Benefits
Sprint power after a single pre-exercise dose
In a crossover study of 24 resistance-trained men, one 4.5 g dose taken 90 minutes before testing gave 2.6 to 15% higher peak and mean power across five 10-second cycle sprints than placebo, and 16% less blood lactate 14 minutes after. Thirty-two men enrolled, and the maker funded the study.
Sprint power after 4 weeks of daily use
Of 45 resistance-trained men enrolled, 38 completed testing; 4 weeks at 1.5, 3 or 4.5 g a day had no significant overall effect on sprint power. At 3 or 4.5 g, power in the later sprints was 2 to 5% below the men's placebo baseline; at 1.5 g it was 3 to 6% above. The 4-week phase had no placebo group, and the maker funded the study.
Aerobic fitness and muscle carnitine during endurance training
In 32 healthy men and women doing 8 weeks of aerobic training, 1 or 3 g a day did not change peak oxygen uptake, time to fatigue, anaerobic power or muscle carnitine compared with placebo. The researchers concluded that GPLC was ineffective for these outcomes at these doses.
Plasma nitrate and nitrite, a marker of nitric oxide
In 15 resistance-trained men, 4.5 g a day for 4 weeks gave about 31% higher plasma nitrate/nitrite than placebo, though 4 of the 15 did not respond. In an 8-week training study the marker rose at 3 g but not 1 g a day, and a single 4.5 g dose before a workout did not change it. This marker is not a measure of blood flow.
Resting oxidative stress marker during aerobic training
In 30 untrained adults doing 8 weeks of supervised aerobic exercise, malondialdehyde (MDA), a blood marker of lipid peroxidation (oxidative damage to fats), fell at rest with 1 or 3 g a day but not with placebo; the paper reports changes within each group, not a difference between groups. Blood lipids did not change. A related paper from the same lab found little effect on oxidative stress markers right after exercise tests.
Blood markers after a forearm cuff occlusion test
In 15 trained men, 4.5 g a day for 4 weeks did not reduce the rise in blood oxidative stress markers, such as MDA, hydrogen peroxide and oxidized glutathione, after a period of arm cuff occlusion and release, compared with placebo. The rise itself was small and short-lived.
Muscle oxygen and bench press volume before a workout
In a crossover of 19 resistance-trained men, a 4.5 g pre-workout dose gave higher muscle oxygen saturation at the start of a bench press test than one of three commercial pre-workout products, but no performance measure differed significantly from placebo. The device used does not measure blood flow directly.
Mechanism of action
Part of the body's carnitine pool
Propionyl-L-carnitine is one of the body's own carnitine forms, alongside L-carnitine and acetyl-L-carnitine. In a drug study that infused propionyl-L-carnitine into the veins of healthy men, blood L-carnitine and acetyl-L-carnitine rose too, and its half-life was about 1 hour. That study did not test oral GPLC.
Proposed link to nitric oxide
The trial authors propose that GPLC raises nitric oxide, a signal that helps blood vessels widen. A review of nitric oxide related supplements noted that the way GPLC might do this has not been worked out. The blood marker used, nitrate/nitrite, reflects nitric oxide production only indirectly.
Propionyl group as a cell fuel (theory)
Trial authors describe the propionyl part as a source of propionyl units that cells can turn into succinate, an intermediate of the energy-producing citric acid cycle. This is a biochemical rationale; the GPLC trials did not measure it, and one found no rise in muscle carnitine after 8 weeks.
Clinical trials
Double-blind, placebo-controlled crossover; one 4.5 g dose of GPLC or cellulose 90 minutes before five 10-second cycle sprints with 1-minute recoveries, one week apart. Funded by Sigma-tau HealthSciences. (Jacobs et al. 2009, J Int Soc Sports Nutr)
24 resistance-trained men (mean age 25) completed, of 32 enrolled.
Peak and mean power were 2.6 to 15% higher across the sprints with GPLC (P < 0.05), and blood lactate was 16.2% lower 14 minutes after exercise (P < 0.05; 15.7% lower at 4 minutes, P = 0.09). The authors noted that the smaller final sample reduced statistical power. A single-dose lab test, not a sport outcome.
Placebo and single-dose GPLC sessions in random order, then random assignment to 1.5, 3.0 or 4.5 g/day for 28 days with no placebo group, comparing results with each man's placebo baseline. Funded by Sigma-tau HealthSciences. (Jacobs et al. 2010, J Int Soc Sports Nutr)
45 resistance-trained men aged 18 to 35 enrolled; 38 completed (13 at 1.5 g, 11 at 3 g and 14 at 4.5 g a day).
No significant effect of condition on peak or mean power. In the later sprints, 3 or 4.5 g/day was followed by 2 to 5% lower peak power and greater power drop-off than baseline, while 1.5 g/day gave 3 to 6% higher peak power and 24% less lactate per unit of power (P < 0.05). The authors called the effects dose dependent.
Randomized, double-blind trial of placebo, 1 g/day or 3 g/day GPLC for 8 weeks during an aerobic training program, with muscle biopsies. (Smith et al. 2008, Int J Sport Nutr Exerc Metab)
32 healthy men and women aged 18 to 44 (placebo 9, 1 g 11, 3 g 12).
No significant differences between or within groups for peak oxygen uptake, time to fatigue, anaerobic threshold, anaerobic power, total work or muscle carnitine. The authors concluded that up to 3 g/day for 8 weeks was ineffective for raising muscle carnitine or improving performance.
Double-blind crossover of GPLC 4.5 g/day (3 g propionyl-L-carnitine plus 1,044 mg glycine) or placebo for 4 weeks each, 2-week washout; blood taken at rest and after upper-arm cuff occlusion. Funded in part by Sigma-tau HealthSciences. (Bloomer et al. 2007, J Int Soc Sports Nutr)
15 healthy resistance-trained men (mean age 24).
Plasma nitrate/nitrite, an indirect marker of nitric oxide, averaged 45.6 with GPLC versus 34.9 micromol/L with placebo (P = 0.0008). Differences at rest were not significant; they were significant 3 and 10 minutes after cuff release. 11 of 15 men had a rise. Blood flow itself was not measured.
Randomized, double-blind trial of 1 g or 3 g GPLC a day or placebo during 8 weeks of supervised aerobic exercise; fasting blood at rest before and after. (Bloomer et al. 2009, Int J Vitam Nutr Res)
30 untrained adults with normal blood lipids.
MDA fell from before to after with 1 g (1.08 to 0.69) and 3 g (0.94 to 0.66 micromol/L) but not with placebo. Nitric oxide rose with 3 g (21.3 to 29.5 micromol/L) but not with 1 g or placebo. Blood lipids did not change. Results are within-group changes in a small sample.
Randomized, double-blind crossover of placebo, 4.5 g GPLC or one of three commercial pre-workout supplements before bench press power and endurance tests, one week apart. Funded by Sigma-tau HealthScience; the lead author disclosed research grants from the company since 2005. (Bloomer et al. 2010, J Int Soc Sports Nutr)
19 resistance-trained men.
GPLC gave higher muscle oxygen saturation at the start of exercise than one of the other products. No significant effects on nitrate/nitrite, MDA, heart rate, effort ratings or any performance measure; a 3.3% greater total volume load versus placebo was not significant. The authors concluded the products tested were ineffective for raising blood flow and acute upper-body performance.
Double-blind crossover of GPLC 4.5 g/day or placebo for 4 weeks each with a 2-week washout; blood taken at rest and after an ischemia-reperfusion (cuff occlusion and release) protocol. (Bloomer et al. 2010, Int J Vitam Nutr Res)
15 healthy men who trained regularly.
No effect of condition and no condition by time interaction for any marker, including lactate, MDA, F2-isoprostanes, hydrogen peroxide, xanthine oxidase and glutathione. The authors concluded that GPLC did not blunt the small, short-lived rise in these markers.