Lipids: Structure, Function, Digestion, and Health Implications

Chapter 5: Lipids

Introduction to Lipids

Lipids are a diverse group of hydrophobic compounds essential for human health. They include triglycerides, phospholipids, and sterols, each with unique structures and functions. Lipids are a major source of energy, play structural roles in cell membranes, and are involved in hormone synthesis and nutrient transport.

Classification and Structure of Lipids

Types of Lipids

• Triglycerides: Composed of three fatty acids attached to a glycerol backbone. They are the most common dietary and body fat.

• Phospholipids: Contain a glycerol backbone, two fatty acids, and a phosphate group. They are key components of cell membranes.

• Sterols: Have a complex ring structure. Cholesterol is the most well-known sterol, serving as a precursor for vitamin D, bile acids, and steroid hormones.

Fatty Acids: Chain Length and Saturation

Fatty acids differ by carbon chain length, degree of saturation, and shape:

• Short-chain: 2–4 carbons

• Medium-chain: 6–10 carbons

• Long-chain: 12 or more carbons (most common in foods)

The degree of saturation refers to the presence of double bonds:

• Saturated fatty acids: No double bonds; solid at room temperature; mainly from animal sources.

• Unsaturated fatty acids: One or more double bonds; liquid at room temperature; mainly from plant and fish sources.

• Monounsaturated fatty acids (MUFA): One double bond (e.g., olive oil).

• Polyunsaturated fatty acids (PUFA): Two or more double bonds (e.g., walnuts, flaxseed).

• Trans fats: Unsaturated fats chemically altered to behave like saturated fats; associated with negative health effects.

Essential Fatty Acids and Omega Classification

• Omega-3 fatty acids: First double bond at the third carbon from the methyl end (e.g., alpha-linolenic acid).

• Omega-6 fatty acids: First double bond at the sixth carbon from the methyl end (e.g., linoleic acid).

These essential fatty acids must be obtained from the diet and are important for inflammation regulation and cell membrane structure.

Functions of Lipids in the Body

Physiological Roles

• Energy storage (9 kcal/g, more than double that of carbohydrates and proteins)

• Insulation and protection of organs

• Component of cell membranes (phospholipid bilayer)

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

• Transport of fat-soluble vitamins (A, D, E, K)

Digestion, Absorption, and Transport of Lipids

Digestion Process

Lipid digestion begins in the mouth with lingual lipase and continues in the stomach with gastric lipase. Most digestion occurs in the small intestine, where bile acids emulsify fats, allowing pancreatic lipase to break them down into fatty acids and monoglycerides.

Absorption and Transport

• Short- and medium-chain fatty acids are absorbed directly into the bloodstream.

• Long-chain fatty acids are reassembled into triglycerides and packaged into chylomicrons for transport via the lymphatic system.

• Lipoproteins (chylomicrons, VLDL, LDL, HDL) transport lipids in the blood.

Lipoprotein Types and Health Implications

• Chylomicrons: Transport dietary lipids from the intestine to tissues.

• VLDL (Very-Low-Density Lipoprotein): Delivers triglycerides from the liver to tissues.

• LDL (Low-Density Lipoprotein): Delivers cholesterol to cells; high levels are associated with increased heart disease risk.

• HDL (High-Density Lipoprotein): Removes cholesterol from cells and returns it to the liver; high levels are protective.

Dietary Recommendations and Food Sources

Recommended Intake

• Acceptable Macronutrient Distribution Range (AMDR) for fat: 20–35% of daily calories

• Limit saturated fat and trans fat intake

• Emphasize unsaturated fats, especially omega-3 fatty acids

• The liver synthesizes all the cholesterol the body needs; dietary cholesterol intake should be moderate

Food Sources of Lipids

• Good sources: Extra virgin olive oil, raw nuts and seeds, avocado, fatty fish, grass-fed meats, eggs, grass-fed dairy

• Poor sources: Refined/processed vegetable oils, margarine, fried foods, hydrogenated/trans fats

Lipids and Cardiovascular Health

Atherosclerosis and Heart Disease

Cardiovascular disease (CVD) is the leading cause of death in adults. Atherosclerosis, the buildup of fatty plaques in arteries, is a primary cause. Risk factors include high LDL cholesterol, low HDL cholesterol, hypertension, smoking, and poor diet.

Blood Cholesterol Levels and Interpretation

Total Cholesterol (mg/dL)	Interpretation
< 200	Desirable
200–239	Borderline high
≥ 240	High

LDL Cholesterol (mg/dL)	Interpretation
< 100	Optimal
100–129	Near or above optimal
130–159	Borderline high
160–189	High
≥ 190	Very high

HDL Cholesterol (mg/dL)	Interpretation
> 60	Desirable
40–60	Adequate
< 40	Low

Strategies to Lower LDL Cholesterol

Dietary Changes	Lifestyle Changes
Consume less saturated fat	Lose excess body weight
Strictly limit trans fats	Exercise more
Consume less dietary cholesterol	Quit smoking
Consume more foods rich in soluble fiber
Consume a plant-based diet

Exercise and Smoking Cessation

• Regular exercise raises HDL, lowers LDL, and reduces blood pressure and insulin resistance.

• Smoking damages arteries and accelerates atherosclerosis; quitting smoking significantly reduces heart attack risk.