Lipids

• Triglyceride


  • The caloric value of triglyceride is over twice that of glycogen or protein (per unit mass)
  • Triglyceride provides more metabolic water upon oxidation than glycogen
• When body fat content is high, total body water content is low
• Lipids


  • They act as both thermal and electrical insulators
  • Some lipids help to regulate biologic reactions
• Acetyl-CoA


  The source of carbon atoms in all mammalian lipids, and it can be derived from carbohydrate, lipid, or amino acid catabolism
• Major structural lipids


  • Cholesterol
  • Glycerophospholipids
  • Sphingolipids
• Types of lipids


  • Saturated and unsaturated fatty acids
  • Phospholipids
  • Bile acids
• Lipids


  A heterogenous group of compounds which have the common property of being relatively insoluble in water, yet soluble in nonpolar solvents such as ether, chloroform, and benzene
• Lipids of metabolic significance in mammalian organisms

  • Fat-soluble vitamins
  • Triglycerides
  • Phospholipids
  • Glycolipids
  • Lipoproteins
  • Sphingolipids
  • Steroids
  • Saturated and unsaturated fatty acids
  • Eicosanoids
  • Ketone bodies
  • Volatile fatty acids
• Simple lipids


  Saturated and unsaturated fatty acids, as well as esters of these fatty acids with various alcohols, such as glycerol (e.g., mono-, di- and triglycerides)
• Complex lipids


  Phospholipids, sphingolipids, glycolipids, lipoproteins, sulfolipids, and aminolipids
• Derived lipids


  Lipids that cannot be neatly classified into either simple or complex lipids, and are generally derived from other lipids by hydrolysis, including steroids, eicosanoids, ketone bodies, and fat-soluble vitamins
• Triglycerides present in fat cells were once considered to be inactive storehouses of calorigenic material, called upon only in times of energy shortage
• Experiments showed that a considerable proportion of depot fat (triglyceride) is formed from dietary lipid within 4 days, while the total mass of triglyceride remains relatively constant, demonstrating the dynamic state of body fat
• Fatty acids derived from triglyceride may be the major source of energy for many tissues, and may be used as fuel in preference to carbohydrate and protein in certain organs
• Adipose cells (fat cells or adipocytes)


  • They are specialized for the assembly, synthesis, and storage of triglyceride, and for its mobilization into fuel molecules (free fatty acids) that are transported to other tissues bound to albumin in blood
• Triglyceride


  The principal form in which energy is stored in the body, with advantages over carbohydrate and protein due to its higher caloric value per unit mass and more anhydrous nature
• A gram of dry glycogen binds about two grams of water, so a gram of nearly anhydrous fat stores more energy than a gram of hydrated glycogen, which is probably the reason why triglyceride, rather than glycogen, was evolutionarily selected as the major energy reserve</b>
• Fatty acids also provide more metabolic water upon oxidation than other biologic fuels, which is advantageous to hibernating animals and those occupying dry environments
• When body fat content is high


  Total body water content is low
• When fat content is high

  Total body water (intracellular water plus extracellular water) is low
• When fat content is low


  Fluid content is high
• Lipids are important dietary constituents not only because of their high energy value, but also because of fat-soluble vitamins and essential fatty acids contained in the fat of natural foods
• Lipids serve to protect cells from infection as well as excess loss or gain of water
• Lean body mass (LBM)


  TBW plus solids, held within narrow limits in most normal, adult animals
• Total body water (TBW)


  May vary with age and sex, due principally to the amount of body fat an animal possesses
• Fat content is high


  TBW (ICW plus ECW) is low
• Fat content is low


  Fluid content is high
• Lipids


  • Important dietary constituents due to high energy value, fat-soluble vitamins and essential FAs
  • Protect cells from infection and excess loss/gain of water
  • Adipose tissue cushions vital organs against physical damage
  • Act as thermal insulators in subcutaneous tissue and around certain organs
  • Nonpolar lipids act as electrical insulators, allowing rapid propagation of depolarization waves along myelinated nerves
  • Important components of all biological membranes
  • Combinations of fat and protein (lipoproteins) are important cellular constituents and means of transporting lipids in blood
• Phospholipid (lecithin) and lipid metabolites


  Important components of bile, help facilitate digestion and absorption of dietary lipid
• Lipid metabolites


  Play important roles in regulating biologic reactions, including steroid hormones and derivatives of essential fatty acids known as prostaglandins
• Overview of major metabolic pathways in lipid metabolism

  1. Fatty acids derived from dietary lipid or de novo biosynthesis from acetyl-CoA

  2. Unsaturated FAs (e.g. arachidonic acid) used in biosynthesis of eicosanoids

  3. Acetyl-CoA can be derived from carbohydrate, lipid, or amino acid catabolism

  4. Fatty acids may be oxidized to acetyl-CoA (β-oxidation) or esterified to acylglycerols or sphingolipids

  5. Acetyl-CoA is source of carbon atoms in cholesterol, steroid hormones, and bile acid molecules

  6. Acetyl-CoA can also be used to form ketone bodies in the liver
• Types of fatty acids

  • Saturated fatty acids
  • Unsaturated fatty acids
• Saturated fatty acids


  • Unbranched, straight-chain derivatives
  • Even number of carbon atoms
  • Synthesized from acetyl-CoA
• Unsaturated fatty acids


  • Contain one or more double bonds
  • Generally predominate over saturated fatty acids, particularly in higher plants and cold environments
  • Addition of double bonds lowers melting point and increases membrane fluidity
• Cis configuration


  Groups being considered are on the same side of the double bond
• Trans configuration


  Groups being considered are on opposite sides of the double bond
• Most naturally occurring unsaturated long-chain fatty acids are in the cis configuration
• Trans-unsaturated fatty acids are found in ruminant fat, where they arise from the action of rumen microbes
• Presence of large amounts of trans-unsaturated fatty acids in partially hydrogenated vegetable oils has raised questions about their safety as food additives
• Trans-unsaturated fatty acids tend to raise blood LDL and lower HDL levels, and are associated with hypercholesterolemia, atherosclerosis, and coronary artery disease