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Lecture 14
Chapter 26: Lipid Metabolism
Sections 1-
Lipids Pack More Energy
-! Triglycerides are the richest energy
source in the human diet
-! The oxidation of -! 1g of a typical fat/oil liberates ~9.5 kcal of energy -! 1g of carbohydrate liberates ~4.2 kcal of energy -! Lipid molecules are more highly reduced
(contain more C-H bonds)
-! Lipids have a greater capacity to
combine with O 2 and consequently have
a higher heat content
-! Lipids are our long-term energy storage
molecule
Nutritional Aspects of Lipids
-! The nutritional aspects of lipids are not completely understood -! An adult human can survive on a nearly fat free diet, consuming only protein and carbohydrate for energy -! But certain fats are essential or necessary: -! Linoleic and linolenic acids are required for normal growth and development (are essential must be obtained from diet) -! Arachidonic acid , in humans, can only be synthesized from linoleic acid, so it is also classified as essential, most importantly it is the precursor to prostoglandins -! Essential fatty acids are key components of biological membranes and are necessary for the efficient transport & metabolism of cholesterol
Linoleic Acid
Linolenic Acid
Arachidonic Acid
Differences in Fat Storage
-! Men and women differ in their capacity to store fat: -! An average adult male will have ~16% body fat -! An average adult female will have ~25% body fat -! Athletes in superb condition have less -! A male athlete can have less than 7% body fat -! A female athlete can have less than 12% body fat
This is a measure of % body fat, measurements need
to be taken to determine a personās specific value
Methods to measure body fatness include skin fold thickness measurements (with calipers), underwater weighing, bioelectrical impedance, dual-energy x-ray absorptiometry (DXA), and isotope dilution. However, these methods are not always readily available, and they are either expensive or need highly trained personnel. Furthermore, many of these methods can be difficult to standardize across observers or machines, complicating comparisons across studies and time periods.
BMI
-! According to the CDC website: āBody Mass Index (BMI) is
a number calculated from a person's weight and height.
BMI provides a reliable indicator of body fatness for most
people and is used to screen for weight categories that
may lead to health problemsā
BMI Formula: [weight (lb) / [height (in)]^2 x 703
BMI
Weight Status
Below 18.5 Underweight
18.5 ā 24.9 Normal
25.0 ā 29.9 Overweight
30.0 and Above Obese
This is an easy
inexpensive way to
determine relative
healthiness
Lipid Consumption
-! Americans consume ~100-125g of lipids/day, which
represents ~34% of the daily calorie requirement
-! Health authorities advise people to reduce lipid
consumption to less than 30% of their total caloric intake
-! The daily diet should, however contain 4-6g of the
essential fatty acids
-! Infants lacking essential fatty acids in their diet are at risk
of weight loss and eczema
-! Nuts, seeds, soybean, and soybean oil are good sources
of both linoleic and linolenic acids
Fat vs.. Glycogen
-! Fat reserves in the average person provides sufficient
energy to survive starvation for 30-40 days
-! In comparison, the glycogen stores in the liver are
depleted within 1 day
-! Glycogen is extremely hydrated- 2g of water is bound per
every 1g of glycogen stored
-! Fat is not hydrated, so 1g of body fat is 6x more energy
rich than 1g of hydrated glycogen
-! The ability to store greater amounts of fat is especially
important for migrating birds and some terrestrial animals
Fat Storage
-! Fat is primarily stored in fat cells (or adipocytes), found in adipose tissue -! In these cells most of the cytoplasm is replaced by a large fat droplet of triglycerides (~90% of the cell) -! Adipocytes produce, store and mobilize triglycerides and are among the largest cells in the body -! Adults have ~ 30-40 billion fat cells that swell and shrink depending on the amount of fat inside them -! Adipose tissue is the only tissue in which free triglycerides occur in appreciable amounts -! In non-adipose tissue lipids are bound to protein
Fat Depots
-! Are body locations containing large amounts of
adipose tissue
-! There are certain regions where accumulation occurs: -! Subcutaneous fat depot- is important for insulation -! Abdominal fat depot- long term energy storage -! Fat deposits around vital organs- serve as a protective cushions
Triglyceride Metabolism
-! In order for the chemical
energy in triglycerides to be
released, they must be cleaved
by lipases
-! Fat metabolism is also
regulated by hormones, such
as epinephrine & glucagon
-! These hormones are secreted when blood glucose level is low -! They bind to receptors on adipocytes to stimulate hydrolysis of triglycerides by lipases, and release of fatty acids and glycerol -! Lipids recently consumed can be: -! transported by lipoproteins for storage OR -! can be hydrolyzed by lipases in the capillary walls to also produce glycerol and FAs
Dietary Lipids (absorbed from the intestine)
Lipid Transport (circulatory system)
Triglycerides (synthesized in the liver)
Stored Fatty Acids (from adipose tissue)
Insulation^ Fat Depots against heat loss (^) Protection of vital organs against injury
Storage of energy reserves
Degradation and synthesis of other lipids Conversion to brain and nervous tissue Fatty acid oxidation to produce energy
Fate of Lipid
Products
-! The fatty acids produced are transported by serum albumin in the blood to other tissues for oxidation -! The glycerol is transported to the liver where it is converted to dihydroxyacetone phosphate
Glycerol Kinase! Glycerol PhosphateDehydrogenase!
Can enter glycolysis or gluconeogenesis
Efficiency of Fatty Acid Metabolism
-! The combustion of 1 mol of palmitic acid releases:
C 16 H 32 O 2 + 23 O 2 16 CO 2 + 16 H 2 O + 2340 kcal
-! % Efficiency of the Cell:
Energy Conserved
Total Energy Available X 100% =^
(106 ATP)(7.5kcal/ATP) 2340 kcal X 100% = 34%
-! The efficiency of fatty acid metabolism (34%) is comparable to carbohydrate metabolism (35%)
Complete Oxidation of Triglycerides
-! What is the total amount of ATP produced from the complete oxidation of 1 mol of triglyceride to CO 2 and H 2 O, using glycerol palmitate as an example:
Glycerol palmitate is composed of 3 mol of palmitic acid + 1 mol of Glycerol
Oxidation of 1 mol of palmitic acid produces 106 mol of ATP Since there are 3 mol of palmitic acid 318 mol of ATP are produced
Oxidation of 1 mol of glycerol produces between 16.5-18.5 mol of ATP , as a consequence of glycolysis + Krebs cycle + e -^ transport chain + oxidative phosphorylation
Thus the total yield of ATP would be 334.5 to 336.5 mol of ATP
Amount of ATP Produced from
Glycerol Breakdown
-! Glycerol !! dihydroxyacetone phosphate -! 1 ATP is consumed and 1 NADH is produced (1.5-2.5 ATP, depends on cell type) = .5-1.5 ATP -! Dihydroxyacetone phosphate !! Glycolysis -! Produces 1 NADH (1.5-2.5 ATP, depends on cell type) + 2 ATP = 3.5 - 4.5 ATP -! Pyruvate !! Acetyl CoA -! Produces 1 NADH = 2.5 ATP -! Acetyl-CoA !! Krebs Cycle -! Produces 1GTP (1ATP), 3NADH (3 x 2.5ATP), & 1FADH (1.5ATP) = 10 ATP -! Total = 1.5 + 4.5 + 2.5 + 10 = 18.5 ATP -! OR Total = 0.5 + 3.5 + 2.5 + 10 = 16.5 ATP
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