LIPIDS Biochemistry and Health Guide to Lipids

Overview and Classification of Lipids

• Definition and Family of Lipids

  • Lipids represent a family of organic compounds that are insoluble in water (hydrophobic). The lipid family includes:

  • Triglycerides (fats and oils)

  • Phospholipids

  • Sterols

• Clinical Significance of Blood Lipids

  • The concentration and proportion of lipids circulating in the bloodstream directly reflect the statistical probability of developing cardiovascular disease.

  • A lipid profile is a clinical blood test that measures:

  • Total cholesterol

  • High-density lipoprotein (HDL) cholesterol

  • Low-density lipoprotein (LDL) cholesterol

  • Triglycerides

Structure and Chemistry of Fatty Acids and Triglycerides

• Triglycerides

  • Represent the most abundant class of lipids, comprising approximately $95\%$ of all lipids found in both food and the human body.

  • Molecular composition consists of one glycerol backbone attached to three fatty acids.

  • Fatty acids vary significantly in chain length and degree of saturation.

  • Most dietary fatty acids consist of long-chain fatty acids.

• Condensation Synthesis of Triglycerides

  • Triglycerides are formed through a series of three condensation reactions:

  • An hydrogen ($\text{H}$) atom from a hydroxyl group on glycerol combines with a hydroxyl ($\text{OH}$) group from the carboxylic acid end of a fatty acid.

  • This reaction forms a molecule of water ($\text{H}_2\text{O}$) and leaves an oxygen atom bridging the glycerol backbone to the carbon at the acid end of the fatty acid.

  • The complete assembly of one triglyceride molecule from one glycerol and three fatty acids yields three molecules of water ($3\,\text{H}_2\text{O}$).

  • Most natural triglycerides contain a mixture of different fatty acids (mixed triglycerides).

• Chemical Nature and Saturation of Fatty Acids

  • Every carbon atom in a fatty acid chain must form four covalent bonds.

  • The degree of saturation is determined by the presence or absence of double bonds ($\text{C=C}$), which represent points of unsaturation.

  • Saturated Fatty Acids:

  • Fully loaded with hydrogen atoms; contain zero double bonds ($\text{C=C}$).

  • Possess a straight, linear chemical structure allowing molecules to stack tightly together.

  • Solid or firm at room temperature.

  • Examples include stearic acid ($18$ carbons, saturated).

  • Abundant in animal fats (butter, lard, beef tallow, chicken fat) and tropical oils (coconut oil, palm oil, palm kernel oil).

  • Monounsaturated Fatty Acids:

  • Contain exactly one double bond ($\text{C=C}$).

  • The double bond introduces a bend or kink in the hydrocarbon chain, preventing tight packing.

  • Liquid at room temperature.

  • Examples include oleic acid ($18$ carbons, monounsaturated).

  • Abundant in olive oil, canola oil, peanut oil, and avocados.

  • Polyunsaturated Fatty Acids:

  • Contain two or more double bonds ($\text{C=C}$).

  • Possess multiple kinks in their structure, maintaining a liquid state at room temperature.

  • Major classes include Omega-3 and Omega-6 fatty acids (defined by the position of the first double bond relative to the methyl/omega carbon end):

    • Linoleic Acid (Omega-6):

    • An $18$-carbon polyunsaturated fatty acid with $2$ double bonds, where the first double bond is located $6$ carbons from the methyl end.

    • Abundant in vegetable oils (corn, safflower, soybean, cottonseed, sunflower), meats, pine nuts, and walnuts.

    • Serves as the precursor for arachidonic acid.

    • Linolenic Acid (Omega-3):

    • An $18$-carbon polyunsaturated fatty acid with $3$ double bonds, where the first double bond is located $3$ carbons from the methyl end.

    • Abundant in flaxseed oil, walnuts, soybeans, canola oil, and fatty fish.

    • Serves as the precursor for eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).

    • Direct sources of EPA and DHA include human breast milk, cold-water fish (salmon, mackerel, herring, tuna), and shellfish.

• Physical Characteristics and Stability of Fats

  • Firmness:

  • Polyunsaturated plant oils are liquid at room temperature.

  • Saturated animal fats are solid at room temperature.

  • Tropical plant oils (palm oil, palm kernel oil, coconut oil) are high in saturated fatty acids and are solid at room temperature.

  • Stability and Oxidation:

  • Unsaturated fatty acids are vulnerable to oxidation due to double bonds.

  • Oxidation causes fats to spoil, producing compounds with rancid odors and flavors.

• Eicosanoids:

  • A biologically active, hormone-like group of signaling compounds synthesized from $20$-carbon omega-3 and omega-6 polyunsaturated fatty acids.

  • Omega-3 Derived Eicosanoids:

  • Provide systemic health benefits: reduce blood pressure, prevent blood clot formation, protect against irregular heartbeats, and suppress inflammatory responses.

  • Omega-6 Derived Eicosanoids:

  • Promote inflammatory responses, stimulate blood clot formation, and induce blood vessel constriction.

• Hydrogenation and Trans-Fatty Acids

  • Hydrogenation:

  • A chemical process where hydrogen atoms ($\text{H}^+$) are added to double bonds of polyunsaturated fatty acids to convert them into saturated fatty acids.

  • Advantages: Increases product shelf-life by protecting against oxidation/rancidity; improves food texture and aesthetic properties; converts liquid vegetable oils into solid or spreadable fats (e.g., margarine, shortening).

  • Disadvantages: Eliminates health benefits of unsaturated fats; generates hazardous trans-fatty acids.

  • Cis vs. Trans Isomer Configurations:

  • Cis-fatty acids: Hydrogens attached to the double bond carbons are on the same side, producing a U-like bent conformation. Most naturally occurring unsaturated fatty acids in foods are in the cis configuration.

  • Trans-fatty acids: Hydrogens attached to the double bond carbons are on opposite sides, producing a straight, linear structure similar to saturated fats. Occurs primarily in partially hydrogenated processed foods.

  • Sources of Trans Fats: Commercial baked goods (doughnuts, cakes, cookies), fried foods, hydrogenated shortening, hydrogenated/partially hydrogenated margarines, non-dairy creamers, fast food, and snack foods (chips, crackers, microwave popcorn).

Phospholipids and Sterols

• Phospholipids

  • Chemical Structure:

  • Consists of a glycerol backbone attached to two fatty acids and one phosphate group joined to a molecule of choline.

  • Amphipathic Properties: Fatty acid tails make phospholipids fat-soluble (lipophilic), while the phosphate group and choline head make them water-soluble (hydrophilic).

  • Biological and Industrial Roles:

  • Essential structural components of cellular membranes (lipid bilayers).

  • Act as versatile emulsifiers in the food industry, allowing fat and water mixtures to blend (used in candy bars, mayonnaise).

  • Natural food sources include eggs, liver, soybeans, peanuts, and wheat germ.

  • Lecithin:

  • The most widely recognized phospholipid, synthesized endogenously by the liver.

  • Yields $9\,\text{kcal/g}$ of energy.

  • Supplementation is ineffective because oral lecithin is hydrolyzed by the enzyme lecithinase in the digestive tract.

• Sterols

  • Chemical Structure:

  • Compounds featuring a complex ring system of carbon, hydrogen, and oxygen atoms with various side chains attached.

  • Examples include cholesterol, bile acids, steroid hormones (sex and adrenal hormones), and Vitamin D3.

  • Distribution in Foods:

  • Both plant and animal sources contain sterols (plant sterols compete with cholesterol absorption).

  • Only foods of animal origin contain cholesterol; plant foods (grains, fruits, vegetables, legumes) contain zero cholesterol.

  • Endogenous Cholesterol Synthesis and Function:

  • Synthesized in the liver at a rate of $800\text{--}1500\,\text{mg/day}$.

  • Serves as an essential structural constituent of cell membranes.

  • Acts as the metabolic precursor for Vitamin D3, bile acids, estrogen, testosterone, and cortisol.

  • Becomes detrimental when present in excess concentrations, depositing inside arterial walls to initiate and drive atherosclerosis.

  • Common Misconceptions About Cholesterol:

  • Substituting margarine for butter does not guarantee reduced cholesterol or lower heart disease risk if the margarine contains trans fats.

  • Lean or thin individuals can still exhibit elevated blood cholesterol levels.

  • Absence of cholesterol on a nutrition label does not automatically make a food item "heart-healthy" (it may still be high in saturated or trans fats).

  • Cholesterol-lowering medication does not negate the necessity of maintaining a healthy diet.

  • Women are vulnerable to high cholesterol and cardiovascular disease, not just men.

  • Lipid profile screening is necessary before reaching middle age.

• Cholesterol Content in Common Foods

  • Hard-cooked egg ($1\text{ egg}$, $77\,\text{kcal}$): ~$211\,\text{mg}$

  • Boiled shrimp ($3\,\text{oz}$, $85\,\text{kcal}$): ~$150\,\text{mg}$

  • Broiled lean ground beef ($3\,\text{oz}$, $237\,\text{kcal}$): ~$78\,\text{mg}$

  • Roasted chicken breast ($3\,\text{oz}$, $141\,\text{kcal}$): ~$73\,\text{mg}$

  • Lean roasted ham ($3\,\text{oz}$, $123\,\text{kcal}$): ~$118\,\text{mg}$

  • Lean broiled sirloin steak ($3\,\text{oz}$, $171\,\text{kcal}$): ~$76\,\text{mg}$

  • Poached cod ($3\,\text{oz}$, $88\,\text{kcal}$): ~$55\,\text{mg}$

  • Water-packed canned tuna ($3\,\text{oz}$, $99\,\text{kcal}$): ~$30\,\text{mg}$

  • Whole milk ($1\,\text{cup}$, $150\,\text{kcal}$): $24\text{--}33\,\text{mg}$

  • Reduced-fat $2\%$ milk ($1\,\text{cup}$, $121\,\text{kcal}$): ~$20\,\text{mg}$

  • Butter ($1\,\text{tsp}$, $36\,\text{kcal}$): ~$11\,\text{mg}$

Roles of Lipids in the Human Body

• Energy Provision and Storage:

  • Provide energy to cells; adipose tissue possesses a virtually unlimited storage capacity for fat energy.

  • Adipose tissue supplies approximately $60\%$ of the body's resting energy expenditure.

• Thermal Insulation and Protection:

  • Provides a subcutaneous insulation layer to maintain body temperature.

  • Acts as a mechanical shock absorber to protect vital internal organs from physical trauma.

• Structural and Metabolic Roles:

  • Maintains cellular membrane integrity and fluidity.

  • Required for the intestinal absorption, transport, and utilization of fat-soluble vitamins ($A$, $D$, $E$, and $K$).

  • Phospholipids (such as lecithin) act as metabolic emulsifiers.

  • Cholesterol forms structural membrane components and serves as the precursor for steroid hormones, bile, and Vitamin D.

Lipid Digestion, Absorption, and Transport

• The Digestive Challenge:

  • Lipids are hydrophobic, whereas biological digestive enzymes (lipases) and GI fluids are hydrophilic.

  • Emulsification by phospholipids and bile salts (possessing both lipophilic and hydrophilic regions) is required to break down large fat globules and expose triglycerides to enzymatic hydrolysis.

• Step-by-Step GI Tract Lipid Digestion:

  • Mouth and Salivary Glands:

  • Hard fats begin melting upon reaching body temperature.

  • The sublingual salivary gland at the base of the tongue secretes lingual lipase.

  • Lingual lipase causes minor hydrolysis of most fats, but provides appreciable hydrolysis of milk fats.

  • Stomach:

  • Strong muscular churning actions mix fat with water and gastric acid.

  • Gastric lipase accesses and hydrolyzes a small fraction of triglycerides into monoglycerides and fatty acids.

  • Small Intestine:

  • Fat entry into the duodenum triggers the release of the hormone Cholecystokinin (CCK) into the blood.

  • CCK signals the gallbladder to contract and release stored bile through the common bile duct into the small intestine.

  • CCK signals the pancreas to release pancreatic lipase via the pancreatic duct.

  • Bile emulsifies fat, converting large globules into small suspension droplets that repel one another.

  • Pancreatic and intestinal lipases hydrolyze emulsified triglycerides into monoglycerides, glycerol, and free fatty acids:       $\text{Triglyceride} + 2\,\text{H}_2\text{O} \rightarrow \text{Monoglyceride} + 2\,\text{Fatty Acids}$

  • Large Intestine:

  • Unabsorbed fat and cholesterol trapped within dietary fiber exit the body in feces.

• Lipid Absorption Dynamics:

  • Short-Chain & Medium-Chain Fatty Acids and Glycerol:

  • Water-soluble compounds that do not require micelle formation.

  • Absorbed directly across enterocyte membranes into intestinal capillaries and transported to the liver via the portal vein.

  • Long-Chain Fatty Acids and Monoglycerides:

  • Insoluble in water; merge with bile salts and phospholipids to form spherical complexes termed micelles.

  • Micelles diffuse through the watery lumen fluid to contact the brush border of intestinal mucosal cells (enterocytes).

  • Lipids detach from micelles and diffuse passively into enterocytes.

  • Inside enterocytes, monoglycerides and free fatty acids are reassembled into new triglycerides.

  • Reassembled triglycerides, phospholipids, and cholesterol are packaged with proteins to form chylomicrons.

  • Chylomicrons exit enterocytes via exocytosis into the lymphatic system (lacteals), bypassing the liver initially before entering the systemic bloodstream near the heart.

Lipoproteins and Enterohepatic Circulation

• Lipoprotein Structure and Function:

  • Lipoproteins consist of an inner hydrophobic core of triglycerides and cholesterol esters surrounded by an outer hydrophilic shell of phospholipids, free cholesterol, and apolipoproteins.

• The Four Major Classes of Lipoproteins:

  • Chylomicrons:

  • The largest size and lowest density lipoprotein (composed primarily of diet-derived triglycerides).

  • Transport dietary lipids from intestinal cells throughout the lymphatic and systemic circulation.

  • Cells extract triglycerides via lipoprotein lipase; liver cells absorb remaining chylomicron remnants from the blood.

  • Very-Low-Density Lipoproteins (VLDL):

  • Synthesized in the liver; composed of approximately $50\%$ triglycerides.

  • Deliver endogenously produced lipids to peripheral body cells.

  • As tissues strip VLDL of triglycerides, its composition shifts, density increases, and it transforms into LDL.

  • Low-Density Lipoproteins (LDL):

  • Derived from VLDL; composed of approximately $50\%$ cholesterol.

  • Deliver cholesterol and triglycerides to peripheral tissues for cellular needs, or return to the liver.

  • Elevated circulating LDL is strongly linked to arterial plaque formation and cardiovascular disease ("bad cholesterol").

  • Specific LDL receptors on liver cells clear LDL from circulation, regulating blood cholesterol levels.

  • High-Density Lipoproteins (HDL):

  • Synthesized by the liver; smallest and densest lipoprotein due to being composed of approximately $50\%$ protein.

  • Mediates reverse cholesterol transport: removes excess cholesterol from tissue cells and returns it to the liver for recycling or excretion.

  • Possesses anti-inflammatory properties that protect against heart attacks ("good cholesterol").

• Enterohepatic Circulation of Bile:

  • Liver synthesizes bile from cholesterol.

  • Bile is stored and concentrated in the gallbladder.

  • Gallbladder secretes bile into the small intestine to emulsify dietary fat.

  • In the terminal ileum, most bile salts are reabsorbed into the bloodstream and recycled back to the liver.

  • In the colon, bile trapped by soluble dietary fibers is excreted in feces, driving the liver to utilize circulating cholesterol to synthesize replacement bile salts.

Health Implications, Cardiovascular Disease, and Lipids

• Target Blood Lipid Values for CVD Prevention:

  • Total Cholesterol: $< 200\,\text{mg/dL}$ (optimal)

  • LDL Cholesterol: Optimal $< 100\,\text{mg/dL}$ ($< 130\,\text{mg/dL}$ standard threshold)

  • HDL Cholesterol: Optimal $> 60\,\text{mg/dL}$ ($> 45\,\text{mg/dL}$ minimum threshold)

  • Triglycerides: $< 150\,\text{mg/dL}$ (optimal $< 100\,\text{mg/dL}$)

  • Total Cholesterol Calculation Formula:     $\text{Total Cholesterol} = \text{HDL} + \text{LDL} + (0.20 \times \text{Triglycerides})$

• Pathophysiology of Cardiovascular Disease (CVD):

  • General term encompassing diseases of the heart and blood vessels; Coronary Heart Disease (CHD) is the most common form.

  • Atherosclerosis:

  • Accumulation of fatty streaks along inner arterial walls that progressively harden into fibrous plaques, reducing vessel elasticity and lumen diameter.

  • A diet high in saturated fat and trans fat acts as a major driver of plaque formation.

  • The critical phase of heart disease begins when fibrous plaques cover $60\%$ of coronary arterial walls.

  • Thrombosis and Embolism:

  • Arterial plaques are recognized as tissue injuries, triggering blood platelet aggregation and clot formation.

  • Thrombosis: A stationary blood clot grows large enough to block blood flow through a vessel (coronary thrombosis causes myocardial infarction/heart attack; cerebral thrombosis causes ischemic stroke).

  • Embolism: A clot detaches, travels through the circulation, and lodges in a smaller downstream vessel.

  • Omega-3 fatty acids protect against abnormal clot formation.

  • Hypertension and Aneurysm:

  • Narrowed, rigid arteries increase resistance to blood flow, causing blood pressure to rise.

  • High blood pressure (hypertension) inflicts mechanical stress on arterial walls, damaging endothelial linings and accelerating plaque development (atherosclerosis and hypertension worsen each other).

  • An aneurysm is a ballooning out of a weakened arterial wall; rupture leads to severe internal hemorrhage and death.

  • Ischemia and Tissue Necrosis:

  • Restricted blood flow causes angina (severe chest pain and pressure).

  • Complete obstruction of blood flow starves tissue of oxygen, causing cell death (heart attack in cardiac tissue, stroke in brain tissue).

• Established Risk Factors for Atherosclerosis:

  • Elevated blood lipids (high LDL, high triglycerides, low HDL)

  • Family history / heredity

  • Glucose intolerance / Diabetes mellitus

  • Hypertension

  • Sedentary lifestyle

  • Obesity

  • Cigarettes / smoking

  • Estrogen deficiency (post-menopause)

Dietary Recommendations and Practical Guidelines

• Dietary Reference Intakes (DRIs) & Dietary Guidelines:

  • Minimum requirement: $3\%$ of total daily calories from essential fatty acids to prevent deficiency symptoms.

  • Acceptable Macronutrient Distribution Range (AMDR): $20\text{--}35\%$ of total daily energy intake from fat.

  • Saturated Fat: Limit to $< 10\%$ of total daily calories (saturated fat is the primary dietary driver of elevated LDL cholesterol).

  • Trans Fat and Cholesterol: Consume as little as possible (previous dietary target for cholesterol was $< 300\,\text{mg/day}$).

  • Linoleic Acid (Omega-6) DRI: $5\text{--}10\%$ of daily energy intake.

  • Linolenic Acid (Omega-3) DRI: $0.6\text{--}1.2\%$ of daily energy intake.

  • Omega-3 Supplementation: Baseline recommendation is $500\,\text{mg/day}$ of combined EPA and DHA; therapeutic heart health targets require $2\text{--}4\,\text{g/day}$ ($2000\text{--}4000\,\text{mg/day}$).

  • Note on Supplement Labels: A capsule labeled as $1000\,\text{mg}$ fish oil concentrate may only contain $300\,\text{mg}$ of active omega-3 fatty acids (e.g., $180\,\text{mg}$ EPA and $120\,\text{mg}$ DHA).

• Caloric and Saturated Fat Savings via Dietary Modifications:

  • Trimming visible fat off a pork chop (reducing fat from $19\,\text{g}$ to $9\,\text{g}$):

  • Savings: $110\,\text{kcal}$, $10\,\text{g}$ fat, $4\,\text{g}$ saturated fat.

  • Choosing a plain baked potato over a potato with $1\,\text{tbs}$ butter and $1\,\text{tbs}$ sour cream:

  • Savings: $150\,\text{kcal}$, $13\,\text{g}$ fat, $10\,\text{g}$ saturated fat.

  • Replacing $1\,\text{cup}$ whole milk ($150\,\text{kcal}$, $8\,\text{g}$ fat) with $1\,\text{cup}$ fat-free milk ($90\,\text{kcal}$, $<1\,\text{g}$ fat):

  • Savings: $60\,\text{kcal}$, $7\,\text{g}$ fat, $4\,\text{g}$ saturated fat.

• Replacing Saturated Fats with Unsaturated Fats (~$100\,\text{kcal}$ Portions):

  • Replace Butter ($1\,\text{tbs}$: $7\,\text{g}$ sat, $4\,\text{g}$ unsat) with Olive Oil ($1\,\text{tbs}$: $2\,\text{g}$ sat, $9\,\text{g}$ unsat).

  • Replace Bacon ($2$ slices: $3\,\text{g}$ sat, $6\,\text{g}$ unsat) with Sunflower Seeds ($2\,\text{tbs}$: $1\,\text{g}$ sat, $7\,\text{g}$ unsat).

  • Replace Potato Chips ($10$ chips: $2\,\text{g}$ sat, $5\,\text{g}$ unsat) with Mixed Nuts ($2\,\text{tbs}$: $1\,\text{g}$ sat, $8\,\text{g}$ unsat).

  • Replace Hard Cheese ($1$ slice: $4\,\text{g}$ sat, $4\,\text{g}$ unsat) with Avocado ($6$ slices: $2\,\text{g}$ sat, $8\,\text{g}$ unsat).

  • Replace Steak ($1.5\,\text{oz}$: $2\,\text{g}$ sat, $3\,\text{g}$ unsat) with Salmon ($2\,\text{oz}$: $1\,\text{g}$ sat, $3\,\text{g}$ unsat).

• Categorization of Protein Foods by Fat Content:

  • Very Lean Options: Skinless chicken breast, cod, flounder, trout, water-packed canned tuna, egg whites, legumes.

  • Lean Options: Beef or pork round/loin cuts, skinless chicken drumsticks/legs, herring, salmon, oil-packed canned tuna.

  • Medium-Fat Options: Ground beef, whole eggs, tofu.

  • High-Fat Options: Bacon, hot dogs, lunch meats, sausage, peanut butter, nuts.

The Mediterranean Diet

• Nutritional Characteristics:

  • Low in saturated fat and extremely low in trans fat.

  • High in monounsaturated fats (olive oil) and polyunsaturated omega-3 fatty acids (fish, nuts).

  • High in dietary fiber, complex carbohydrates, essential vitamins, minerals, and active antioxidant phytochemicals.

  • Clinically proven to reduce total cholesterol, LDL cholesterol, and overall cardiovascular disease risk.

• Mediterranean Diet Pyramid Hierarchy:

  • Base (Every Meal): Fruits, vegetables, whole grains, olive oil, beans, nuts, legumes, seeds, herbs, and spices. Regular physical activity and social dining.

  • Frequent (At least 2 times per week): Fish and seafood.

  • Moderate Portions (Daily to Weekly): Poultry, eggs, cheese, and yogurt.

  • Infrequent (Less often): Red meats and sweets.

  • Hydration & Beverages: Water as the primary beverage; wine consumed in moderation.