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Exercise Metabolism

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Exercise vs. Fed Fast Cycle

  • fluctuation in activity levels

  • difference in energy demand

  • all energy derived from nutrients

  • the energy demands of sports resemble the fed-fast cycle

  • understanding of integration of metabolism required

  • keep in mind the changes in level of hormones

Energy Metabolism in Exercise

  • Energy systems at cellular level

    • immediate, intermediate, long term​

  • ultimately all provide ATP from substrate level oxidation (anaerobic, glycolysis) and/or oxidative phosphorylation (aerobic)

  • ATP generated by oxidation o​f: 

    • CHO, fat and C-skeleton of AAs​

  • source of substrates: 

    • food​

    • body fuel stores

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Energy Systems

  1. Immediate

  • 5-10 seconds

  • ATP in muscle cells (minimal)

    • we don't store it​

  • creatine-P (phosphocreatine)

    • 5X the amount of energy as pre-formed ATP​

  • very important in sprints and weight lifting

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2. Intermediate​

  • short term

  • 30 seconds - 2-4 mins

  • anaerobic glycolysis

    • quick but limited release of energy​

    • high intensity (800m springtin, 100m swim)

    • weight

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3. Long term (oxidative)

  • aerobic metabolism

    • takes longer to activate​

    • lasts longer

    • pyruvate from glucose

      • directly from glycolysis or via the cori cycle​

    • acetyl CoA via beta-oxidation of FA

    • protein: C-skeletons from different AA can enter TCA at different location

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  • blood is directly away from internal organs to: 

    • muscle (O2 delivery)​

    • skin (heat removal)

      • blood vessels dilate​

      • release heat

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Limits to Exercise

  • fuel limitations

    • increased H+ and Lac-​

  • probably not O2 delivery due to lungs

    • probably is blood flow​

      • deliver to O2 to muscle - in prolonged exercise, can lead to a return to anaerobic glycolysis​

      • removal of heat

  • most of us don't exercise that intensely

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  • Carbohydrate

    • plasma glucose​

    • muscle glycogen

  • Fat

    • free FA​

    • TAG in lipoproteins

    • IMTG (intramuscular triacylglycerols)

  • Protein/Amino Acid

Muscle Fiber Types

Type I Slow Twitch - high oxidative, moderate glycolytic

  • aerobic - lots of mitos

  • fat is the primary fuel (never alone)

  • "red muscle" due to hyoglobin (heme)

Type IIa - oxidative/glycolytic

  • anaerobic and aerobic

  • both glucose/glycolytic and fat/aerobic

  • intermediate contractility

Type IIb - high glycolytic, low oxidative

  • anaerobic glycolysis

  • white fibers

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  • exercise can increase the size of muscle cells

  • high intensity endurance --> increase in % of type I fiber

  • strength/resistance --> increase in volume of both type I and type II

  • the portion of each type of muscle fibers a person has is defined by genes 

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Muscle Fuel Use During Rest

  • at rest, low demand, low expenditure

    • skeletal muscle needs less energy than internal organs​

  • use both CHO and fat

  • glucose mainly from blood

    • breakdown of muscle glycogen minimal at rest​

  • fatty acids

    • albumin bound FFA​

    • hydrolysis of TAG in lipoproteins

    • LPL in skeletal muscle increases when fasted

    • hydrolysis of IMTG (low because HSL is at rest)

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Postprandial to Fasted

  • Postprandial: HIGH INSULIN

    • increased GLUT4 --> increased glucose uptake by muscle --> increased glycolysis --> increased levels of pyruvate and acetyl-CoA --> high malonyl CoA --> decreased CPT-1 --> decreased beta-oxidation​

  • When transitioning to fasted state

    • decreased insulin --> decreased GLUT4 --> decreased glucose uptake --> decreased glycolysis​ --> decreased pyruvate and acetyl CoA --> decreased malonyl CoA --> decreased inhibition on CPT-1 --> increased beta-oxidation of FA

    • decreased insulin --> decreased inhibition of HSL in adipose tissue --> increased FFA from lipolysis --> increased FFA in plasma

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Randall Cycle​​ (glucose-fatty acid cycle)

  • inhibition of glucose utilization when fatty acids are available as an alternative fuel and vice versa

  • proposed that increased beta-oxidation --> increased acetyl-CoA --> increased inhibition of PDH + increased citrate --> inhibition of glycolysis and oxidation of pyruvate from glucose in mito

When fasted

  • in skeletal muscle, LPL activity increases

  • in adipose tissue, LPL activity decreases

When re-fed

  • in skeletal muscle, LPL activity remains elevated

Therefore

  • when fed, FA in chylomicrons and VLDL directed to adipose

  • when fasted, FA in VLDL directed to muscle for oxidation

Fuel Utilization in Working Muscle

  • A person's max aerobic capacity is VO2 max

    • the point when further increase in exercise intensity no longer leads to increase in oxygen uptake​

      • low intensity: 25% VO2 max​

      • moderate intensity: 60% VO2 max

      • high intensity: 80% VO2 max

  • athletic training increases VO2 max

    • via cardiovascular fitness​

    • slows heart rate​​

    • increased volume pumped

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Exercise increases FA availability to muscle

 

 

 

 

 

Net effects: increased FFA in muscle for beta-oxidation

  • exercise also increases beta-oxidation capacity​​

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Utilization of CHO in Working Muscle

  • increased glucose uptake by muscle: exercise increases GLUT4 via insulin-dependent pathway

  • plasma glucose comes from

    • hepatic glyceogenolysis​

    • dietary CHO

  • muscle glycogenolysis

    • glycogen depleted muscle can oxidize FA more efficiently​

      • low acetyl CoA (and malonyl CoA​​

      • low PDH activity

      • high free carnitine

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Utilization of a Mixture of Fuels

  • muscle always uses a mixture of substrates as fuel

  • each source of fuel contributes a different proportion to the total energy expenditure in muscle

  • at rest, fat plays an important role

  • fat burns most at moderate intensity

  • as intensity increases, CHO (especially glycogen) becomes more important

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Adaptation to Exercise

Cardiovascular

  • increased cardiac output

  • increased gas exchange in lungs

  • increased capillary density in skeletal muscle

increased mito in skeletal muscle

  • 5X increase

  • seasonal variation: during season vs off season

increased activity of enzymes involved in oxidation

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decreased lactate production

  • aerobic oxidation is more efficient in producing ATP

  • same amount of glycogen can last longer

  • lower H+ --> less exercise stress and fatigue

Reduced reliance on CHO, early mobilization of fat

  • in athletes, IMTG is higher

  • exercise inhibits acetyl-CoA carboxylase --> less malonyl CoA -> more FA oxidized

Initial Muscle Glycogen

  • Exercise uses fatty acids as a major fuel

  • we know (from fasting states) that our cells need CHO to burn fat because OAA is needed to bring acetyl-CoA from fatty acid oxidation into the TCA cycle

  • therefore, no matter how much, how fast our bodies can mobilize fat, we need to have enough CHO in the system

  • in addition, as intensity increases, CHO contributes more to fuel

  • glycogen depletion is the single most consistently observed factor that contributes to fatigue

  • high initial muscle glycogen level correlates to longer time to exhaustion in submaximal workload

  • does not apply in low intensity exercise becaues glycogen is not a limiting factor

Carbohydrate Loading

  • used to maximize muscle glycogen contents

  • common practice for endurance athletes

  • start about one week before event

  • general strategy: CHO depletion followed by CHO reloading

  • desired results: higher muscle glycogen level than before start

  • specific regimens vary

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©2023 by Syracuse University Dr.Margaret Voss

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