The Lipids: Triglycerides, Phospholipids, and Sterols Chapter Five

Definition of lipid A water insoluble biomolecule

Classes of lipids Triglycerides Phospholipids Sterols

Types of Lipids

Triglycerides Comprises about 95% of dietary lipid intake Made of glycerol molecule—a 3 carbon alcohol three fatty acids—carboxylic acids containing from 4-24 carbons (18 most common) Short chain less than 6 carbons Medium chain 6-10 carbons Linked by condensation reactions (esters)

Figure 6.7

Fatty acid types Saturated—full of hydrogen, no double bonds, usually found in animals, fats Unsaturated—having at least one double bond, usually found in plants, oils Monounsaturated—one double bond Polyunsaturated—many double bonds (three or more) Trans—misshapen double bond

Figure 6.1

Fatty acid designation Designation indicates position of final double bond in molecule Omega Most unsaturated fatty acids are either omega-3, omega-6, or omega-9. Omega-3—fish and flax (poly) Omega-6—vegetable oils (poly) Omega-9—olive and canola oil (mono)

A Saturated Fatty Acid

Monounsaturated Fatty Acid

Polyunsaturated Fatty Acid (corn oil)

Polyunsaturated Fatty Acid (fish)

Table 6.1

Essential fatty acids Alpha linolenic—an omega-3 Linoleic—an omega-6 Both form eicosanoids (prostaglandins), but each initiates the synthesis of antagonistic prostaglandins.

Figure 6.4

Partial hydrogenation (trans fatty acids) Polyunsaturated fatty acid  more saturated (add hydrogens) Produces trans fatty acids Shape of molecule is altered—cis to trans— may have various negative health consequences Also extends food shelf life—rancidity occurs when oxygen reacts with double bonds Average intake: 6g/day!

Table 6.5

Dietary triglycerides Mostly are composed of mixtures of fatty acid types—mixed triglycerides

Roles of triglyceride in the body Energy—immediate and stored 60% of energy at rest supplied by fat Body fat stores = about 70,000 kcals (carb storage = about 2,000 kcals) 1 lb fat = 3500 kcals Adipose cells Insulation and protection Transportation of fat soluble nutrients

Phospholipids Commonly called lecithins Found in eggs, liver, soy, wheat germ, peanuts Made of glycerol condensed with two fatty acids and a phosphorous containing group Have a small water soluble portion Major uses include cell membranes, emulsifiers, formation of bile acids

Pg 201b

Sterols Do not resemble triglycerides or phospholipids but like them are not water soluble Have characteristic multiringed structure Present in all food sources, but cholesterol only in animal products Used to make sex hormones, vitamin D, cell membranes, bile acids Cholesterol not essential. Liver makes 5 x molecules/sec or mg/day

Sterols

Fat digestion Average intake g triglyceride 4-8 g phospholipid g sterols 95% of fat consumed is absorbed

Figure 6.9

Digestion step by step Mouth: fat melts. triglyceride  some fatty acids and mono and diglycerides (sublingual lipase) Stomach: floats, little digestion (gastric lipase) Small intestine: triglyceride  droplets (bile triggered by CCK) Triglyceride  fatty acids and glycerol (pancreatic lipases) Phospholipids follow same process Sterols need no digestion and are absorbed as is

Absorption Fatty acids and glycerol move into the villi Once inside are reassembled into triglyceride again Triglyceride complexed with protein (chylomicron) Chylomicron added to lymph Lymph eventually enters blood stream

How is fat transported in blood? Lipoproteins Identified by how they layer in centrifuged blood

Types of lipoproteins Chylomicrons—carry dietary lipids (triglyceride) from G.I. tract to rest of body VLDL—carry liver made lipids (all classes) from liver to body LDL—carry mostly liver made cholesterol from liver to body bad cholesterol HDL—carry excess circulating cholesterol back to liver good cholesterol

Types of Lipoproteins

Fat storage/fat release Lipoprotein lipase (LPL)— allows fat to enter cells (muscle, adipose) Hormone-sensitive lipase— allows fat to be released from cells

Interpreting blood lipid profiles Total cholesterol— under 200mg/dL (screening test) Total:HDL ratio should be 4.5 or less (Framingham study)

Recommendations for fat intake Total dietary fat: no more than 30% total kcals Saturated fat: 10% or less of total kcals Essential fatty acids: at least 5% of energy should come from w-6, and around 1% from w-3 Cholesterol: no more than 300 mg/day

Roles of various fats on health Fatty acidHeart disease Cancerobesity Saturated >LDL, <HDL: risk increases PromoterAll can lead to obesity if eaten in excess Omega-6 Lowers both LDL and HDL, risk unchanged Promoter Omega-9 < LDL, HDL unchanged? Risk lower Not a promoter Omega-3 >HDL, <LDL, risk lower Antipromoter trans Same as sat?Promoter?

Fat Substitutes Water Starch derivatives and fiber Z-trim Oat-trim Protein based Dairy-lo Engineered fats olestra

From Guidelines to Groceries Meat group—choose lean cuts, free- range, grass-fed, watch for invisible fat, try soy Milk—fat free, lowfat, more yogurt (lowers blood cholesterol) More vegetables, fruits and whole grains