FAT METABOLISM DURING EXERCISE SSE

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FAT METABOLISM DURING EXERCISE SSE59, Volume 8
(1995), Number 6

• Edward F. Coyle, Ph.D.Professor, Department of
  Kinesiology and Health EducationThe University
  of Texas at AustinAustin,Texas

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Key Points

• People store large amounts of body fat in the
  form of triglycerides within fat (adipose) tissue
  as well as within muscle fibers (intramuscular
  triglycerides).When compared to carbohydrate
  stored as muscle glycogen, these fat stores are
  mobilized and oxidized at relatively slow rates
  during exercise.
• As exercise progresses from low to moderate
  intensity, e.g., 25-65 VO2max, the rate of fatty
  acid mobilization from adipose tissue into blood
  plasma declines, whereas the rate of total fat
  oxidation increases due to a relatively large use
  of intramuscular triglycerides. Intramuscular
  triglycerides also account for the characteristic
  increase in fat oxidation as a result of habitual
  endurance-training programs.
• Dietary carbohydrate intake has a large influence
  on fat mobilization and oxidation during
  exercise when dietary carbohydrate produces
  sufficient carbohydrate reserves in the body,
  carbohydrate becomes the preferred fuel during
  exercise. This is especially important during
  intense exercise because only carbohydrate(not
  fat) can be mobilized and oxidized rapidly enough
  to meet the energy requirements for intense
  muscular contractions.

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Introduction

• Carbohydrates and Fats during exercise
• Carbs are most important
• Fats are too slow, therefore
• glycogen depletion leads to fatigue
• With training, fat utilization can be increased
  during exercise

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Table 7-2. Blood Lipid Levels in the Normal
Resting Blood

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Fat Mobilization
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Activation and Translocation

• Figure 7-5
• FA bind to acyl-CoA (1) Fatty acyl-CoA
• Move to inner membrane
• CoA is removed and FA attaches to carnitine
• (2) Fatty acyl-carnitine to the inside of the
  mitochondrion
• Carnitine is removed
• Another CoA added, (3) Fatty acyl-CoA

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?-Oxidation

• Degrades the (3) fatty acyl-CoA to (4) acetyl-CoA
  by cleaving two carbons at a time
• First carbon is the alpha ? carbon
• Second carbon is the ? carbon
• Palmitate (16 carbons) will provide 8 acetyl CoAs

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?-Oxidation

• Acetyl Co-A enters Krebs cycle
• Each acetyl Co-A results in 12 ATP
• 1 NADH and 1 FADH
• Palmitate (16 carbons) will provide 129 ATP
• Triglyceride of stearate (18 C) 460 ATP

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Burning Fats During Exercise

• Mobilization of Fats
• Circulation and Uptake
• Activation
• Translocation
• Beta (?) Oxidation
• Krebs/ETC Oxidation

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Ingesting Fats During Exercise
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Medium Chain Triglycerides

• Medium-chain triglycerides (MCTs) are fats with
  an unusual chemical structure that allows the
  body to digest them easily. Most fats are broken
  down in the intestine and remade into a special
  form that can be transported in the blood. But
  MCTs are absorbed intact and taken to the liver,
  where they are used directly for energy. In this
  sense, they are processed very similarly to
  carbohydrates.
• MCTs are different enough from other fats that
  they can be used as fat substitutes by people
  (especially those with AIDS), who need calories
  but are unable to absorb or metabolize normal
  fats.
• MCTs are also popular among athletes as a
  proposed performance enhancer, although there is
  little evidence as yet that they really work.

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Training Effects
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Training Effects
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Dietary Influences

• Eating carbs before exercise increases insulin
  levels
• Insulin inhibits lipolysis
• High fat diet increases fat oxidation but limits
  exercise intensity

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Fat Utilization After Exercise

• Intramuscular fat utilization is critical after
  exercise
• Allow for repletion of muscle glycogen

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Summary

• People store large amounts of body fat in the
  form of triglyceride within adipose tissue as
  well as within muscle fibers. These stores must
  be mobilized into FFA and transported to muscle
  mitochondria for oxidation during exercise. Fatty
  acids from adipose tissue are mobilized into
  plasma and carried by albumin to muscle for
  oxidation. FATS ARE A SLOW FUEL
• As exercise intensity increases from low (25
  VO2max) to moderate (65 VO2max) to high (85
  VO2max), plasma FFA mobilization declines.
  However, total fat oxidation increases when
  intensity increases from 25 to 65 VO2max, due
  to oxidation of intramuscular triglycerides,
  which provide about one-half of the fat for
  oxidation. INTENSITY DOESNT MATTER
• Endurance training characteristically increases
  fat oxidation during moderate intensity exercise
  by accelerating the oxidation of intramuscular
  triglyceride without increasing the mobilization
  or oxidation of plasma FFA. TRAINING INCREASES
  FAT OXIDATION

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Summary

• Similarly, during low-intensity exercise with
  little intramuscular triglyceride oxidation, the
  increased fat oxidation of trained people does
  not appear to be caused by increased mobilization
  of FFA into plasma, but rather by a greater rate
  of oxidation of the FFA removed from the blood
  during exercise. Therefore, it seems that
  untrained people have greater abilities to
  mobilize FFA than they do to oxidize it when they
  exercise in the fasted state. GREATER FAT
  OXIDATION REQUIRES MORE THAN INCREASED
  MOBILIZATION
• Carbohydrate ingestion during the hours before
  exercise, even in relatively small amounts,
  reduces fat oxidation during exercise largely
  through the action of insulin. CARBS CAN LIMIT
  FAT USE
• Fat supplementation and special diets have
  limited ability to increase fat oxidation in
  people, especially during sport competitions.
  Therefore, fat from body stores and/or dietary
  supplementation cannot adequately replace muscle
  glycogen and blood glucose as fuels for intense
  exercise. FAT USE NOT AFFECTED BY DIET OR
  SUPPLEMENTS DURING EXERCISE

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Fat Burns in a Carbohydrate Flame

• Beta oxidation produces acetyl-CoA
• Acetyl-CoA must bind with oxaloacetate (OAA) in
  order to enter the Krebs cycle
• OAA needs to constantly be replaced.
• Pyruvate can be converted to OAA
• Without adequate pyruvate, there is insufficient
  OAA needed for acetyl-CoA to enter the Krebs
  cycle.

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Beta Oxidation
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Fat Utilization During Exercise

• Rest
• 60 fat, 35 carb, 5 protein
• Exercise
• Burn more of everything
• Greater percentage comes from carbohydrates
• Active muscle cells burn mostly all carbohydrates
• Other inactive cells burn fat
• Very hard exercise
• Burn less fat due to an increase in lactic acid
  which inhibits L-HSL and fat mobilzation

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Fat Burning During Exercise

• A Function of Percentage and Rate

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J. A. Romijn1, E. F. Coyle2, L. S. Sidossis3,4,
J. Rosenblatt3,4, and R. R. Wolfe3,4 Substrate
metabolism during different exercise intensities
in endurance-trained women Vol. 88, Issue 5,
1707-1714, May 2000.We have studied eight
endurance-trained women at rest and during
exercise at 25, 65, and 85 of maximal oxygen
uptake. The rate of appearance (Ra) of free fatty
acids (FFA) was determined by infusion of
2H2palmitate, and fat oxidation rates were
determined by indirect calorimetry. Glucose
kinetics were assessed with 6,6-2H2glucose.
Glucose Ra increased in relation to exercise
intensity. In contrast, whereas FFA Ra was
significantly increased to the same extent in
low- and moderate-intensity exercise, during
high-intensity exercise, FFA Ra was reduced
compared with the other exercise values.
Carbohydrate oxidation increased progressively
with exercise intensity, whereas the highest rate
of fat oxidation was during exercise at 65 of
maximal oxygen uptake. After correction for
differences in lean body mass, there were no
differences between these results and previously
reported data in endurance-trained men studied
under the same conditions, except for slight
differences in glucose metabolism during
low-intensity exercise (Romijn JA, Coyle EF,
Sidossis LS, Gastaldelli A, Horowitz JF, Endert
E, and Wolfe RR. Am J Physiol Endocrinol Metab
265 E380-E391, 1993). We conclude that the
patterns of changes in substrate kinetics during
moderate- and high-intensity exercise are similar
in trained men and women.
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ABSTRACT ACHTEN, J., M. GLEESON, and A. E.
JEUKENDRUP. Determination of the exercise
intensity that elicits maximal fat oxidation.
Med. Sci. Sports Exerc., Vol. 34, No. 1, 2002,
pp. 92-97. Purpose The aim of this study was to
develop a test protocol to determine the exercise
intensity at which fat oxidation rate is maximal
(Fatmax). Method Eighteen moderately trained
cyclists performed a graded exercise test to
exhaustion, with 5-min stages and 35-W increments
(GE35/5). In addition, four to six continuous
prolonged exercise tests (CE) at constant work
rates, corresponding to the work rates of the GE
test, were performed on separate days. The
duration of each test was chosen so that all
trials would result in an equal energy
expenditure. Seven other subjects performed three
different GE tests to exhaustion. The test
protocols differed in stage duration and in
increment size. Fat oxidation was measured using
indirect calorimetry. Results No significant
differences were found in Fatmax determined with
the GE35/5, the average fat oxidation of the CE
tests, or fat oxidation measured during the first
5 min of the CE tests (56 3, 64 3, 58 3
O2max, respectively). Results of the GE35/5
protocol were used to construct an exercise
intensity versus fat oxidation curve for each
individual. Fatmax was equivalent to 64 4
O2max and 74 3HRmax. The Fatmax zone (range of
intensities with fat oxidation rates within 10
of the peak rate) was located between 55 3 and
72 4 O2max. The contribution of fat oxidation
to energy expenditure became negligible above 89
3 O2max (92 1HRmax). When stage duration
was reduced from 5 to 3 min or when increment
size was reduced from 35 to 20 W, no significant
differences were found in Fatmax, Fatmin, or the
Fatmax zone. Conclusion It is concluded that a
protocol with 3-min stages and 35-W increments in
work rate can be used to determine Fatmax. Fat
oxidation rates are high over a large range of
intensities however, at exercise intensities
above Fatmax, fat oxidation rates drop markedly.
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Question

• Would you rather have 10 of person As money or
  90 of person Bs money?

• Which burns more fat exercising at a low
  intensity or at a moderate intensity?

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Fat Burning Comparison
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Bottom line

• 3500 kcals a pound of fat
• Burning fat during exercise is a function of
  percentage of fat burned and the rate at which
  fat is burned (kcal/min)
• Percentage is greater at lower intensities
• Rate is greater at higher intensities

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Training

• Increase in L-HSL
• Increase in FA translocation with increase in
  number of mitochondria
• ST fibers fat utilization 8 to 10 times more FA
  than FT fibers
• Training can double a cells ability to utilize
  fat
• Decrease in epinephrine
• Decrease in lactic acid
• Increase in mitochondria
• Crossover of fuels occurs at a higher intensity
  level