Carnitine Levels and Acute Exercise

Acute exercise is characterized by a single session that may be repeated for several days, not usually longer than a week. This type of exercise has been widely used in the scientific literature on carnitine to study the changes in carnitine concentrations that occur prior to and after a single exercise session. These types of studies are effective in quantifying carnitine concentrations within the body as well as providing an insight into its metabolism during exercise. Acute studies can set the framework for long-term research because the acute data are used to predict how carnitine will act with larger volumes of training.

Lennon et al.48 examined subjects during submaximal cycle ergometry at 55% of VO2 max and found that there was a significant decrease (about 20%) in muscle carnitine levels. They reported that as the muscle carnitine dropped there was a significant and concurrent increase in plasma carnitine from rest to the termination of 40 min of exercise. These results led the authors48 to suggest that some acylcar-nitines are lost to the plasma from the muscle during acute exercise bouts. They further suggested48 that more intense exercise could severely deplete muscle carnitine levels.

Conflicting results were seen by Carlin et al.,49 who found that with 90 min of cycling (50% VO2 max) there were no decreases in total muscle carnitine levels, but increased levels of acylcarnitines and decreased free carnitine within the muscle were observed. Plasma levels of acylcarnitine were shown to increase progressively with exercise as a result of the decline in free carnitine, and not the transfer of carnitine from the muscle to the plasma.49 Again, these studies help to illustrate the importance of measuring and analyzing carnitine pools separately. These results are supported by Harris et al.,50 who found that both intermittent electrical stimulation and cycling resulted in no change in total muscle carnitine but a significant fall in free carnitine and a concurrent increase in acylcarnitine levels. In addition, Harris's group50 was able to quantify that the resting mean carnitine concentration in muscle is approximately 20.0 mmol/kg dry muscle and that 77% of this was in the form of free carnitine and 19% was acylcarnitines. Their analysis helped make quantifying the concentrations more accurate as well as improved the validity of results.

To further examine the acute effects, Soop et al.51 used femoral artery and vein catheters in seven healthy, moderately trained male cyclists who exercised for 2 h at 50% VO2 max. The purpose was twofold: first, to examine fatty acid utilization during oral carnitine supplementation (5 g/day) during exercise and, second, to quantify and examine the changes that occur in plasma and muscle carnitine levels. Results from this study showed that despite a twofold increase in plasma carnitine levels with supplementation, there was no change in FA turnover, and thus no influence over substrate metabolism.51 In addition, there were no differences between supplemented and nonsupplemented individuals with regard to plasma carnitine levels. In addition, free carnitine was observed to fall, but there was a release from the leg muscles during exercise. Furthermore, there was an increase in acylcarnitines in the plasma with no evidence of release from the leg muscles. Soop et al.51 further conclude that there is an alternate site for acylation of carnitine, which they suggested is the liver. Keep in mind that these results do coincide with those of Lennon et al.48 and Carlin et al.,49 both of whom showed that there were increases in acylcarnitines and decreases in free carnitine. Soop et al.51 have been able to shed more light on the intricate biochemical pathways that are involved in carnitine homeostasis during exercise. These results48,49,51 support the notion that within the muscle adequate carnitine levels can be maintained during exercise and further deficiencies are not likely to occur in the muscle.

It seems evident from the numerous studies that there is no drastic effect on total muscle carnitine concentrations during acute exercise because of the redistribution that occurs; specifically, there is an increase in acylcarnitine and decrease in free carnitine concentrations. Thus, it seems likely that supplementation of carnitine for acute bouts of exercise will not be effective and is subsequently unnecessary.

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