risk by up to 30%.

Summary Table: Types of Lipids

Type	Main Structure	Key Functions	Examples
Triglycerides	Glycerol + 3 fatty acids	Energy storage, insulation	Fats, oils
Phospholipids	Glycerol + 2 fatty acids + phosphate group	Cell membrane structure, emulsification	Lecithin
Sterols	Four carbon rings	Cell membrane, precursor for hormones	Cholesterol

Key Equations

• Energy from Fat:

$\text{Calories from fat} = \text{grams of fat} \times 9$

• AMDR for Fat:

$\text{Recommended fat intake} = 0.20 \times \text{total calories} \text{ to } 0.35 \times \text{total calories}$

Additional info: Some content was expanded for clarity and completeness, including the summary tables and equations.