Chronic oral ingestion of L-carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans: the dual role of muscle carnitine in exercise metabolism — J Physiol.
Insulin increases muscle total carnitine (TC) content during acute i.v. L-carnitine infusion (Stephens et al., 2006a). Here we determined the effects of chronic L-carnitine and carbohydrate (CHO; to elevate serum insulin) ingestion on muscle TC content and exercise metabolism and performance in humans. On 3 visits, each separated by 12 weeks, 14 healthy male volunteers (age 25.9 ± 2.1 yr, BMI 23.0 ±0.8 kg.m2) performed an exercise test comprising 30 min cycling at 50% VO2max, 30 min at 80% VO2max, then a 30 min work output performance trial. Muscle biopsies were obtained at rest and after exercise at 50% and 80% VO2max on each occasion. Following visit one, volunteers ingested either 80 g of CHO (Control) or 2 g of L-carnitine L-tartrate and 80 g of CHO (Carnitine) twice daily for 24 weeks in a randomised, double blind manner. All significant effects reported occurred after 24 weeks. Muscle TC increased from basal by 21% in Carnitine (P<0.05), and was unchanged in Control. At 50% VO2max, the Carnitine group utilised 55% less muscle glycogen compared to Control (P<0.05) and 31% less pyruvate dehydrogenase complex activation (PDCa) compared to before supplementation (P<0.05). Conversely, at 80% VO2max, muscle PDCa was 38% higher (P<0.05), acetylcarnitine content showed a trend to be 16% greater (P<0.10), muscle lactate content was 44% lower (P<0.05) and the muscle PCr/ATP ratio was better maintained (P<0.05) in Carnitine compared to Control. The Carnitine group increased work output 11% from baseline in the performance trial, while Control showed no change. This is the first demonstration that human muscle TC can be increased by dietary means and results in muscle glycogen sparing during low intensity exercise (consistent with an increase in lipid utilisation) and a better matching of glycolytic, PDC and mitochondrial flux during high intensity exercise, thereby reducing muscle anaerobic ATP production. Furthermore, these changes were associated with an improvement in exercise performance.