Comprehensive Notes on Lipids: Triglycerides, Phospholipids, and Sterols

Lipids: Triglycerides, Phospholipids, and Sterols

Learning Objectives

  • Recognize the chemistry of fatty acids and triglycerides, and understand the differences between saturated and unsaturated fats.
  • Describe the chemistry, food sources, and roles of phospholipids and sterols.
  • Summarize fat digestion, absorption, and transport.
  • Outline the major roles of fats in the body, including a discussion of essential fatty acids and the omega fatty acids.
  • Explain the relationships among saturated fats, trans fat, and cholesterol and chronic diseases, noting recommendations.
  • Explain the relationships between monounsaturated and polyunsaturated fats and health, noting recommendations.

Lipids

  • Triglycerides: Most common in food and the body.
    • Fatty acids are components of triglycerides.
  • Phospholipids
  • Sterols

Chemist’s View of Triglycerides

  • Triglycerides: Consist of glycerol and three fatty acids.
  • Fatty acids:
    • Can be saturated or unsaturated.
    • Include omega-3 and omega-6 fatty acids.
    • 18-carbon polyunsaturated fatty acids.

Introduction to Fats

  • Having too much or too little fat can lead to poor health.
  • Fatty acids contain an even number of carbons.
  • Triglycerides consist of glycerol and three fatty acid tails.
  • 1 gram of fat contains 9 kcalories.

Fatty Acids

  • Saturated fat: All carbons are linked to hydrogens only, contains no double bonds.
  • Monounsaturated fat: Has one double bond.
  • Polyunsaturated fat: More than one double bond is present.

18-Carbon Fatty Acids

NameNumber of Carbon AtomsNumber of Double BondsSaturationCommon Food Sources
Stearic acid180SaturatedMost animal fats
Oleic acid181MonounsaturatedOlive and canola oils
Linoleic acid182PolyunsaturatedSunflower, safflower, corn, and soybean oils
Linolenic acid183PolyunsaturatedSoybean and canola oils, flaxseed, walnuts
  • Shorthand notation for fatty acids: The first number indicates the number of carbon atoms; the second, the number of double bonds. For example, the notation for stearic acid is 18:0.

Condensation of Glycerol and Fatty Acids to Form a Triglyceride

  • Glycerol + three fatty acids → Triglyceride + three water molecules
  • An H atom from glycerol and an OH group from a fatty acid combine to create water, leaving the O on the glycerol and the C at the acid end of each fatty acid to form a bond.
  • Three fatty acids attached to a glycerol form a triglyceride and yield water.
  • In the example, the triglyceride includes (from top to bottom) a saturated fatty acid, a monounsaturated fatty acid, and a polyunsaturated fatty acid.

Saturated and Unsaturated Fatty Acids

  • Saturated fatty acids tend to stack together, making saturated fats solid or more firm at room temperature.
  • Unsaturated fatty acids bend at the double bond(s), making unsaturated fats liquid or less firm at room temperature.

Essential Fatty Acids

  • Alpha-linolenic acid:
    • An omega-3 fatty acid.
    • First double bond is located on the 3rd carbon from the omega end.
  • Linoleic acid:
    • An omega-6 fatty acid.
    • First double bond is located on the 6th carbon from the omega end.

Eicosanoids

  • Eicosanoids are made from essential fatty acids and act like hormones.
  • Health benefits:
    • Blood pressure
    • Blood clotting
    • Immune system
    • Regulate inflammation
    • Brain and retina development in fetus, and more.
  • Omega-6 to Omega-3 ratio is important.
  • Fatty acid deficiencies are rare in the United States.
  • Symptoms of fatty acid deficiencies can occur.

Fatty Acid Composition of Common Food Fats

  • Animal fats and the tropical oils of coconut and palm contain mostly saturated fatty acids.
  • Some vegetable oils, such as olive and canola, are rich in monounsaturated fatty acids.
  • Many vegetable oils are rich in omega-6 polyunsaturated fatty acids.
  • Only a few oils provide significant omega-3 polyunsaturated fatty acids.

Cis- and Trans-Fatty Acids

  • Cis-fatty acid: Hydrogens on the same side of the double bond; cis molecules bend into a U-like formation. Most naturally occurring unsaturated fatty acids in foods are cis.
  • Trans-fatty acid: Hydrogens on the opposite sides of the double bond; trans molecules are more linear. The trans form typically occurs in partially hydrogenated foods when hydrogen atoms shift around some double bonds and change the configuration from cis to trans.

Hydrogenation

  • Unsaturated vegetable oil (liquid) can be partially hydrogenated by adding hydrogen under pressure to create a hydrogenated fat (solid or semisolid).
  • Usually liquid fats are only partially hydrogenated.

Phospholipids

  • Solubility in fat and water.
  • Emulsifiers in the food industry.
  • Lecithin is best-known phospholipid.
  • Food sources of phospholipids.
  • Roles:
    • Emulsifiers
    • Part of cell membranes

Sterols

  • Food sources:
    • Cholesterol
    • Plant sterols
  • Roles of sterols:
    • Made in the body
    • Structural component of cell membranes
    • Precursors to many hormones
    • Bile

Lipid Digestion

  • Fats are hydrophobic.
  • Digestive enzymes are hydrophilic.
  • Goal of fat digestion: Dismantle triglycerides into:
    • Monoglycerides
    • Fatty acids, and glycerol

The Beginning of Fat Digestion

  • Mouth: Lingual lipase plays a role in fat digestion in infants.
  • Stomach:
    • Strong muscle contractions.
    • Gastric lipase (very little fat digestion).
  • Small Intestine:
    • Bile
    • Pancreatic lipase

Emulsification of Fat by Bile

  • Fat and watery GI juices tend to separate; enzymes in the GI juices can't get at the fat.
  • When fat enters the small intestine, the gallbladder secretes bile. Bile has an affinity for both fat and water, so it can bring the fat into the water.
  • Bile's emulsifying action converts large fat globules into small droplets that repel one another.
  • After emulsification, more fat is exposed to the enzymes, making fat digestion more efficient.

Hydrolysis of a Triglyceride

  • The triglyceride and two molecules of water are split.
  • The H and OH from water complete the structures of two fatty acids and leave a monoglyceride.
  • These products may pass into the intestinal cells, but sometimes the monoglyceride is split with another molecule of water to give a third fatty acid and glycerol. Fatty acids, monoglycerides, and glycerol are absorbed into intestinal cells.

Enterohepatic Circulation

  • In the liver, bile is made from cholesterol.
  • In the gallbladder, bile is stored.
  • In the small intestine, bile emulsifies fats.
  • Bile is reabsorbed into the blood.
  • In the colon, bile that has been trapped by soluble fibers is excreted in feces.

Absorption of Fat

  • Small intestine
  • Stomach
  • Glycerol and small lipids such as short- and medium-chain fatty acids can move directly into the bloodstream.
  • Large lipids such as monoglycerides and long-chain fatty acids combine with bile, forming micelles that are sufficiently water-soluble to penetrate the watery solution that bathes the absorptive cells. There the lipid contents of the micelles diffuse into the cells.
  • Intestinal cells
  • Capillary network
  • Lacteal (lymph)
  • Blood vessels
    *

Lipoprotein Types

  • Chylomicrons:
    • Made from dietary lipid.
    • Contain the highest amount of triglyceride and are therefore the least dense.
  • Very-low-density lipoproteins (VLDL):
    • Made in the liver then shrink in the bloodstream as cells take up triglycerides.
  • Low-density lipoproteins (LDL):
    • Higher in cholesterol, lower in triglycerides, taken up by cells after binding to receptors.
    • Excess levels can have negative health implications.
  • High-density lipoproteins (HDL):
    • Carry excess cholesterol back to the liver for recycling or excretion.

Lipid Transport via Lipoproteins

  • Intestinal cells form chylomicrons from dietary lipids.
  • Chylomicrons deliver dietary lipids to most of the body's cells.
  • Liver cells receive small lipids directly from the intestine.
  • As cells remove lipids from the chylomicron, it becomes a smaller chylomicron remnant.
  • Liver cells remove chylomicron remnants from the blood.
  • Liver cells synthesize lipids.
  • Liver cells form VLDL, which deliver lipids to the body's cells.
  • As cells remove lipids from the VLDL, it forms a smaller LDL.
  • LDL deliver lipids to body cells or return to the liver.
  • Liver cells form HDL, which pick up cholesterol from the body's cells.
  • HDL deliver cholesterol to the liver for excretion.

Roles of Triglycerides

  • Provide the cells with energy.
    • Virtually unlimited ability to store fat energy.
  • Other uses of fat in the body:
    • Skin insulation
    • Temperature regulation
    • Shock absorption
    • Cell membrane material
    • Cell signaling pathways

An Adipose Cell

  • Newly imported triglycerides first form small droplets at the periphery of the cell, then merge with the large, central globule.
  • As the central globule enlarges, the fat cell membrane expands to accommodate its swollen contents.

Health Effects of Saturated Fats, Trans Fats, and Cholesterol

  • Focus on quality and quantity.
  • The current U.S. diet contains excessive solid fats.
  • Blood lipid profile:
    • Desirable: Total < 200 mg/dl, LDL < 100 mg/dl, HDL > 60 mg/dl
  • Heart disease:
    • Elevated LDL cholesterol is a risk factor.
      • Saturated fats increase LDL cholesterol and promote blood clotting.
        • Ways to lower dietary saturated fat.
    • Trans fats increase LDL and lower HDL cholesterol.
    • Dietary cholesterol.

Fat Links to Cancer and Obesity

  • Cancer:
    • Promotion rather than initiation of cancer.
    • Dietary fat and cancer risk differs for various types of cancer.
  • Obesity:
    • Cutting fat from the diet often reduces kcalories.

Recommended Intakes of Saturated Fat, Trans Fat, and Cholesterol

  • DRI and Dietary Guidelines:
    • 20 to 35 percent of daily energy from fat.
      • Less than 10 percent from saturated fat.
      • As little trans fat as possible.
    • No longer limit to <300 mg cholesterol.
  • Average U.S. consumption of fat.
  • Avoid getting too little fat.
    • Recommendation: one teaspoon of fat at meals.

Health Effects of Monounsaturated and Polyunsaturated Fats

  1. Replacing saturated fats with unsaturated fats helps reduce the risk of heart disease.
  2. Regular consumption of omega-3 fatty acids suppresses inflammation, which may protect against some cancers.

Replacing Saturated Fat with Unsaturated Fat

  • Portion sizes have been adjusted so that each of these foods provides approximately 100 kcalories. Notice that for a similar number of kcalories and grams of fat, the second choices offer less saturated fat and more unsaturated fat.

Two Meals Compared

  • Saturated Fats Meal:
    • 1 c fresh broccoli topped with 1 tbs butter
    • 1 c mixed baby greens salad with 2 strips bacon (crumbled)
    • 1 oz blue cheese crumble
    • 1 tbs light Italian dressing
    • 4 oz grilled steak
    • Energy = 600 kcal
  • Unsaturated Fats Meal:
    • 1 c fresh broccoli sautéed in 1 tbs olive oil
    • 1 c mixed baby greens salad with avocado
    • 2 tbs sunflower seeds
    • 1 tbs light Italian dressing
    • 4 oz grilled salmon
    • Energy = 600 kcal
  • To lower saturated fat and raise monounsaturated and polyunsaturated fats

Cutting Fat Cuts kcalories and Saturated Fat

  • Pork chop with fat (340 kcal, 19 g fat, 7 g saturated fat) vs. Pork chop with fat trimmed off (230 kcal, 9 g fat, 3 g saturated fat). Savings: 110 kcal, 10 g fat, 4 g saturated fat
  • Potato with 1 tbs butter and 1 tbs sour cream (350 kcal, 14 g fat, 10 g saturated fat) vs. Plain potato (200 kcal, <1 g fat. 0 g saturated fat). Savings: 150 kcal, 13 g fat, 10 g saturated fat
  • Whole milk, 1 c (150 kcal, 8 g fat, 5 g saturated fat) vs. Fat-free milk, 1 c (90 kcal, <1 g fat, <1 g saturated fat). Savings: 60 kcal, 7 g fat, 4 g saturated fat

Guidelines for Fat Intake

  • Lower saturated and trans fat.
  • Moderate kcalories.
  • Enough fats for good health.
  • Not too much of the harmful fats.
  • Consumer education on types of fat.
  • DRI recommendations:
    • Compatible with low rates of disease.

High-Fat Foods and Heart Health

  • Olive oil:
    • Benefits for heart health.
    • Replace saturated fats.
  • Nuts:
    • Fat composition.
    • Benefits for heart health:
      • Blood pressure, LDL, inflammation
    • Cautious advice for dietary inclusion.

Fish

  • DRI:
    • Linolenic acid: 0.6–1.2 percent of daily energy
      • No DRI for EPA and DHA
  • Omega-3 fatty acid:
    • Benefits for heart health
      • Benefits for hypertension or atherosclerosis
    • Dietary recommendations
  • Environmental contaminants:
    • Mercury

High-Fat Foods and Heart Disease

  • Saturated fat is the main determinant of LDL cholesterol.
    • Sources of saturated fat in the United States:
      • Meats
      • Whole milk products
      • Tropical oils
    • Zero saturated fat is not possible.
  • Trans fat:
    • Limit hydrogenated foods

The Mediterranean Diet

  • Features of a traditional Mediterranean diet:
    • Low in saturated fat
    • Very low in trans fat
    • Diet composition:
      • Whole foods—vegetables, legumes, fruits
      • Limited meat
      • Monounsaturated and polyunsaturated fats
      • Complex carbohydrates, fiber, and phytochemicals
  • Benefits for heart disease risk

Mediterranean Diet Pyramid

  • Base: Fruits, vegetables, whole grains, healthy fats such as olive oil, beans, nuts and peanuts, legumes and seeds, herbs and spices (Every day)
  • Olive Oil Often, at least two times per week
  • Fish and seafood: Often, at least two times per week
  • Poultry, eggs, cheese, and yogurt: Moderate portions, daily to weekly
  • Meats and sweets: Sometimes (no more than a few times a month)
  • Wine: In moderation
  • Water: Throughout the day
  • Be physically active, enjoy meals with others