Lipids: Not Just Fat

BIOL 103, Chapter 6-2

Today’s Topics

• Lipids Digestion and Absorption• Lipids in the Body• Lipids in the Diet• Lipids and Health

Lipid Digestion

• Digestion of triglycerides (TG), cholesterol, and phospholipids (PL)– Mouth• Chewing + lingual lipase secreted by salivary glands

tiny fat droplets

– Stomach• 30% Triglycerides + gastric lipase FFA and DG

Lipid Digestion (cont.)

• Small Intestine– Emulsified by bile (bile salts and lecithin/PL in the body)– Rest of the “exposed” TG and DG are digested by

pancreatic lipase MG, glycerol and FFA– Phospholipids + pancreatic phospholipase

lysolecthicin/LsL and FFA– Cholesterol ester + cholesterol esterase FFA +

cholesterol– Bile salts surround MG, FFA, cholesterol, and

lysolecthicin forming micelles (tiny emulsified fat packets made up of phospholipids)

Lipid Digestion (cont.)

• Micelles carry MG, LyL, cholesterol, and long-chain FA through the watery environment to microvilli/small intestine– The lipid components are then absorbed into the

intestinal cells at the microvilli– Bile salts return to the liver to be used again

• Short-chain FA and glycerol are directly absorbed into your intestinal cells without the help of micelles.

Lipids Absorption

• To travel in the bloodstream, lipids are packaged into lipoprotein carriers

• How? – In your intestinal cell: MG + FFA rejoin TG

• Cholesterol esters and phospholipids form here also

– Then, TG, PL, cholesterol esters join protein carrier lipoprotein.

– Lipoprotein leaves the intestinal cell chylomicron goes to lymph system bloodstream.

Lipid Digestion and Absorption(Figure 6.21)

Chylomicron Transport

• In the blood stream, chylomicrons are large, fatty lipoproteins (90% fat) circulate in the body while releasing TG

• When reaching capillaries, lipoprotein lipase “attacks” chylomicrons to remove TG FFA and glycerol:– FFA enter adipose cells reassembled into TG– OR FFA enter muscle cells used for energy– Glycerol liver or kidney

• Your liver picks up “left-over” chylomicron and uses them to build VLDL

Lipids in the Body1. Very-low-density lipoproteins (VLDL):– Deliver triglycerides to cells

2. Intermediate-density lipoproteins (IDL) – Returns to liver and is converted to LDL

3. Low-density lipoproteins (LDL)– Deliver cholesterol to cells, which body uses to

synthesize membranes, hormones, etc.– Returns to liver afterwards

4. High-density lipoproteins (HDL)– Pick up cholesterol for removal or recycling

Lipoprotein density

• Lipoproteins differ by size, density, and composition of their lipid cores– In general, larger the lipoprotein, the less dense it

is.– Size: HDL < LDL < IDL < VLDL < Chylomicron

Lipids in the Diet

• Recommended intake:– Reduce saturated fat and trans fat– Total fat: 20-35% of calories– Less than 10% of calories from saturated fat

Lipids in the Diet

• Essential FA requirements– Linoleic acid/omega-6 FA should provide about 2%

of calories– Requirements for omega-3 FA is less well-known

• Omega-6 and omega-3 FA balance– Ratio of 6:1 (omega-6 to omega-3 FA)– While omega-3 FA is good for you, consuming too

much of the omega-3 FA can suppress immune function and prolong bleeding time