Table 1.
The use of L-carnitine supplementation in healthy subjects: characteristics and outcomes of studies included in the systematic review according to PICOS (population, intervention, comparison, outcome and study design) criteria for the inclusion of studies.
| Authors, yr | Study Design; Evidence Levels |
Population, Y |
Comparison Samples | Intervention: L-Carnitine Dosage | Outcomes |
|---|---|---|---|---|---|
| Marconi 1985 | RT; Level 2 |
6 competitive walkers m 25.3 y |
A: Before supplementation B: After supplementation |
4 g/d, 2 w | Increased serum L-carnitine, no change in blood lactate concentrations and R at fixed workload. The 6% increase in VO2 was not significantly related to carnitine intake. |
| Cooper 1986 | CT; Level 1 |
10 marathon runners m 19–25 y |
A: Before supplementation B: After supplementation |
4 g/d, 10 d | L-carnitine supplementation increased the tissue content of oxidized glutathione. |
| Drăgan 1987 | RT; Level 2 |
7 athletes | A: Before supplementation B: After supplementation |
1 g/d for 6 w + 2 g/d for 10 d | Carnitine group showed better obtained higher performances. |
| Greig 1987 | CT; Level 1 |
19 healthy subjects, 7 m/12 f 27.1 ± 4.6 y |
A: 9 s, 3 m, 6 f B: 10 s, 4 m, 6 f |
A: 2 g/d, 2 w B: 2 g/d, 4 w |
No change in maximum oxygen uptake (VO2, R). |
| Drăgan 1988 | RT; Level 2 |
110 healthy subjects | A: Before supplementation B: After supplementation |
1 g/d for 3 w | Improved athletic performance, lower lactic acid. |
| Oyono-Enguelle 1988 | RT; Level 2 |
10 exercising subjects - |
A: Before supplementation B: After supplementation |
2 g/d, 4 w | After L-carnitine intake, the levels returned to their initial values 6–8 w after cessation of the supply. |
| Soop 1988 | RT; Level 2 |
7 moderately trained subjects 19–31 y |
A: Before supplementation B: After supplementation |
5 g/d, 5 d | No change in O2 uptake and arterial levels and turnover of FFA after L-carnitine supplementation. |
| Gorostiaga 1989 | RT; Level 2 |
10 trained athletes, 9 m/1 f 25.8 ± 2.2 y |
A: Before supplementation B: After supplementation |
2 g/d, 4 w | Reduced R during submaximal exercise after supplementation. Increased lipid use by muscles during exercise. Oxygen uptake, heart rate, blood glycerol and resting plasma free fatty acid concentrations presented a nonsignificant trend. |
| Siliprandi 1990 | CT; Level 1 |
10 moderately trained subjects 23–30 y |
A: Before supplementation B: After supplementation |
2 g/d, 2 d | L-carnitine supplementation decreased plasma lactate and pyruvate concentration. |
| Vecchiet 1990 | CT; Level 1 |
10 moderately trained subjects 22–30 y |
A: Before supplementation B: After supplementation |
2 g/d, one single dosage | Increased VO2, maximal oxygen uptake, power output and reduced pulmonary ventilation and plasma lactate after L-carnitine supplementation. |
| Wyss 1990 | RT; Level 2 |
7 healthy subjects 22.2 ± 2.3 y |
A: Before supplementation B: After supplementation |
3 g, 1 w | Decreased R, and rate of carbohydrate transformation during hypoxia after L-carnitine supplementation. |
| Arenas 1991 | CT; Level 1 |
24 athletes 19–27 y |
A: 13 and 11 s placebo B: 11 s sprinters L-carnitine C: 13 long-distance runners |
1 g/d, 24 w | The supplementation avoids the decrease of total and free muscle carnitine due to training in athletes. |
| Huertas 1992 | CT; Level 1 |
14 athletes | A: Before supplementation B: After supplementation |
2 g/d, 4 w | Increase in respiratory-chain enzyme activities in the muscle. |
| Natali 1993 | CT; Level 1 |
20 healthy subjects A: 29.5 ± 1.7 years B: 29 ± 2 y |
A: 8 healthy m B: 12 healthy |
A: 1 g + 0.5 g/h iv B: 3 g, iv. 40′ before exercise |
No changes during exercise with L-carnitine intake, but increased fatty acid oxidation during recovery. |
| Arenas 1994 | CT; Level 1 |
16 long-distance runners 28.3 ± 7.1 |
A: 8 s placebo B: 8 s L-carnitine |
2 g/d, 4 w | The supplementation increased pyruvate dehydrogenase complex activities. |
| Brass 1994 | CT; Level 1 |
14 athletes 23–40 y |
A: Before supplementation B: After supplementation |
185 μmol/kg/1 d iv | No effect on skeletal muscle carnitine homeostasis during exercise: on R, muscle lactate accumulation, plasma lactate concentration, muscle glycogen utilization, plasma p-hydroxybutyrate concentration after L-supplementation. |
| Colombani 1996 | CT; Level 1 |
7 m runners 36 ± 3 y |
A: Before supplementation B: After supplementation |
4 g/d, 1 d | No effect on performance, and no changes in running time and in R after L-carnitine supplementation. |
| Giamberardino 1996 | RT; Level 2 |
6 healthy untrained subjects, 26 ± 3.8 y |
A: Before supplementation B: After supplementation |
A: 3 g/d, 3 w | Protective effect against pain and damage. |
| Swart 1997 | RT; Level 2 |
7 marathon runners - |
A: Before supplementation B: After supplementation |
2 g/d, 6 w | After L-carnitine supplementation: increase of peak treadmill running speed of 5.68%, average VO2, free carnitine levels, decreased heart rate and respiratory exchange ratio values. |
| Nuesch 1999 | RT; Level 2 |
9 athletes | A: Before supplementation B: After supplementation |
1 g/d after treadmill | In athletes without L-carnitine intake, plasma free carnitine concentration decreased significantly 10 min after exercise compared with baseline. In athletes with oral L-carnitine supplementation, the elevated plasma concentration of free carnitine at baseline did not decrease after maximal exercise. |
| Muller 2002 | RT; Level 2 |
10 healthy untrained subjects 5 m/5 f 36.4 ± 12.8 y |
A: Before supplementation B: After supplementation |
3 g/d, 10 d | Significant increase in fatty acid oxidation, muscle weight, total body water and metabolic rate. |
| Wachter 2002 | RT; Level 2 |
8 healthy subjects 23–25 y |
A: Before supplementation B: After supplementation |
4 g/d, 12 w | Significant increase in physical performance after L-carnitine supplementation. |
| Stuessi 2005 | CT; Level 1 |
12 m athletes, 25 ± 3 y |
A: Before supplementation B: After supplementation |
2 g/d, lump sum | No enhanced performance in endurance after L-carnitine supplementation. |
| Malaguarnera 2007 | CT; Level 1 |
64 healthy elderly subjects A: 101 ± 1.3 y B: 101 ± 1.4 y |
A: 32 s L-carnitine B: 34 s placebo |
2 g/d, 24 w | Improvements in fat mass, muscle mass, blood carnitine, acylcarnitine, physical and mental fatigue. |
| Chun 2011 | RT; Level 2 |
36 m soccer players 20.67± 1.21 y |
A: 6 s, 2g L-carnitine B: 6 s, 3g L-carnitine, C: 6 s, 4g L-carnitine, D: 6 s,5g L-carnitine E: 6 s, 6g L-carnitine F: 6 s, no intake |
2–6 g/d, 4 w | L-carnitine can enhance endurance and recovery from fatigue in athletes, increase VO2 and decrease lactate concentration |
| Orer 2014 | CT; Level 1 |
26 footballers 18.4 ±0.5 y |
A: 12 athletes placebo B: 14 athletes L-carnitine |
A; 3 g/d, 1 d B: 4 g/d, 1 d |
Supplementation affected performance positively in terms of running speed corresponding to specific lactate concentrations, lactic acid and Borg scale responses corresponding to running speeds. |
| Parandak 2014 | CT; Level 1 |
21 healthy subjects A: 22.2 ± 1.1 y B: 22.0 ± 1.0 y |
A: 10 L-carnitine B: 11 placebo |
2 g/d, 2 w | TAC increased significantly 14 days after supplementation. Serum MDA-TBARS, CK, TAC, LDH were significantly lower 24 h after exercise. The supplementation alleviated the effects on lipid peroxidation and muscle damage markers. |
| Bradasawi 2016 |
CT; Level 1 |
50 healthy subjects A: 68.2 ± 6.3 y B: 68.2 ± 6.5 y |
A: 26 s L-carnitine B: 24 s placebo |
1.5 g/d, 10 w | No significant changes in free L-carnitine, total L-carnitine, acyl L-carnitine blood level and frailty biomarkers (IL-6, TNF-α, and IGF-1) between the placebo group and carnitine group. |
| Koozehchian 2018 | CT; Level 1 |
23 trained subjects A: 24.5 ± 1.5 y B: 25.5 ±1.5 y |
A: 11 s placebo B: l2 s L-carnitine |
2 g/d, 9 w | No significant influence on muscle mass though upper/lower body strength improved. |
| Mor 2018 | CT; Level 1 |
16 taekwondo players 18–28 y |
A: 8 s L-carnitine B: 8 s placebo |
1 g/d, 1 w | Low body fat mass. |
CT: Clinical trial, RT: Retrospective study, PS: Prospective study, s: Subjects, y: Years old, A: Group 1, B: Group 2, g: Gram, d: Days, w: Week, m: Months, h: Hours, IL: Interleukin, TNF: Tumor necrosis factor, ILGF: Insulin-like growth factor, FIS: Frailty Index score, PASE: Physical Activity Scale for Elderly, WST: walking speed test, 2MST: 2-min step test, TUG: time up and go test, CST: chair stand test, RPT: rapid pace test, SST: shoulder strength test, PEFR: peak expiratory flow rate, ADL: Activities of daily living, IADL: Instrumental activities of daily living, RT: Resistance training, VO2: maximum oxygen uptake, FFA: Free Fatty Acids, h: Hour, MRI: magnetic resonance imaging, R: respiratory exchange ratio, m: males, f: females, MMSE: Mini-Mental State Examination, TAC: Plasma total antioxidant capacity, MDA: malondialdehyde, TBARS: thiobarbituric acid-reactive substance, CK: creatine kinase, LDH: lactate dehydrogenase, S: Supplementation, TMAO: trimethylamine-N-oxide, 3IGF-BP3: Serum insulin-like growth factor-binding protein, iv: Intravenously.