Lipids: A Broad Category of Nutrients
•Insoluble in water
•Soluble in ether, hexane, alcohol, or other organic (carbon-based) solvents
Classification
•Triglycerides--what we normally think of as fats
–___ Calories/g
•Phospholipids
•Cholesterol
•Fat-soluble vitamins (we’ll save these for later in the quarter)
Triglycerides
•Three fatty acids esterified (bound) to one molecule of glycerol
•Character of fat dependent on:
–Number of carbon atoms in fatty acids
–Degree of unsaturation (number of double bonds) of fatty acids
–Cis or trans double bonds in fatty acids
Fatty Acid Chain Length
•Short Chain Fatty Acids (SCFA) C2-C6
•Medium Chain Fatty Acids (MCFA) C8-C12
•Long Chain Fatty Acids (LCFA) C14 and up
•(Most naturally occurring fatty acids have an even number of carbon atoms)
Degree of Saturation
•Saturated fatty acids-no double bonds
•Monounsaturated fatty acids-1 double bond
•Polyunsaturated fatty acids-≥2 double bonds
•Double bonds are usually separated by at least two single bonds
Cis vs. Trans Double Bonds
•In nature, most double bonds are cis, when hydrogen atoms are on same side of fatty
acid
•Trans double bonds are formed during hydrogenation (making a polyunsaturated fat
more saturated)
Melting points
•Lower in short chain and polyunsaturated fatty acids
•Higher in long chain and saturated fatty acids
At Room Temperature (70° F)
•Fats with more saturated fatty acids are solid
–Butter and other animal fats, margarine, shortening, cocoa butter, coconut oil
•Fats with predominantly monounsaturated and polyunsaturated fatty acids are liquid
–Olive, soybean, and canola oils
Double Bonds are Counted from the Omega (CH3) End
•Oleic acid is an omega-9 fatty acid
•Linoleic acid is an omega-6 fatty acid with two double bonds
•Alpha-linolenic acid is an omega-3 fatty acid with 3 double bonds
•Humans cannot add double bonds before omega-9 position, so linoleic and alphalinolenic acids are essential fatty acids
Why hydrogenate fats?
•Polyunsaturated fatty acids are prone to oxidation, leading to rancidity
•Hydrogenation saturates fatty acids to improve shelf life
•Hydrogenation also improves the texture of certain baked products
Phospholipid
•One fatty acid of a triglyceride is replaced by a phosphate group and a nitrogencontaining endgroup
•Example: Phosphatidyl choline (lecithin)
•Phospholipids emulsify fat (think mayonnaise)
Cholesterol
•Found only in animal products
•Humans can make cholesterol, so it is not essential in the diet
•We can’t use it for energy
•Required for cell membranes, nerves, and as a precursor to important steroid hormones
(estrogen, testosterone, cortisol, aldosterone) and vitamin D
Triglyceride Digestion & Absorption
•There is a small amount of lingual lipase in the mouth to produce small droplets of fat
•Gastric lipase hydrolyzes about 30% of triglycerides to diglycerides and fatty acids
•Pancreatic lipase hydrolyzes the rest to monoglycerides, glycerol, and free fatty acids
•Bile and phospholipid emulsify lipid into micelles
Glycerol, SCFA, and MCFA Absorption
•Not dependent on bile salts
•SCFA and MCFA diffuse directly into intestinal cell and pass into portal vein to liver
•Medium chain triglycerides (derived from coconut oil) are used for patients with fat
malabsorption
–Not suitable for cooking
Monoglyceride and LCFA Absorption
•Diffuse into intestinal cells
•Re-constructed into triglycerides, then packaged with phospholipids, fat-soluble
vitamins, and some protein into chylomicrons
•Pass into lacteal and are transported in lymph
•Dumped into circulation at thoracic duct
The Fate of Chylomicrons
•Chylomicrons circulate through blood and triglycerides are removed from them in
capillary walls by action of the enzyme lipoprotein lipase (LPL)
•Chylomicron remnants are taken up by the liver
•Usually cleared from blood within 10 hours
The VLDL Pathway
•Liver packages triglycerides into very-low-density lipoprotein (VLDL) & send it into
circulation
•LPL removes TG, resulting in intermediate-density lipoprotein (IDL)
•With more removal of TG, IDL becomes low-density lipoprotein (LDL)
LDL is predominantly cholesterol
•LDL is picked up by specific receptors on cell surfaces for cholesterol
•If these receptors are lacking or defective, LDL cholesterol increases
The Oxidized LDL Theory of Atherosclerosis
•Atherosclerosis-- “hardening of the arteries”
•LDL can be oxidized by cigarette smoke or other oxidants
•White blood cells gorge on LDL via the scavenger receptor and become foam cells
•Foam cells accumulate in the arterial wall & set stage for more advanced damage
What is HDL?
•High density lipoprotein
•High in protein
•Picks up cholesterol from peripheral tissues and other lipoproteins and returns it to liver
•“You want your highs high and your lows low”--Cara East, MD
Beneficial Functions of Lipids
•Concentrated energy source
–Primary fuel at rest
•Insulation, cushion
•Needed for skin and hair
•Required for synthesis of cell membranes, steroid hormones, and eicosanoids (local
hormones like prostaglandins)
Beneficial Lipid Functions, cont.
•Carrier of fat-soluble vitamins and phytochemicals
•Provides satiety
•Mouthfeel
•Phospholipids are important emulsifiers in food and the body
National Cholesterol Education Program (NCEP), 2001 (table 5.6)
•Total Cholesterol
•Desirable < 200 mg/dL
•Borderline-high 200-239 mg/dL
•High > 240 mg/dL
•HDL Cholesterol
•Low < 40 mg/dL
•Desirable > 60 mg/dL
NCEP, part deux--LDL Cholesterol
•Optimal < 100 mg/dL
•Above optimal 110-129
•Borderline high 130-159
•High 160-189
•Very High ≥ 190
NCEP July 2004 Addendum
•In high risk patients, LDL-cholesterol should be reduced to < 70 mg/dL
•Requires high doses of statin drugs
•These drugs are NOT benign
•Many of the scientists on panel have financial relationships with companies that market
statins
NCEP, numero three-oh
Triglycerides
•Normal < 150 mg/dL
•Borderline-high 150-159
•High 200-499
•Very high ≥ 500
The Good, the Bad, and the Ugly of Dietary Fats
•How they affect blood LDL and HDL levels
Cis-Monounsaturated Fats
•Good guys
•Decrease total and LDL cholesterol
•Also decrease triglycerides
•In large doses, decrease HDL cholesterol
Omega-6 PUFA
•For example, linoleic acid
•Found in most garden-variety vegetable oils
•Good guys up to a point
•Decrease LDL cholesterol
•Can also reduce HDL cholesterol
•Problem--more prone to oxidation
Omega-3 PUFA
•Found in fish oils, canola oil, and some nuts
•Also a “good” fat
•Has a neutral effect on total cholesterol
•Increase LDL cholesterol modestly
•Decrease triglycerides
•Precursors to prostaglandins that inhibit blood clotting
Saturated Fatty Acids
•More or less bad
•Increase LDL and HDL cholesterol
•Primary bad saturated fatty acid is palmitic (C16)
•Stearic acid (C18) has a neutral effect on blood lipid levels
Trans Fatty Acids
•We’re talking ugly
•Found in partially hydrogenated oils, and in small amounts in animal fats
•Increase LDL cholesterol
•Decrease HDL cholesterol
•Typical US diet contains 3% of Calories from trans fatty acids
Conjugated Linoleic Acid (CLA)
•A new good guy, maybe
•Conjugated--two double bonds are separated by only one single bond
•Trans-10, cis-12 CLA reduces fat and glucose uptake by cultured fat cells
•Human studies have been inconsistent so far
What are the risk factors for heart disease?
2000 American Heart Association Guidelines
•Maximum 30% of Calories from fat
•Maximum 7% of Calories from saturated fat
•Maximum 200 mg/day cholesterol
2002 DRI’s for Fat
•Acceptable Macronutrient Distribution Range
–Total fat 20-35% of Calories
–5-10% of Calories from omega-6 PUFA
–0.6-1.2% of Calories from omega-3 PUFA
Utilizing Fat for Energy
•Glycerol can go through part of pathway for carbohydrate metabolism
•Fatty acids are metabolized via beta-oxidation
–Two-carbon units are taken off fatty acids in sequence, then can go through TCA cycle