Week 6: Lipids (Ch. 05):
Section 1: Lipid Metabolism Section 2: Lipids and Health
Lipid Metabolism
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There are three major components of dietary lipids
u Triglycerides- the most common component of dietary lipids u Phospholipids
u Sterols-cholesterol
The health-related properties of triglycerides are determined by the fatty acids that are attached to the glycerol molecule.
u Categories of fatty acids:
uShort, medium, and long chain fatty acids u Saturated
u Unsaturated
uMonounsaturated & polyunsaturated
uOmega 3, 6, & 9 fatty acids u Cis
We have already discussed some health- related properties of fats:
u Diets low in saturated fat and high in unsaturated fat reduce the risk of cardiovascular disease. Canada’s Food Guide recommends foods containing “healthy fats” which are foods high in unsaturated fat.
u Trans fatty acids, created by the partial hydrogenation of vegetable oils, were banned from the Canadian food supply in 2018 because they increased the risk of cardiovascular disease,
u There are trans fatty acids that are naturally occurring in beef and dairy products, but they differ in chemical composition from the banned trans fats and are not considered a health concern.
The steps in the digestion of triglycerides are shown here. Please note the typo that has been corrected in this figure. Lipases from the pancreas break down triglycerides to fatty acids and monoglycerides. Because these compounds do not readily dissolve in the watery environment of the lumen, bile is secreted by the liver. This bile emulsifies the digested lipids, forming small droplets called micelles which can be absorbed. Other fat-soluble compounds also enter the micelle and are also absorbed.
Note: Correction of typo in text.
monoglycerides.
chylomicrons
Lipoproteins
u Lipids are insoluble in blood, which is water-soluble
u To be able to circulate in blood they are organized into lipoproteins
u Inner core: triglycerides and cholesterol
u Outside: water-compatible phospholipid and proteins
Bile: Mixture of cholesterol and bile acids (made from cholesterol)
After a meal- postprandial state- chylomicrons bring lipids to body cells
fatty acids
Between meals (fasting state) the liver releases lipids in VLDL particles; fatty acids from VLDL are taken up by body cells
Reverse cholesterol transport
Blood Lipids Panel
Serum Triglycerides
Total Cholesterol
Additional Measurements
Non-HDL cholesterol
Ratio: Total-C: HDL-C
Ratio: LDL-C: HDL-C
Lipid Functions in the Body
u Structure
u Adipose tissue –storage of lipids à insulation & cushioning to protect organs u Brain and nervous system: insulation of myelin sheath
u Cell membranes
Lipid functions
u Essential fatty acids
u Linoleic acid (omega-6)
u Alpha-linolenic acid (omega-3)
u Role in cell membranes, red blood cells, skin, fertility
u Linoleic acid à Converted to arachidonic acid (omega-6)
u Alpha-linolenic acid à Converted to EPA (omega 3) & DHA (omega-3) u Eicosanoid synthesis and function
u Ecosanoids = hormone-like molecules
One of the main functions of lipids is as a source of energy.
Fatty acids undergo beta-oxidation in tissue producing Acetyl Co-A.
Acetyl CoA enters the TCA cycle ultimately to produce ATP.
Ketone bodies are produced when carbohydrate intake is very low or during fasting or starvation.
Carbohydrate must be present in the diet in order for oxaloacetate to
be present
Storing and retrieving energy from adipose tissue
Uptake of lipids by adipose tissue stimulated by insulin; hormone sensitive lipase is suppressed
Between meals, when insulin levels drop, hormone sensitive lipase, breaks down lipids in adipose tissue, releasing it into the blood
These lipids are called free fatty acids (or non- esterified fatty acids) because they are not packaged in lipoproteins but circulate in the blood bound to the protein, albumin.
19, from either chylomic rons or VLDL,
Why LDL isn’t just “bad”
• Synthesis of important hormones
• Testosterone in males
Integration of carbohydrate and fat metabolism
Post prandial
Between meals
Lipids and Health
Development of atherosclerosis
Development of atherosclerosis
Effect of fatty acids on cholesterol levels: Saturated and trans fat
Effect of fatty acids on cholesterol levels: unsaturated fats
Dietary recommendation:
uFoods high in unsaturated fat should replace foods high in saturated fat.
Total cholesterol:HDL cholesterol ↑ à ↑ CVD
Saturated Fat –Scientific Evidence
u Saturated fat intake linked to an increased risk of cardiovascular disease
uBased on studies beginning in the late 1950s uSaturated fat à increased risk uPolyunsaturated fat à decreased risk
uMajor message in public health nutrition
uReduce saturated fat intake, increase polyunsaturated
u More recent studies have shown that increased saturated fat intake is not associated with an increased risk of cardiovascular disease (CVD), but these studies did not consider replacement nutrient
Replacement nutrient
The importance of replacement nutrients.The isocaloric replacement of saturated fat in the typical Western diet with trans fat increases the risk of cardiovascular disease by 5%, while replacement with refined carbohydrates does not appreciable change the risk. Only when saturated fat is replaced with monounsaturated fatty acids (MUFAs), polyunsaturated fatty acids (PUFAs), or carbohydrates from whole grains, is there a statistically significant decrease in risk. Statistical significance is indicated by
30 CI (black line) not crossing zero.
Source: Li Y, , M, H, D, Chiuve SE, , M, Rimm EB, C, Hu FB. Saturated Fats Compared With Unsaturated Fats an Sources of Carbohydrates in Relation to Risk of Coronary Heart Disease: A Prospective Cohort Study. J Am . 66(14):1538-1548, 2015.
How does saturated fat compare to other known dietary risk factors?
Danaei G, L, Mozaffarian D, , , J, Ezzati M.
The preventable causes of death in the United States: comparative risk assessment
of dietary, lifestyle, and metabolic risk factors. PLoS Med. 2009 Apr 28;6(4):e1000058. doi: 10.1371/journal.pmed.1000058.
Dietary cholesterol
u Dietary cholesterol might lead to slightly elevated blood cholesterol u Saturated fats is the much more significant risk factor
u Most foods high in cholesterol are also high in saturated fat
u Reducing saturated fat, therefore, also reduces dietary cholesterol u Exception: Eggs
u Low in saturated fat, very high in cholesterol
u Cohort Studies: one egg/day à no change in CVD risk u RCTs: generally agree with cohort studies
Required Textbook Reading
u Critical Thinking: Fish Consumption and Heart Disease
u pg. 220, A46: Note it does not include a p trend; assume that the association
shown is significant.
u Appendix page numbers (A36, A46), where answers to critical thinking questions, in the reading, are located, are for print text; for e-text look for an active link to the answers.
Dietary patterns that may reduce cardiovascular disease:
Dietary Patterns Effective in Cardiovascular Disease:
Cohort studies: Many studies report inverse association between adherence to Mediterranean diet and CVD
RCTs: Many studies report a reduction in risk factors, such as blood lipids
Dietary Approaches to Stop Hypertension
Moderate levels of sodium Foods high in potassium, magnesium, calcium
RCTs: Lowering of blood pressure
Also: lowers blood cholesterol
https://www.health.harvard.edu/diet-and-weight-loss/dash-or- mediterranean-which-diet-is-better-for-you
https://food-guide.canada.ca/en/
Jenkins et al 2005
http://childnutrition.utoronto.ca/sites/default/files/Portfolio%20Diet%20Scroll%20editable_eng2.pdf
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