Food Components to Enhance Performance An Evaluation of Potential Performance-Enhancing Food Components for Operational Rations (1994) / Chapter Skim
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21. The Role of Carnitine in Enhancing Physical Performance
21. The Role of Carnitine in Enhancing Physical Performance
Pages 433-452
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From page 433... ...
During endurance exercise, the oxidation of fatty acids spares the use of muscle glycogen and delays the onset of fatigue. Muscle tissue cannot synthesize carnitine and thus is dependent on the transport of carnitine from the bloodstream.
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From page 434... ...
As diagramed in Figure 21-2, at the completion of the action of the carnitine transport system, the carbon chain is located on the opposite side of the membrane and is esterified to a different CoA molecule. Long-chain fatty acids activated to the CoA level were the first acylcarnitines to be documented as being transported across a membrane, as diagramed in Figure 21-3.
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From page 435... ...
Short Chain Acvl Long Chain Acyl, CoA ~ \ AcylCoA ~ Id Ca~ //,/iVnitine `~ / . ///,// Acyl carnitine CoA _' AcylCo ~ FIGURE 21-3 The carnitine transport systems function in the transport of both short-chain and long-chain acyl-coenzyme A (acylCoA)
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From page 436... ...
An alternative pathway is for pyruvate to form acetyl-CoA, which in turn forms acetylcarnitine, which does not deplete limiting concentrations of CoA and which can be removed from the cell. Carnitine Stimulates Utilization of Carbohydrate and Amino Acids Since a common metabolite in the oxidation of fatty acids, amino acids, and glucose is acetyl-CoA, the role of Carnitine in the transport of acetyl-CoA as acetylcarnitine across membranes places Carnitine in the role of facilitating the oxidation of all three fuels used by the body.
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From page 437... ...
, allowing the acyl-CoA to be esterified on the opposite side of the membrane without the use of an additional ATP molecule. The overall effect of the action of the carnitine transport system is the delivery of both the acyl group and high energy to the new site on the opposite side of the membrane.
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From page 438... ...
The published methods require research-grade instruments and procedures that are not practical for measuring acylcarnitine levels in a large number of samples. However, ongoing work in several laboratories should permit measurement of specific acylcarnitines with benchtop instruments.
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From page 439... ...
Low-intensity exercise was performed for 60 min at a work load equal to 50 percent of the lactate threshold, and high-intensity exercise was performed for 30 min at a work load between the lactate threshold and the maximal work capacity for the individual (Hiatt et al., 1989~. The data in Table 21-1 show that low-intensity exercise for 60 min has no detectable effect on muscle carnitine composition, but high-intensity exercise for 10 min decreases the free carnitine concentration and dramatically increases the short-chain acylcarnitine concentration of muscle.
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From page 440... ...
Although the contribution of extracellular substrates to thigh oxidative metabolism increased during the third hour of exercise in both groups of subjects, only in the thighs of the trained group was there an increased utilization of the plasma free fatty acids (Turcotte et al., 1992~. Carnitine Affects Pyruvate Metabolism Pyruvate oxidation was measured in intact mitochondria isolated from fresh human skeletal muscle obtained from the excision of pectoralis minor in the course of a mastectomy for breast cancer; the muscle was free of macroscopic neoplastic infiltration, however.
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From page 441... ...
(Siliprandi et al., 1990~. Ninety minutes after carnitine administration and with the subjects at rest, plasma free carnitine levels increased, and short-chain carnitine levels did not change.
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From page 442... ...
The authors interpreted the increase in plasma and urinary short-chain acylcarnitine levels with carnitine administration as reflecting a washout of short-chain acyls that accumulate in tissues as the CoA ester during exercise. They suggested that in the placebo trial, pyruvate dehydrogenase was inhibited, which caused an energy crisis, and that branched-chain amino acids were used for energy, which gave rise to the C4 compound.
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From page 443... ...
. Provision of high levels of free fatty acids while at rest increased acetylCoA and acetylcarnitine contents, despite an unchanged oxygen uptake, suggesting that little regulation of free fatty acid metabolism exists in skeletal muscle while at rest.
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From page 444... ...
to 99.1 percent (red gastrocnemius) of the extra acetyl-CoA produced by adding free fatty acid to the perfusion medium was buffered by the formation of acetylcarnitine.
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From page 445... ...
Muscle biopsy specimens were taken at rest, at the end of the work period, and after 10 min of recovery. During the incremental exercise test, there was an increase in muscle lactate levels, an increase in the levels of the active form of the pyruvate dehydrogenase complex, and an increase in both acetyl-CoA and acetylcarnitine levels, with a corresponding decrease in CoA and free carnitine levels.
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From page 446... ...
Tests were terminated after muscular exhaustion or upon reaching the theoretical maximal heart rate (Vecchiet et al., 1990~. As exercise intensity increased, both VO2 maX and the blood lactate level increased.
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From page 447... ...
Muscle carnitine concentrations did not change with time and did not correlate with carnitine intake, but they showed consistent interindividual differences (Decombaz et al., 1992~. After exercise, the total muscle carnitine concentration was unaltered, but the free carnitine concentration decreased 20 percent and the short-chain carnitine concentration increased 108 percent.
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From page 448... ...
During exercise there was a progressive increase in total and acylated plasma carnitine concentrations and a decrease in the concentrations of plasma free carnitine. At 40 min postexercise, there was a decline in total and acylated carnitine levels in comparison with those at 120 min.
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From page 449... ...
Thus, the observed fall in plasma free carnitine levels and the rise in acylated carnitine levels in response to exercise is difficult to explain solely in terms of alterations in muscle carnitine exchange. Results suggest that during exercise, free carnitine is released from muscle and acylated at a site other than contracting muscle.
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From page 450... ...
Some individuals consume up to 300 mg of dietary carnitine per day. Most of the studies evaluating the effect of carnitine supplementation on exercise performance have used quantities that are severalfold higher than the typical dietary carnitine intakes.
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From page 451... ...
Maurer, and J.P. Eclache 1989 Decrease in respiratory quotient during exercise following ~-carnitine supplementation.
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From page 452... ...
Wahren 1988 Influence of carnitine supplementation on muscle substrate and carnitine metabolism during exercise.
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