Lipid metabolism during endurance exercise1.docx

8/9/2019 Lipid metabolism during endurance exercise1.docx

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Lipid metabolism during endurance

exercise1,2,3

1. Jeffrey F Horowitzand

2. Samuel Klein

+Author Affiliations

1. 1From the Department of Internal Medicine, Washington University School of

Medicine, St Louis.

Abstract

Endogenous triacylglycerols reresent an imortant source of fuel for endurance e!ercise.

"riacylglycerol o!idation increases rogressi#ely during e!ercise$ the secific rate is

determined %y energy re&uirements of wor'ing muscles( fatty acid deli#ery to muscle

mitochondria( and the o!idation of other su%strates. "he catecholamine resonse to e!ercise

increases liolysis of adiose tissue triacylglycerols and( resuma%ly( intramuscular

triacylglycerols. )n addition( increases in adiose tissue and muscle %lood flow decrease fatty

acid reesterification and facilitate the deli#ery of released fatty acids to s'eletal muscle.

Alterations in fatty acid mo%ilization and the relati#e use of adiose and intramusculartriacylglycerols during e!ercise deend( in large art( on degree of fitness and e!ercise

intensity. *omared with untrained ersons e!ercising at the same a%solute intensity( ersons

who ha#e undergone endurance training ha#e greater fat o!idation during e!ercise without

increased liolysis. A#aila%le e#idence suggests that the traininginduced increase in fat

o!idation is due rimarily to increased o!idation of nonlasmaderi#ed fatty acids( erhas

from intramuscular triacylglycerol stores. Fat o!idation is lower in highintensity e!ercise

than in moderateintensity e!ercise( in art %ecause of decreased fatty acid deli#ery to

e!ercising muscles. ,arenteral liid sulementation during highintensity e!ercise increases

fat o!idation( %ut the effect of ingesting longchain or mediumchain triacylglycerols on

su%strate meta%olism during e!ercise is less clear. "his re#iew discusses the relation %etween

fatty acid mo%ilization and o!idation during e!ercise and the effect of endurance training(e!ercise intensity( and liid sulementation on these resonses.

Adiose tissue

intramuscular triacylglycerol

liolysis

fatty acids

glycerol
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mediumchain triacylglycerol

sta%le isotoes

e!ercise

endurance training

liid sulementation

INTRODUTION

Endogenous triacylglycerols reresent the largest fuel reser#e in the %ody. -ost

triacylglycerols are stored in adiose tissue /10 mmol in a lean adult man3( %ut they are

also resent in s'eletal muscle /4 mmol3 and lasma /. mmol3. "he total amount of

energy stored as triacylglycerol /5 -J3 is 65 times the amount stored as glycogen /7

-J3. "hus( fatty acid o!idation during endurance e!ercise ermits sustained hysical acti#ity

and delays the onset of glycogen deletion and hyoglycemia. "he use of fatty acids as a fuel

re&uires hydrolysis of triacylglycerols ie( liolysis3 from adiose tissue( muscle( and lasma

and the deli#ery of the released fatty acids to s'eletal muscle mitochondria for o!idation. )n

this article( we will discuss the relation %etween fatty acid mo%ilization and fat o!idation

during e!ercise in humans and re#iew the influence of liid sulementation on su%strate

meta%olism during e!ercise.

,re#ious Section8e!t Section

LI!ID "IN#TI$ DURIN% R#$T AND #RI$#

After an o#ernight fast( most energy needs at rest are ro#ided %y o!idizing fatty acids

deri#ed from adiose tissue triacylglycerols 13. Adiose tissue liolytic acti#ity is regulated

%y the %alance %etween hormones that stimulate rimarily catecholamines3 and those that

inhi%it hormonesensiti#e liase rimarily insulin3( which hydrolyzes triacylglycerols to fatty

acids and glycerol. At rest( the amount of fatty acids released from adiose tissue tyically

e!ceeds the amount o!idized$ fatty acid rate of aearance into lasma 9a3 is aro!imately

twice the rate of fatty acid o!idation 23. "herefore( a large ortion of fatty acids li%erated %y

liolysis of adiose tissue triacylglycerols are reesterified %ac' into triacylglycerols(

rincially %y the li#er.

-ild or moderateintensity e!ercise :2;5< of ma!imal o!ygen consumtion =V 2ma!3>

is associated with a ;1fold increase in fat o!idation a%o#e resting amounts 43 %ecause of

increased energy re&uirements of muscle and enhanced fatty acid a#aila%ility. A large ortion

of the increased suly of fatty acids is ro#ided %y liolysis of adiose tissue

triacylglycerols( which increases 2;4fold ?( 3 and is mediated %y increased @adrenergic

stimulation 5( 03. )n addition( the ercentage of released fatty acids that are reesterified

decreases %y half ?3( resuma%ly %ecause of alterations in %lood flow that facilitate thedeli#ery of fatty acids from adiose tissue to wor'ing muscles. -oderateintensity e!ercise
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dou%les adiose tissue %lood flow ( 73 and causes a 61fold increase in s'eletal muscle

%lood flow 13. )ncreasing the remo#al of fatty acids from adiose tissue %y increasing

adiose tissue %lood flow may also %e necessary to re#ent otentially to!ic regional fatty

acid accumulation. Hodgetts et al 113 found that the ratio of fatty acid to al%umin in #enous

%lood coming from su%cutaneous adiose tissue increased from 2B1 at rest to nearly 5B1 at the

end of e!ercise. "heoretically( greater increases in local fatty acid concentration couldo#erwhelm a#aila%le fatty acid %inding sites on al%umin 123 and cause harmful increases in

the concentration of un%ound fatty acids.

"he relation %etween liolysis defined as 4 C glycerol 9a( %ecause 4 fatty acids and 1

glycerol molecule are released from 1 triacylglycerol during liolysis3( lasma fatty acid

tissue uta'e :fatty acid rate of disaearance from lasma 9d3>( and fatty acid o!idation at

rest and during rolonged moderateintensity e!ercise ?< of =V 2ma!3 is shown in 'igure

13. During the first 12 min of e!ercise( the liolytic rate is aro!imately twice the rate

of fatty acid o!idation. Howe#er( lasma fatty acid uta'e fatty acid 9d3 is similar to the rate

of fat o!idation during this eriod. )n fact( se#eral in#estigators ha#e reorted that fatty acid

9d is lower than fatty acid o!idation during the first 1;2 h of e!ercise 14;13. "his e#idencesuggests that another fat source( resuma%ly lasma or intramuscular triacylglycerols

)-"s3( is %eing o!idized in addition to lasma fatty acid deri#ed from adiose tissue.

iew larger #ersionB

)n this age

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F)G9E 1.

9ates of liolysis 4 C the glycerol rate of aearance in lasma3( fatty acid uta'e( and fatty

acid o!idation at rest and during ? h of treadmill e!ercise erformed at ?< of ma!imal

o!ygen uta'e =V 2ma!3 in untrained su%ects. Data adated from reference .

Se#eral studies suggest that )-"s reresent a considera%le ortion of the total fat used

during endurance e!ercise 1?;213. Estimates of )-" use( calculated indirectly with

isotoic tracer methods( indicate that nonlasmaderi#ed fatty acids resuma%ly from

)-"s3 ro#ide 6< of the total fat o!idized during e!ercise 13 and muscle contraction

223. Se#eral studies in which muscle %iosies were ta'en %efore and after e!ercise found that)-" concentration declines %y 2;?< after 1;2 h of moderateintensity cycle ergometer
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e!ercise( which could account for 5;0< of the total amount of fat o!idized 15;17( 213. )n

contrast( others found that )-" concentration decreases minimally or not at all after

rolonged e!ercise and therefore does not contri%ute significantly to total energy roduction

24;203. "he reason for the discreancies %etween the studies is not clear %ut may %e related

to differences in e!ercise rotocols( #aria%ility when measuring )-" concentration in

muscle %iosies 203( and differences in the inter#al %etween the last e!ercise %out and thee!erimental trial.

Few studies ha#e e#aluated the contri%ution of lasma triacylglycerols to total energy

roduction. "he a#aila%le data suggest that during resting conditions( lasma triacylglycerols

may account for ;1< of total fat o!idation 2( 273. "here is also indirect e#idence that

only a small fraction of total energy roduction is deri#ed from lasma triacylglycerols

during e!ercise 4;423. For e!amle( Kiens and Iithell 423 found that IDI

triacylglycerol uta'e %y s'eletal muscle is negligi%le during e!ercise. e are unaware of any

studies that ha#e &uantified the o!idation of IDI triacylglycerols or lasma chylomicron

triacylglycerols during e!ercise in humans.

Iiolysis of adiose tissue triacylglycerols( lasma fatty acid uta'e( and fatty acid o!idation

increase rogressi#ely throughout a %out of e!ercise Figure 13. After /2 h of e!ercise( the

rate of lasma fatty acid uta'e %ecomes greater than the rate of fatty acid o!idation(

suggesting that fatty acids released into lasma from adiose tissue can suly all fatty acids

used %y acti#e muscles. "hus( as e!ercise duration increases( it is li'ely that the relati#e

contri%ution of )-"s to total fat o!idation declines and the contri%ution from lasma fatty

acid increases.

#NDURAN# #RI$# TRAININ%

Endurance e!ercise training increases the o!idation of fat during su%ma!imal e!ercise 2(

44( 4?3. Se#eral factors contri%ute to this adati#e resonseB increased density of the

mitochondria in the s'eletal muscles( which increases the caacity for fat o!idation 43$ a

roliferation of caillaries within s'eletal muscle( which enhances fatty acid deli#ery to

muscle 453$ an increase in carnitine transferase( which facilitates fatty acid transort across

the mitochondria mem%rane 403$ and an increase in fatty acid %inding roteins( which

regulate myocyte fatty acid transort 4(473.

Data from %oth in #itro and in #i#o studies suggest that increased liolysis of adiose tissue

triacylglycerols is not resonsi%le for the traininginduced increase in whole%ody fat

o!idation. Although se#eral in#estigators found that ma!imally stimulated liolytic acti#ityat einehrine concentrations %etween 15 and 1? molLI3 was greater in adiocytes

o%tained from endurancetrained su%ects than in those from untrained su%ects ?;?43(

liolytic acti#ity was the same or slightly lower in adiocytes from endurancetrained

su%ects at hysiologic einehrine concentrations %etween 11and 1molLI3 ?1( ?23.

-oreo#er( %y using microdialysis ro%es to measure regional glycerol release in #i#o(

Stall'necht et al ??3 found the liolytic resonse of a%dominal su%cutaneous adiose tissue

to einehrine infusion was the same in trained and untrained su%ects.

Similarly( crosssectional studies of trained and untrained su%ects and longitudinal training

studies showed that endurance training does not increase the whole%ody liolytic resonse

during e!ercise erformed at the same a%solute e!ercise intensity ie( same ower outut3.
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For e!amle( liolytic rates 4 C glycerol 9a3 measured during e!ercise erformed at the

same a%solute intensity were similar in endurancetrained athletes and untrained #olunteers

3 'igure 23. )n addition( -artin et al 13 found that after 12 w' of endurance training(

lasma fatty acid 9d decreased %y 4


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resonse to einehrine infusion than sedentary control su%ects ??3 and thus may ha#e a

greater catecholamine deli#ery to adiose tissue during e!ercise desite similar lasma

catecholamine concentrations. )n addition( an increase in )-" liolysis after endurance

training may %e resonsi%le for the increased glycerol 9a in trained su%ects.

#RI$# INT#N$IT(

"he relati#e contri%utions of lasma fatty acid and )-"s to total fat o!idation during

e!ercise at different intensities ha#e %een studied in highly trained %ut not in untrained

su%ects 1?3. "he estimated relati#e contri%ution of lasma fatty acid and )-"s to total fat

o!idation during e!ercise at low( moderate( and high intensities 2


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rolonged e!ercise 04( 0?3. an Nyl et al 043 found that( comared with car%ohydrate

ingestion alone( the ingestion of -*" /4 gLh3 added to car%ohydrate during 2 h of cycling

at 5< of =V 2ma! reduced the calculated rate of muscle glycogen o!idation and slightly

imro#ed erformance /4

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Lipid metabolism during endurance exercise1.docx