8.2 Lipids (nutrition)

Question 1

What are the main lipids in the diet?

Answer 1

• The main dietary lipids are triglycerides (~95%), phospholipids (~4%) and sterols (~1%)
• Whether they are a fat or an oil, triglycerides are still digested and absorbed the same way, only the palatability and texture is different

Question 2

What is the source of most saturated fat in the Australian diet?

Answer 2

• Most fats come from animal sources and most oils come from vegetable sources
• Most saturated lipids were thought to come from animal foods in our diet, however with lowering of fat in meat and use of tropical oils by the food industry, this is not the case!
• Most saturated fat in our (Australian) diet is now coming from palm oil.
• Palm oil and coconut oil are highly saturated, and are semi-solid at room temperature.

Question 3

How is the use of palm oil increasing globally and what are the consequences?

Answer 3

• Global production of palm oil has doubled over the last decade. By 2000, palm oil was the most produced and traded vegetable oil (FAO 2002), accounting for 40% of all vegetable oils traded internationally. By 2006, the percentage had risen to 65% (FAO).
• New plantations are being developed and expanded in Indonesia, Malaysia and other Asian countries, as well as in Africa and South America.
• But this expansion comes at the expense of tropical forest – which forms critical habitat for a large number of endangered species, as well as an important atmospheric carbon trap.

Question 4

What fatty acid is found in the saturated fat of palm oil?

Answer 4

• Virtually all the saturated fat in palm oil is palmitic acid (16:0), one of the most hypercholesterolemic fatty acid, whereas in butter, cocoa butter, lard and tallow the dominant saturated fatty acid is stearic acid, which has no effect on blood cholesterol.

Question 5

What is the fatty acid breakdown of coconut oil?

Answer 5

• *Hypercholesterolemic fatty acids (see later)
• Almost 70% of the lipid in coconut oil is dangerous

Question 6

How does a triglyceride form?

Answer 6

Formed by condensation reactions between glycerol and three fatty acids

Question 7

What is the difference in the structure of a triglyceride and a phospholipid?

Answer 7

Similar in structure to triglycerides but have only two fatty acids attached to a glycerol backbone. The third position is occupied by a combined phosphate/base group, of which there are four types :

Question 8

What are the four options for the phospholipid base group?

Answer 8

• The four base groups that make up phospholipid molecules are:

1. Serine
2. Choline
3. Ethanolamine
4. Inositol

• Hence there are 4 types of PL’s: PS, PC, PE, PI

Question 9

What properties of phospholipids are handy in the food industry?

Answer 9

• Apart from their presence in cell membranes, phospholipids are used in the food industry, to suspend lipids in an aqueous environment (emulsifiers)
• This property is due to the fact that phospholipids have a non polar region (the two fatty acids) that is attracted to other lipids, and a polar region (the phosphate/base group) that is attracted to water.

Question 10

What are the two different sources of sterols?

Answer 10

• Cholesterol is found in animal foods only – meat, eggs, fish, etc.
• Plant-based sterols include Stigmasterol and campesterol + others
• Plant sterols are present naturally in small quantities in many fruits, vegetables, nuts, seeds, cereals, legumes and vegetable oils
• Plant-based sterols can interfere with cholesterol absorption in the small intestine, thus lowering blood cholesterol levels.

Question 11

What is the role of cholesterol in the body?

Answer 11

• Starting material for bile acids, sex hormones, adrenal hormones and vitamin D.
• Structural component of cell membranes.
• Liver produces 800 to 1500 mg cholesterol per day (much more than we eat).
• Atherosclerosis is a disease that causes heart attacks. It occurs when oxidised cholesterol forms deposits in the artery wall.

Question 12

How would this fatty acid be named in terms of the double bond position?

Answer 12

• Linoleic acid, an 18-carbon polyunsaturated fatty acid (18:2n6) (ie the first double bond from omega end is on carbon 6)

Question 13

What are the different families of polyunsaturated fatty acids?

Answer 13

• There is variation in location of double bonds as well as their number.
• This effects the behaviour of the fatty acid in the body, particularly in how it is metabolised, what function it serves in cell membranes and the effects on over all health

Main PUFA subgroups are:

• Omega-6
• Omega-3

Other families or subgroups are:

• Omega-7
• Omega-9

Question 14

What are the three most important omega 3 PUFAs?

Answer 14

Question 15

How can humans add extra carbons to fatty acids?

Answer 15

• Fatty acids can have extra carbons added to make them longer or more double bonds added to make them more unsaturated by various enzymes in your body.
• Humans do this well for omega-6 and 9 fatty acids , but are very poor at doing this with omega-3 PUFA
• ALA →EPA ~5%
• ALA →DHA ~0.1%
• Also humans cannot interconvert fatty acids between series, ie once an omega-6 fatty acid is consumed , it may become longer and have more double bonds, but remains an omega-6 fatty acid

Question 16

What are the natural trans fatty acids?

Answer 16

A few natural trans forms occur in some animal foods and milk, particularly human breast milk (CLA and vaccenic acid) and have never been shown to cause a health issue.

Question 17

How does hydrogenation give rise to trans fatty acids?

Answer 17

• However some fatty acids are changed from cis to trans configuration during hydrogenation of plant oils to make solid margarine.
• Mainly elaidic acid (18:1 trans-9) is formed
• Acts like some saturated fatty acids in the body, blood cholesterol levels.
• It’s heart disease connection is being researched and results implicate 18:1 trans-9 (but not all trans fatty acids) as being involved in CVD.

Question 18

Which trans fatty acid raises blood cholesterol levels?

Answer 18

• Food sources of elaidic acid (18:1 trans-9) include deep-fried foods using vegetable shortening, cakes, biscuits, doughnuts, pastry, crackers, snack chips, margarine, imitation cheese.
• This trans fatty acid does raise blood cholesterol leve

Question 19

How do natural trans fatty acids affect blood cholesterol levels?

Answer 19

• Food sources of natural trans fatty acids (CLA and vaccenic acid) are meat and dairy products, including breast milk.
• Neither alter blood cholesterol levels and both appear to have health benefits
• Also dietary cholesterol itself has little or no effect on blood cholesterol levels

Question 20

What is the role of lipase in digesting lipids?

Answer 20

Digestion of triglycerides by lipase enzymes in the first section of the small intestine usually leads to a monoglyceride and two fatty acids

Question 21

What is bile?

Answer 21

• For lipids to break down they must be drawn into the watery environment of the GI tract.
• This process is termed emulsification and is made possible by the action of bile.
• Bile is a mixture of liver produced substances, the main ingredient (bile acids) are made from cholesterol.
• Active form is a bile salt or acid that is combined with an amino acid group

Question 22

What is bile made from?

Answer 22

Question 23

What is the action of bile?

Answer 23

Question 24

What is meant by entero-hepatic recirculation of bile?

Answer 24

• Cholesterol in bile is a critically important compound in the body.
• It is produced in the liver for many bodily functions including cell membrane function, and hormone and Vitamin D production etc etc.
• The body has evolved a process by which it conserves cholesterol by reabsorbing bile after it is used for emulsification.
• Cholesterol products in bile are excreted into the duodenum and function as a fat emulsifier. They are then reabsorbed into the blood and returned to the liver for reuse.

Question 25

What happens to lipids that are emulsified?

Answer 25

• Once emulsified lipids are easily broken down into smaller molecular units by pancreatic and intestinal lipase enzymes
• The smaller chemical units are glycerol and short, medium and long chain fatty acids
• Glycerol and smaller fatty acids can enter the intestinal lining cells easily
• Some TG is not fully broken down and form monoglycerides .

Question 26

How are the larger monoglycerides and long chain fatty acids absorbed in digestion?

Answer 26

• These monoglycerides and and long chain fatty acids are large on a molecular scale. They are more water insoluble and form spherical complexes called micelle that then enter intestinal mucosal cells
• Monoglycerides and longer chain fatty acids then combine with cholesterol and proteins to form chylomicrons in the mucosal cells
• Chylomicrons enter the lymph in the villi where they reach the circulatory system at the thoracic duct and subclavian vein in the upper chest region (review Digestion lecture and chapter)

Question 27

What are chylomicrons?

Answer 27

• Chylomicrons are large lipoprotein particles that take larger lipid products via lymph vessels in the villi bypassing the liver.
• The smaller fatty acids enter the blood vessels in the villi along with other nutrients and are taken via the Portal Vein to the liver..

Question 28

What are the three types of lipoproteins (transport of lipids)?

Answer 28

• VLDL (Very-Low-Density Lipoproteins) – composed primarily of triglycerides are produced in the liver.
• LDL (low-density lipoproteins) – composed primarily of cholesterol are derived from VLDL particles that have already distributed their TG load around the body.
• HDL (high-density lipoproteins) – composed primarily of protein, scavenge excess cholesterol from around the body and returns it to the liver for reuse or excretion as bile.

Question 29

Where is each type of transport lipoprotein produced?

Answer 29

Chylomicrons made in the intestinal lining cells, VLDL, LDL, HDL made in the liver

Question 30

What is the structure of lipoproteins (for lipid transport)?

Answer 30

• The protein molecules and PL heads are hydrophilic ie. They like water, hence are on the surface.
• The hydrophobic lipids such as TGs and cholesterol are embedded inside the particles, away from water environments such as blood and lymph.

Question 31

What is the comparative size of each of the transport lipoproteins?

Answer 31

Comparative size of various lipoproteins, indicating the relatively large size of the triglyceride rich chylomicrons from intestinal digestion compared with the liver derived VLDL, LDL and HDL particles.

Question 32

How do the lipoproteins differ in percentage composition?

Answer 32

• As well as significant size difference, lipoproteins differ in percentage composition.
• The chart to the right indicates the relative % contribution of TG, PL, Cholesterol and protein to the mass of each lipoprotein type.

Question 33

How does LDL/HDL have health implications for cholesterol?

Answer 33

• There is excess cholesterol being transported in LDL particles by people consuming westernised diets. This is due to a high saturated fat intake in the diet.
• Saturated fats do not make cholesterol themselves but up-regulate the production of cholesterol in the liver and decrease its removal from the body.
• The high amounts of LDL-cholesterol are correlated with increased CHD rates
• As cholesterol in HDL is being removed from cells and blood vessels to the liver.
• High HDL cholesterol is correlated with lowered CHD death rates.
• HDL particles are made in the liver and rate of production responds to exercise.

Question 34

How does the ratio of LDL to HDL relate to cardiac events?

Answer 34

• The greater the LDL : HDL cholesterol ratio the greater the chance of coronary events such as heart attacks
• The result of chronically raised LDL : HDL cholesterol levels is arterial blockage by plaque.
• As the coronary arteries are relatively narrow, divided and crucial to heart function, any blockage in these can lead to a heart attack (MI).
• Clear message is to lower LDL cholesterol by decreasing intake of saturated fat and increase HDL cholesterol by increasing exercise.

Question 35

How do triglycerides gain access to cells to be used up for energy?

Answer 35

• Some energy used by cells comes from triglycerides, these TGs are delivered to cells by:
• Chylomicrons directly from food from the intestines and VLDL from liver storage
• Triglycerides are too large to pass from the blood through cell membranes so they are hydrolysed by lipoprotein lipase (LPL) into glycerol and free fatty acids, which are small enough and can be taken up by cells easily
• Once absorbed into functional cells of the body the fatty acids and glycerol are metabolised for energy .

Question 36

How is excess triglycerides dealt with?

Answer 36

Excess TGs are directed to adipocytes (fat cells) where LPL directs the breakdown products into the cells and cellular enzymes then reverse the process and recombine fatty acids and glycerol into triglyceride molecules for energy storage

Question 37

What is the structure of a fat storage cell (adipocyte)?

Answer 37

*

Question 38

How are fatty acids metabolised for energy?

Answer 38

• Hormone sensitivel lipases in adipose cells hydrolyse TG when energy is required. Glycerol and fatty acids formed are transported back into the blood (to form lipoproteins) to be circulated to cells requiring energy
• Fatty acids are converted to acetyl-CoA then oxidised to carbon dioxide and water releasing bond energy in the mitochondria of cells through the TCA cycle / ETC

Question 39

Which saturated fatty acids affect blood cholesterol?

Answer 39

• Some saturated fatty acids cause blood cholesterol levels to rise
• The short chain saturated fatty acids and stearic acid (18:0) do not raise blood cholesterol levels
• Some trans fats behave like saturated fatty acids

Question 40

How do unsaturated fatty acids impact cholesterol?

Answer 40

• PUFA in the diet lowers LDL cholesterol
• Intake of MUFA and cholesterol have no significant effect on blood cholesterol level
• Omega-3 fatty acids in the diet lower blood triglyceride and CHD risk
• Exercise and female sex hormones increase HDL cholesterol levels and therefore decrease risk of CVD

Question 41

What does this equation mean?

Answer 41

• indicates the relative effects on plasma cholesterol of various types of fatty acid
• Where delta TChol = change in plasma total cholesterol level (mmol/L); delta LDLChol = change in plasma low density lipoprotein cholesterol level (mmol/L).
• (delta fatty acid) = change in intake of fatty acid expressed as energy percentage.
• Any positive contribution by a fatty acid will increase plasma cholesterol level, whereas any negative value will have a cholesterol-lowering effect.
• number is magnitude of how much they change it

Question 42

What actually has an effect on blood cholesterol?

Answer 42

• Dietary cholesterol consumption has little or no effect on blood cholesterol
• The liver produces cholesterol and the blood level rises due to saturated fat intake causing the liver to make more cholesterol and remove less

Question 43

What are the recommendations on fat intake of PUFA?

Answer 43

• Replace saturated fats in the diet with MUFA (monounsatured fat) and PUFA (polyunsaturated fat)
• Balance omega-3 and omega-6 PUFA intake (at present our diet is heavily weighted towards the omega-6 type – rich in grains)
• Longer chain forms of omega-3 fatty acids (EPA and DHA) are anti- inflammatory, reduce blood clotting (hence MI) and lower blood pressure
• The longer chain omega-3 fatty acids come from animals only

Question 44

What are the sources of fat in our diet?

Answer 44

• Saturated fats are found mainly in: dairy products, coconut and palm oil, hydrogenated products and eggs and meats (mainly the visible fat on meat).
• Palm oil is common in processed foods and baked products
• Cholesterol is found in eggs, meats, milk products but not plant material
• Meat can be low in fat (approx 2%) if consumed as fat trimmed and from lean meat cuts. High fat meats include hamburgers, sausages, salami and many processed meats, many of which can be up to 25-30% fat by weight.

Question 45

What two essential fatty acids go on to make other fatty acids?

Answer 45

• Linoleic acid (18:2n6) from vegetable oils is from the omega-6 family and can make arachidonic acid (20:4n6) in vivo, a conditionally essential fatty acid, also consumed preformed in meats and fish.
• Alpha-Linolenic acid (18:3n3) from canola, flax and some other plants is essential for health and is from the omega-3 family. It can be turned into EPA (eicosapentaenoic acid 20:5n3) and DHA (docosahexaenoic acid 22:6n3) in vivo, but not well in humans.

Question 46

What is the function of essential fatty acids in the body?

Answer 46

• These long chain omega-3 fatty acids (EPA & DHA) are important for eyes, brain and heart structure and function and can be consumed preformed from animal foods (fish and grass fed red meat).
• Eicosanoids, made from arachidonic acid (20:4n6) and EPA are ‘hormone like substances’ that include prostaglandins, thromboxanes and leukotrienes, which control many cell, tissue and organ functions.

Question 47

What are the recommendations for the consumer for reducing fat intake?

Answer 47

• Lean and very lean options of meats and meat alternates should be chosen (this will limit hypercholesterolaemic fatty acids and ensure adequate Long Chain omega-3 fatty acids in the diet).
• Choose fat-free and low-fat milks and milk products to lower total fat intake below 30%. However the specific fatty acids in dairy foods are not rich in hypercholesterolaemic saturated fatty acids but rather shorter chain forms.
• Butter, cream and some cheeses (all low in water) however can be rich sources of dangerous saturated fatty acids – so limit their intake.
• Choose a wide variety of vegetables, fruits, and wholegrain foods (these are low in hypercholesteroaemic fatty acids and contain many MUFA and PUFA types).