Lipids

Question 1

What are the types of lipids found in food, rich food sources, and their energy value?

Answer 1

Saturated = butter, animal sources
Monounsaturated = olive oil, avocados, peanuts, almonds
Omega 3 PUFA = fish, shellfish flaxseed, hemp seed, walnut, grape-seed, soybean
Omega 6 PUFA = chicken, whole grain wheat, vegetable oils

Question 2

Terminology pertaining to triglyceride formation and breakdown

Answer 2

Esterification, lipolysis, lipogenesis, beta-oxidation

Question 3

Where do dietary trans-fats come from? (production and sources)

Answer 3

Processed foods (e.g. margarine) or ruminant derived foods. Produced via partial hydrogenation of sat FA

Question 4

Describe the digestion, absorption and transport of dietary lipids

Answer 4

Lingual lipase from serous gland. Gastric lipase from chief cells. CCK triggers bile and pancreatic enzyme secretion. Bile emulsifies fat to convert into micelles. Pancreatic enzymes act on micelles: pancreatic lipase (triglycerides to glycerol and FFA), pancreatic phospholipase (phospholipids to FFA, glycerol and phosphoric acid), pancreatic cholesterol esterase (cholesterol esters to cholesterol and FFA)

The water soluble micelles diffuse their contents at the brush border into the enterocytes (distal duodenum and jejunum), moving down a concentration gradient. Long chain FA re-esterified and combine with cholesterol and protein to form chylomicrons. These enter the lymphatic system via lacteals. Short and medium chain FA absorbed directly into the portal vein.

Question 5

Lipids are transported in the circulation via lipoproteins. Describe the characteristics, composition and role of the four main lipoproteins

Answer 5

Chylomicrons = mostly triglycerides. Transport triglycerides from GIT to cells and liver

VLDL = mostly triglycerides. Transport triglycerides from liver to cells

LDL = mostly cholesterol. Transport cholesterol from liver to cells

HDL = mostly protein. Transport cholesterol from cells to liver and excretion

Question 6

Explain the chemical characteristics and specific physiological function of dietary FA

Answer 6

Provide energy
Insulate and protect body structures
Component of membranes (phospholipids), strength (cholesterol and SCFA) and fluidity (PUFA)
Fat soluble digestion and absorption
Precursor of sex hormones
Precursor of eicosanoids

Question 7

What are EFAs? Why are they essential? Describe the omega and delta naming methodologies for these FA

Answer 7

EFSa essential because the body doesn’t produce them endogenously. Lack of ∆12 and ∆15 desaturase enzymes

Delta naming from the carboxyl end 18:1∆^15
Omega naming from the methyl end 18:1n-3

Question 8

Explain the function of EFAs

Answer 8

EFAs important structures within cell membranes = DHA/EPA keep the membrane fluid

DHA/EPA essential in foetal development = assist in development and function of brain, retina, nervous system

DHA/EPA essential throughout life = regulation of nerve transmission, signalling in gene expression, formation of eicosanoids

Question 9

What are eicosanoids? How are they made?

Answer 9

Hormone-like structures formed from EFA. Responsible for numerous cell functions

n-6 PUFA are pro-inflammatory = cause constriction of BV leading to increased BP

n-3 PUFA are anti-inflammatory = cause dilation of BV leading to decreased BP

Question 10

What are 5 enzymes involved in the digestion of lipids?

Answer 10

Lingual lipase, gastric lipase, pancreatic lipase, pancreatic phospholipase, pancreatic cholesterol esterase

Question 11

Explain the entero-hepatic circulation? What is its purpose?

Answer 11

Responsible for recycling of bile. 95% is reabsorbed in the distal ilium into the hepatic portal vein

Question 12

How can the entero-hepatic circulation be used to modulate circulating cholesterol?

Answer 12

Fibre can bind cholesterol and excrete it. This decreases the amount reabsorbed. Liver must generate now bile from cholesterol. Lowers circulating cholesterol

Question 13

What are the sources of circulating cholesterol

Answer 13

Endogenous synthesis in the liver, intestines, gonads, adrenal glands

Question 14

Explain briefly endogenous cholesterol syntheses

Answer 14

37 step process. Starting molecule is acetyl CoA

Synthesis is increased when plasma cholesterol is low and when BGL is increased

Question 15

What exactly is measured when “blood cholesterol” is measured in a routine blood test?

Answer 15

Lipoproteins

Question 16

What is the apolipoprotein in lipoprotein? How is it made?

Answer 16

A “marker” on the surface of the lipoprotein. Assists in cell recognition and facilitates movement across membranes. Synthesised via DNA expression in the liver

Question 17

Describe cholesterol uptake in the peripheral cells

Answer 17

Receptor pathway = B100 LDL receptor recognises the LDL apolipoprotein. LDL is taken up via endocytosis and its content is used by the cell. B100 synthesis is increased when cholesterol concentration in the cell is low and blocked when high. When the system is dysregulated or when there is too much circulating LDL, LDL can build up in the blood

Question 18

Describe plant sterols, where they are found, and their role in reducing dietary cholesterol absorption

Answer 18

Plant sterols are chemically similar to dietary cholesterol from animal products but cannot be converted to steroids in the body.

Found in the cell wall of whole grains, fruits, legumes and leafy vegetables

Reduce dietary cholesterol absorption by competing with cholesterol. Plant sterols are incorporated to micelles during lipid digestion, thus reducing the amount of dietary cholesterol in micelles. 2-3g/day needed

Question 19

Explain the mechanism by which oats can reduce circulating cholesterol

Answer 19

Oats contain B-glucans, a type of soluble fibre. Binds to bile and increases excretion. Lack of reabsorption leads to increased synthesis from cholesterol = decreased plasma cholesterol

Question 20

What is the difference between MCT and LCT metabolism?

Answer 20

MCT absorbed directly into the portal vein bound to albumin. Can be used more rapidly for energy production

LCT absorbed into lymphatic system via lacteals in chylomicrons

Question 21

Describe how EFAs are involved in the inflammatory response of the body

Answer 21

Precursors of eicosanoids. Pro vs anti-inflammatory

Question 22

What is the diet-heart hypothesis: how was it developed; what does it state?

Answer 22

DIetary cholesterol and saturated fat raise blood cholesterol. High blood cholesterol associated with atherosclerosis and thus coronary heart disease. Therefore saturated fat and cholesterol increase risk of CVD

However, study was funded by the sugar industry and involved feeding cholesterol to rabbits (vegetarian)

Question 23

Briefly (but precisely) describe how atherosclerosis develops

Answer 23

Scavenger pathway = LDL not taken away by HDL can become oxidised. This oxidised LDL can burrow under the endothelium and become engulfed (scavenged) by macrophages. This turns the LDL into foam cells. These cells often become trapped in the walls of blood vessels and contribute to atherosclerotic plaque formation => hardening of the blood vessel wall => less flexibility to adapt to changes in blood volume => hypertension

Narrowing of blood vessel => risk of clot => ischemia => stroke

High BGL can damage the endothelium, making cholesterol deposit take place easier

Question 24

Is dietary fat the culprit in the development of atherosclerosis? Explain your answer

Answer 24

No. Dietary fat is broken down via digestion and absorption. Lipoproteins have to be reformed. Increased BGL may actually be the culprit as it can damage the blood vessel endothelium, allowing easier deposition of oxidised LDL

Question 25

What are the Australian recommendations for fat intake?

Answer 25

20-35% EER, no more than 10% sat + trans, 0.2% n-3 PUFA (SDT)

Question 26

What are the essential fatty acids? Explain why specifically they are essential. Explain why the EFAs status (i.e. having sufficient supply for good physiological function) may be problematic in vegan diets

Answer 26

EFAs cannot be synthesised in the body as we lack ∆12 and ∆15 desaturase enzymes

ALA and LA can be elongated or have more double bonds added, but the rate of this is limited by enzymes (genetic variance can result in decreased rates). This is a concern for vegan diets since the only source of DHA and EPA is oily fish and algae