1. Lecture 5: Lipids and Carbohydrates
Craig Kasper
Fish Nutrition

2. Lecture 5: Lipids and Carbohydrates
Craig Kasper

3. Part 1: Lipid Characteristics
Lipid = a compound that is insoluble in water, but soluble in an organic solvent (e.g., ether, benzene, acetone, chloroform)
“lipid” is synonymous with “fat”, but also includes phospholipids, sterols, etc.
chemical structure: glycerol + fatty acids

4. Lipid Molecule

5. Nutritional Uses of Lipids
We already know that lipids are concentrated sources of energy (9.45 kcal/g)
other functions:
1) provide means whereby fat-soluble nutrients (e.g., sterols, vitamins) can be absorbed by the body
2) structural element of cell, subcellular components
3) components of hormones and precursors for prostaglandin synthesis

6. Lipid Classes simple: FA’s esterified with glycerol
compound: same as simple, but with other compounds also attached
phospholipids: fats containing phosphoric acid and nitrogen (lecithin)
glycolipids: FA’s compounded with CHO, but no N
derived lipids: substances from the above derived by hydrolysis
sterols: large molecular wt. alcohols found in nature and combined w/FA’s (e.g., cholesterol)

7. Saturated vs. Unsaturated Fatty Acids
saturated: the SFA’s of a lipid have no double bonds between carbons in chain
polyunsaturated: more than one double bond in the chain
most common polyunsaturated fats contain the polyunsaturated fatty acids (PUFAs) oleic, linoleic and linolenic acid
unsaturated fats have lower melting points
stearic (SFA) melts at 70oC, oleic (PUFA) at 26oC

8. Fatty Acids Commonly Found in Lipids

9. Saturated vs. Unsaturated Fats
saturated fats tightly packed, clog arteries as atherosclerosis
because of double bonds, polyunsaturated fats do not pack well -- like building a wall with bricks (sat.) vs. irregular-shaped objects (unsat.)
plant fats are much higher in PUFA’s than animal fats

10. Saturated vs. Unsaturated FA’s Plant vs. Animal Fat

11. Lipid Digestion/Absorption
Fats serve a structural function in cells, as sources of energy, and insulation
the poor water solubility of lipids presents a problem for digestion: substrates are not easily accessible to digestive enzymes
even if hydrolyzed, the products tend to aggregate to larger complexes that make poor contact with the cell surface and aren’t easily absorbed
to overcome these problems, changes in the physical state of lipids are connected to chemical changes during digestion and absorption

12. Lipid Digestion/Absorption
Five different phases:
hydrolysis of triglycerides (TG) to free fatty acids (FFA) and monoacylglycerols
solubilization of FFA and monoacylglycerols by detergents (bile acids) and transportation from the intestinal lumen toward the cell surface
uptake of FFA and monoacylglycerols into the cell and resynthesis to triglyceride
packaging of TG’s into chylomicrons
exocytosis of chylomicrons into lymph

13. Enzymes Involved in Digestion of Lipids
lingual lipase: provides a stable interface with aqueous environment of stomach
pancreatic lipase: major enzyme affecting triglyceride hydrolysis
colipase: protein anchoring lipase to the lipid
lipid esterase: secreted by pancreas, acts on cholestrol esters, activated by bile
phospholipases: cleave phospholipids, activated by trypsin

14. What about Bile???
These are biological detergents synthesized by the liver and secreted into the intestine
they form the spherical structures (micelles) assisting in absorption
hydrophobic portion (tails of FA) are located to the inside of the micelle, with heads (hydrophillic portion) to the outside
they move lipids from the intestinal lumen to the cell surface
absorption is by diffusion (complete for FA and monoglycerides, less for others)

15. Factors Affecting Absorption of Lipids
amount of fat consumed (fat=digestion=absorption)
age of subject ( age =  digestion)
emulsifying agents ( digestion =  absorption)
chain length of FA’s (> 18C =  digestibility)
degree of saturation of FA ( sat =  digestibility)
overheating and autooxidation (rancidification (rot) at double bond)
optimal dietary calcium = optimal FA absorption (high Ca =  absorption)

16. Lipid Metabolism/Absorption
short chain FA’s are absorbed and enter the portal vein to the liver
those FA’s with more than 10 carbons are resynthesized by the liver to triglycerides
they are then converted into chylomicrons and pass to the lymphatic system
some FA’s entering the liver are oxidized for energy, others stored
blood lipids: 45% P-lipids, 35% triglycerides, 15% cholestrol esters, 5% free FA’s

17. Lipid Digestion/Absorption

18. Lipid Digestion/Absorption

19. Characteristics of Fat Storage
Most of the body’s energy stores are triglycerides
storage is in adipose, source is dietary or anabolism (synthesis) from COH or AA carbon skeletons
remember obesity?
adipose can remove FA’s from the blood and enzymes can put them back

20. Fatty Acid Nomenclature
Nomenclature reflects location of double bonds
also used are common names (e.g., oleic, stearic, palmitic)
linoleic is also known as 18:2 n-6
this means the FA is 18 carbons in length, has 2 double bonds, the first of which is on the 6th carbon
arachidonic = 20:4 n-6

21. Fatty Acid Nomenclature
What’s in a Name??
Fatty Acid Nomenclature

22. Essential Fatty Acids Only recently determined as essential (1930)
body can synthesize cholesterol, phospholipids
research: same as AA’s but via addition (EFA’s added improved growth, NEFA’s didn’t)
requirement determined by depleting fat reserves of subject animal: difficult

23. Essential Fatty Acids (fish)
Most NEAA found in marine food webs
Essential fatty acids (to date):
linoleic (18:2 n-6; terrestrials; fish - not really)
linolenic (18:3 n-3; terrestrials; fish)
arachidonic (20:4 n-6; marine maybe)
eicosopentaenoic acid (20:5 n-3, marine)
docosohexaenoic (22:6 n-3, marine)
Why? Because elongation beyond 18 carbons is very difficult in marine fish (lack pathways)
actual EFA requirement is a matter of whether the fish is FW/SW or WW/CW

24. Essential Fatty Acids (most animals)
salmonids need n-3 FA’s for membrane flexibility in cold water (why does this work?)
trout can elongate and desaturate n-3 FA’s
Linoleic acid (18:2 n-6) is the most essential
addition of arachidonic is also helpful in deficient diets, but can be synthesized from linoleic (maybe sparing effect)
EFA’s, like EAA’s, must be dietary

25. Essential Fatty Acids LINOLEIC CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH
LINOLENIC CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH
18:3 n-3
EICOSOPENTAENOIC ACID
CH3CH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3COOH
20:5 n-3
DOCOSOHEXAENOIC ACID - YOU CAN DO THIS ONE!

26. Lipids as Crustacean Energy Sources
Largely, n-6 FA’s (linoleic) used for energy
as temperature drops, requirement for monounsaturated and PUFA’s increases
change in temperature = change in diet
cold water species = increased dietary HUFA’s
maturation animals: increased requirement for 20:4 n-6, 20:5 n-3 and 22:6 n-3 for proper spawning