Table 1.

Summary of clinical studies investigating the effect of l-carnitine on exercise performance and recovery.

Authors/Title	# Subject	Age (Mean or Range)	Dose Duration	Outcome
Athletes/Well Trained (Professionals, Age 16–36)
[29] Carnitine in muscle, serum, and urine of nonprofessional athletes: effects of physical exercise, training, and l-carnitine administration.	24 athletes	19–27	1 g BID for 6 mo of training	Prevention of training decreased total and free. Carnitine, positive effect on recovery.
[33] Studies concerning the ergogenic value of protein supply and l-carnitine in elite junior cyclists.	7 junior athletes	na	1 g/d for 6 wk and 2 g/d for 10 d (before competition)	Supplemented group showed better stress-induced efforts and obtained higher performances.
[34] Studies concerning chronic and acute effects of l-carnitine in elite athletes.	110 athletes (in 6 studies)	16–33	4 g oral of 1 g iv (single dose) 3 g/d for 3 wk or placebo	Single dose: beneficial effects on physical output, lipid metabolism, muscular function (contraction), lactate accumulation after exercise and urine mucoproteins. 3 week treatment: Beneficial effects on the lipid metabolism, evoked muscular potential, VO2max, behavior and the biological output.
[35] Respiratory chain enzymes in muscle of endurance athletes: effect of l-carnitine.	14 athletes	na	2 g BID for 4 wk of training	Increase in respiratory-chain enzyme activities in the muscle.
[36] The effects of acute l-carnitine supplementation on endurance performance of athletes.	26 athletes	18.42 ± 0.5	12 received 3 g 14 received 4 g	Compared to placebo, l-carnitine supplemented groups showed lower lactate levels and lower heart rate.
[40] Effects of l-carnitine supplementation on physical performance and energy metabolism of endurance-trained athletes: a double-blind crossover field study.	7 athletes	36 ± 3	2 g before start and after 20 km run	Significant increase in l-carnitine plasma concentration. No effect on performance or metabolism.
[54] Effects of l-carnitine l-tartrate supplementation on muscle oxygenation responses to resistance exercise.	9 healthy, previously resistance trained men	25.2 ± 6	2 g/d for 23 d or placebo	Enhanced oxygen consumption => hypoxic stress is attenuated with carnitine supplementation.
[55] Androgenic responses to resistance exercise: effects of feeding and l-carnitine.	10 resistance-trained men	22 ± 1	2 g/d for 21 d or placebo	Increased androgen receptor content and enhanced luteinizing hormone.
[56] Plasma and urine carnitine concentrations in well-trained athletes at rest and after exercise. Influence of l-carnitine intake.	9 athletes	na	1 g before and after treadmill ergometer or placebo	No decrease in serum carnitine levels after exercise in the supplementation group. No effect on maximal exercise. No effect on maximal exercise.
[32] Effects of exercise intensity and altered substrate availability on cardiovascular and metabolic responses to exercise after oral carnitine supplementation in athletes.	15 athletes	Pl: 31 ± 8 LC: 34 ± 10	3 g/d for 15 d or placebo	No significant difference between whole-body rates of CHO and fat oxidation at any workload. At day 15, heart rate and blood glucose concentration were lower during exercise in the l-carnitine group compared to Placebo.
[31] Carbohydrate, protein, and fat metabolism during exercise after oral carnitine supplementation in humans.	20 active male athletes	Pl: 32 ± 9 LC: 34 ± 10	2 g/d for 2 wks or placebo	After 2 wk of l-carnitine supplementation, plasma ammonia response to exercise tended to be suppressed. No effects on fat, carbohydrate, or protein contribution to metabolism during prolonged moderate-intensity cycling exercise
Healthy (recreationally active, age 18–50)
[37] Metabolic changes induced by maximal exercise in human subjects following l-carnitine administration.	10 moderately trained men	18.42 ± 0.50	2 g before high-intensity exercise	Stimulation of PDH activity, and decrease in plasma lactate and pyruvate.
[38] Influence of l-carnitine administration on maximal physical exercise.	10 moderately trained men	22–30	2 g before high-intensity exercise	Increased VO2max.
[39] Effects of four weeks l-carnitine l-tartrate ingestion on substrate utilization during prolonged exercise.	15 trained males	20–46	3 g for 4 wk or placebo	No effect on substrate utilization or endurance performance.
[57] Effects of prolonged l-carnitine administration on delayed muscle pain and CK release after eccentric effort.	6 untrained subjects	26 ± 3.8	3 g/d for 3 wk	Protective effect against pain and damage from eccentric effort.
[58] The effects of l-carnitine l-tartrate supplementation on hormonal responses to resistance exercise and recovery.	10 healthy, recreationally weight-trained men	23.7 ± 2.3	2 g/d for 3 wk	Increased IGFBP-3 concentrations prior to and at 30, 120, and 180 min after acute exercise => protection from muscle damage.
[41] Effect of l-carnitine on submaximal exercise metabolism after depletion of muscle glycogen.	9 healthy males	24.9 ± 1.0	3 g/d for 7 d	No effects on fat oxidation, RQ, perceived exertion, lactate, heart rate during exercise after glycogen depletion.
[42] The effects of l-carnitine supplementation on performance during interval swimming.	20 (swimmers)	20.1 ± 0.6	2 g BID for 7 d or placebo	Elevation in serum l-carnitine and carnitine fractions. No differences in performance times between trials or groups was observed; similar response related to blood pH, LA and BE in both groups during each trial was revealed.
[43] Carnitine supplementation: effect on muscle carnitine and glycogen content during exercise.	8	26.8 ± 2.31	4 g/d for 14 d	Increase in serum carnitine. No effect on muscle carnitine content, lipid oxidation and lactate accumulation.
[59] l-carnitine supplementation results in improved recovery after strenuous exercise—a preliminary study.	12 (trained/untrained)	25.7 ± 4	2 g/d for 5 d	Improved recovery in 9 of 12 subjects.
[60] l-carnitine and the recovery from exhaustive endurance exercise: a randomised, double-blind, placebo-controlled trial.	12	25 ± 3	2 g/d for 14 d or placebo	2 g of l-carnitine taken 2 h before a first of 2 constant-load exercise tests had no influence on the second tests performed 3h after the first test compared with placebo.
[61] l-Carnitine l-tartrate supplementation favorably affects markers of recovery from exercise stress.	30 healthy subjects	30 ± 8	2 g for 3 wk or placebo	Improvement in postprandial vascular functions after a high-fat meal.
[62] Responses of criterion variables to different supplemental doses of l-carnitine l-tartrate.	8 healthy men	22 ± 3	0, 1 g, 2 g for 3 wk	Increase in serum carnitine concentrations. Reduction in post-exercise serum hypoxanthine, serum xanthine oxidase, serum myoglobin, and perceived muscle soreness. Reduced metabolic stress, less muscle damage.
[63] Effects of carnitine supplementation on flow-mediated dilation and vascular inflammatory responses to a high-fat meal in healthy young adults.	30 healthy men and women	30 ± 8	2 g/d for 3 wk or placebo	Improvement in postprandial flow-mediated dilatation after a high-fat meal.
[64] The effect of two-week l-carnitine supplementation on exercise-induced oxidative stress and muscle damage.	21 active healthy men	About 22	2 g/d for 14 d or placebo	Increase in total antioxidant capacity after 14d and 24h post exercise. Lower malondialdehyde-TBARS, creatine kinase and lactate dehydrogenase 24 h post exercise.
[65] l-Carnitine l-tartrate supplementation favorably affects biochemical markers of recovery from physical exertion in middle-aged men and women.	18 healthy men and women	m: 45.4 ± 5.3 f: 51.9 ± 5.0	2 g/d for 24 d	Positive effects on purine metabolism, free radical formation, muscle tissue disruption, muscle soreness. No effect on physical performance.
[30] Long-term administration of l-carnitine to humans: effect on skeletal muscle carnitine content and physical performance.	8 healthy male adults	23–25	2 × 2 g/d for 3 months	No significant differences between VO2max, RERmax, and Pmax between the three time points investigated: pre/post at baseline and post exercise after 3 months. No pre/post difference in muscle carnitine content at baseline and post-exercise at 3 months. Activities of citrate synthase and cytochrome oxidase, as well as the skeletal muscle fiber composition remained unaffected.
[66] Prolonged submaximal exercise and l-carnitine in humans.	10 young males	na	2 g/d for 4 wks; followed by 0 g/d for 6–8 wks	Twenty-five percent increase in free and total L-carnitine plasma levels during supplementation. These levels returned to normal 6–8 weeks after the supplementation stopped. No changes in endogenous lipids for fuel supply, indicating possibly that this population has sufficient levels of L-carnitine
Elderly (age 55–106)
[48] l-Carnitine treatment reduces severity of physical and mental fatigue and increases cognitive functions in centenarians: a randomized and controlled clinical trial.	66 centenarians	100–106	2 g/d or placebo for 6 mo	Reduction of total fat mass, increases total muscular mass, and facilitates an increased capacity for physical and cognitive activity by reducing fatigue and improving cognitive functions.
[49] Levocarnitine administration in elderly subjects with rapid muscle fatigue: effect on body composition, lipid profile and fatigue.	84 elderly subjects	81.5 ± 6.7	2 g BID for 30 day or placebo	Improvements in the following parameters: total fat mass, total muscle mass, total cholesterol, LDL-C, HDL-C, triglycerides, apoAl, and apoB. Decreased physical and fatigue.
[50] Acetyl l-carnitine (ALC) treatment in elderly patients with fatigue.	96 aged subjects	71–88	2 g BID for 180 d or placebo	Reduction in both physical and mental fatigue and improvement of both the cognitive status and physical functions.
[53] Efficacy of a novel formulation of l-Carnitine, creatine, and leucine on lean body mass and functional muscle strength in healthy older adults: a randomized, double-blind placebo-controlled study.	42 healthy older adults	55–70	1.5 g carnitine or carnitine combination or placebo for 8 wk	l-Carnitine combined with creatine, l-leucine, and Vitamin D significantly improved muscle mass and strength compared to placebo; increase in mTOR protein level.

Abbreviation used: BID: twice per day; d: day; wk: week; mo: month; CHO: carbohydrates; Pl: placebo; LC: l-Carnitine; PDH: pyruvate dehydrogenase; VO₂max: maximal oxygen uptake; CK: creatine kinase; IGFBP-3: insulin-like-growth-factor-binding-protein-3; RQ: respiratory quotient; LA: lactate; BE: base excess; TBARS: thiobarbituric acid reactive substances; RER_max: maximal respiratory exchange ratio; P_max: maximal power; LDL-C: low density lipoprotein cholesterol; HDL-C: high density lipoprotein cholesterol; apoA1: Apolipoprotein A1; apoB: apolipoprotein B; mTOR: mechanistic target of rapamycin.