It is widely accepted that chronic-endurance aerobic training can induce changes within the skeletal muscle to enhance performance and increase endurance. Some of these changes include increased capillary density, increased enzymatic concentration, and an increase in the size and number of mitochondria. This is associated with an increased ability of the athlete to oxidize FAs, in the form of intramuscular trigylcerides, for fuel. This phenomenon has led researchers to analyze the potential changes that might occur with carnitine concentrations in all pools, including urinary excretion, plasma, and muscle.
Several studies have documented that there are no differences in the resting plasma concentrations of carnitine between trained individuals and their sedentary counterparts.2861-63 This is practical because carnitine concentrations are governed by dietary intake, and as stated previously, the majority of the population has a diet containing sufficient precursors to promote adequate carnitine synthesis.36 Similarly, it seems rather unlikely to lose carnitine in the urine, as it only contributes to approximately 1% of carnitine loss daily. The chance that a carnitine deficiency develops with training due to changes that occur within the plasma or urinary compartments is low because of the small fraction that they contribute to both exercise and daily excretion.
Since the muscle is the major pool for carnitine, it is important to pay particular attention to the results that have analyzed training and carnitine status. It is important to emphasize here that research findings regarding this topic are divergent in that there are researchers who support both sides of the question that asks whether training causes a change in carnitine concentration. There is literature to support that both changes occur as a result of training, as well as that which does not support changes. The discrepancy seems to be within the methodology of the studies, as there is little uniformity in measurements of carnitine with regard to what pool is studied or the techniques utilized to measure concentrations. In addition, there are differences in carnitine levels for each person within each muscle and levels vary between genders. Finally, the type of exercise does affect the extent of carnitine status, as presented in the previous section. All these possibilities present sources of error and must be thoroughly evaluated.
Lennon et al.48 reported that males have significantly higher levels of muscle carnitine than females in both a high and moderate training group, and this is likely due to the increased muscle mass associated with the male. In addition, they report48 that training status does not affect muscle carnitine concentrations in either the males or females.
Janssen et al.64 further supported these findings with sedentary individuals who completed an 18- to 20-month marathon training program. Muscle carnitine was evaluated prior to, during, and after the completion of the marathon and was not affected during training, nor were sex-related differences observed.65 In addition, running the marathon did not cause a significant decrease in muscle carnitine levels, suggesting that neither the training nor the strenuous exercise alters carnitine muscle levels. Finally, Decombaz et al.47 measured the total muscle carnitine level of skiers for 2 years prior to an Alpine ski race and found that there were no significant changes over the course of their training regiment (mean, 17 vs. 16 ^mol/g dry wt). It is important to point out that there were consistent individual variations (range, 12 to 22 ^mol/g dry wt) that were stable for the course of the training. These results show that with training there is little variation in carnitine levels and that deficiencies will not occur as a result of this training, leading to the suggestion that the diet may provide a sufficient amount of precursors to synthesize adequate amounts of carnitine.
It is important to point out that there are studies that found differences in carnitine concentrations after training. One such study by Arenas et al.66 found that endurance athletes showed a significant decrease in free and total muscle carnitine content after 4 months of training, and that these changes were not as severe as those seen in sprinters. The authors suggest66 these differences were due to the endurance athletes having a higher concentration of type I muscle fibers, and thus a higher mitochondrial content than the type II fibers of the sprinters, leading to the conclusion that there was a wasting of the short-chain acylcarnitines, a result of chronic training, which would deplete the carnitine stores for subsequent training sessions. A limitation in this study was that the researchers did not separate the subjects by gender in their analysis and when reporting their findings. Furthermore, there was not an equal amount of males and females in both groups. The endurance group consisted of all males, whereas the sprint group had 5 females of 11 participants. It has been shown that there are significant differences between muscle carnitine levels in males and females.48,67,68
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