lipolysis. largest storage form of energy provides energy at the slowest rate stored: –adipose...
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Lipolysis
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Lipolysis
• Largest storage form of energy
• Provides energy at the slowest rate
• Stored:– adipose tissue – muscle – Brain, CNS, abdomen,
etc.
• Use of lipids spares glycogen during prolonged work
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Lipids• Substance that
is water insoluble, but soluble in organic solvents
• Most are Non-polar (uncharged)
• Important in a variety of roles
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Cholesterol:•Sterol: Comes from diet or is synthesized in liver•Important: Cell membrane structure•Steroid hormone synthesis
•Testosterone, Estrogen, Corticosteroids
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•Derived from incomplete Fat metabolism•Formed from excess Acetyl-CoA•Kreb’s cycle slows due to low CHO stores•2 Acetyl-CoA molecules
•Acetoacetyl-CoA•Acetoacetate•D-β-Hydroxybutyrate
•Last two used for energy
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Exception to “polarity” rule•Found in cellular membranes•Profers some “selectivity” to the membrane
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• Triglycerides– Biggest
percentage– Cholesterol
and phospholipids
– Digested in small intestine
– Bile: emulsifying agent
Fat digestion
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• Pancreatic lipase– Breaks down fat globule (Micelles)
• Monoglycerides, FFA and glycerol– Taken up by small intestinal cells– Repackaged with intestinal cells as Chylomicrons– Released into lymph
• Different from carbs, most go to heart first
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Chylomicrons and lipoproteins
• Two mechanisms of fat clearance from blood– Transport to liver
• Uses fats for fuel• Converts to
lipoproteins– Mix of trigs,
phospholipids, cholesterol and protein
– Protein allows transport in blood
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Lipoproteins
• Classified by density– VLDL: mostly triglycerides– LDL: mostly cholesterol– HDL: mostly protein
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Uptake of fatty acids: Lipoprotein lipase
• In capillary/cell interface of most tissues– This enzyme
facilitates uptake of FFA from blood after a meal
• Hormone sensitive lipase– Essentially same
enzyme• Breaks down
intracellular lipids in fasted state
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Lipid utilization during exercise• Primarily used:
– Rest, prolonged low-moderate intensity exercise, recovery from exercise
• Complicated – Multi-step
• Mobilization• Circulation• Uptake• Activation
– Fatty-acyl-CoA• Translocation• Β-oxidation• Mitochondrial oxidation
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Mobilization
• HSL– Breaks down
stored triglycerides– Stimulated by
catecholamines (rapid phase)
– Growth hormone (prolonged phase)
– Triglycerides carried in blood by albumin
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Circulation and uptake• FFA circulated in blood
bound to albumin• Uptake
– Directly related to circulating concentration
– Rate of blood flow• Increased flow, increased
delivery, increased uptake and utilization
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Activation and translocation1) FFA are taken up by
FABP2) FAT (fatty acid
transporter)– Brings the FFA into the
cell
3) Attachment of FA to CoA molecule– Fatty acyl-CoA– Outer mitochondrial
membrane
4) Translocation– Into mitochondrial matrix– Carnitine and CAT1 and
CAT2
1
2
3
4
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β-oxidation• Breaks down FA-CoA to acetyl-CoA (2C fragment)• Starts the process of fatty acid oxidation• 16C FA requires:
– 7 cycles of β-oxidation– Each cycle produces 1 Acetyl-CoA, 1 NADH and 1 FADH2
– So 16C FA produces how many ATP?– 8 acetyl-CoA, 7 NADH, 7 FADH2
– WHY 8 Acetyl-CoA?– Each acetyl-CoA = 12 ATP (3 NADH, 1 FADH, 1 ATP)– Activation costs 2 ATP (equivalent, one ATP to AMP)
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Oxidation of fatty acids
• After β-oxidation– Acetyl-CoA
• Enters Kreb’s cycle
– NADH and FADH go to electron transport chain
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Free fatty acids: rest and exercise
• Opposite of CHOs– Fasted state
raises FFA– Most
pronounced during low-to-moderate intensity exercise
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Intramuscular triglycerides• Stored in muscle much like glycogen• Hormone Sensitive Lipase
– Breaks down trigs within cell– Hard to quantify utilization
• Concomitant use by cell and uptake from blood
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Intramuscular lipolysis• Perhaps used in type I fibers
• Results suggest that they are used primarily during recovery from exercise
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Lipid oxidation in muscle
• FFA are taken up by the muscle– Training increases
this ability• Intramuscular TG
– Probably used when glycogen becomes depleted
– Most likely used in recovery
– Used to a great extent by diving mammals
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Tissue specific fat metabolism• Heart and liver specially adapted
to fat utilization• Brain, RBCs use glucose almost
exclusively• Muscle: in between
– Type IIb: use relatively little fat– Type I: use much more fat
• Muscle mitochondrial adaptations– Much greater than those
associated with the cardio-circulatory system (i.e. heart, capillary vol., etc.)
– Increases ability to use fat (particularly when glycogen is low)
– Note how FFA are utilized much more quickly when enzyme content is doubled
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• Biggest factor in Fuel selection– Power output
• Rest– Mostly fat used
• Exercise– Depends on intensity
• Training– Can shift fat curve to left
• Sympathetic nervous system stimulation– Shifts fat curve right
Crossover concept
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Crossover concept
• Note that it is 50% fat-50% CHO at very low power output (~30% Vo2 max)
• As power output rises, fat oxidation slows due to:– The complexity of the FA
oxidation process– Reduced blood flow to
inactive tissues– Sympathetic nervous system
stimulation (which increases CHO utilization)
– Endurance training only affects the percentages slightly
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• Glycerol– Marker of FFA
mobilization from fat stores
– This data suggest slightly greater mobilization after training at 45%
• FFA– Simultaneously
mobilized into blood and taken up by the tissues
– Why are blood levels of FFA lower after training?
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• Glycerol– Rate of appearance
– Measure of mobilization
– Note that mobilization is greater following training
• FFA– Appearance and
disappearance• Measure of turnover
– Note that prior to training
• FFA turnover falls with intensity
– After training• Pattern is different
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Ketosis: Fuel source?• Under starvation
conditions– When carbohydrate
use is minimal– Reduces protein
catabolism for energy needs
– Ketone bodies• Acetoacetate, • β-hydroxybutyrate • Acetone
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• Can be taken up by brain
• Converted to acetoacetate
• Converted to acetyl-CoA and oxidized
• Problems?