In one study, 14 healthy men took 2 grams of L-carnitine-L-tartrate plus 80 grams of carbohydrate per day, or a carbohydrate placebo twice daily. At the start of the study, and at 12 and 24 weeks, the men cycled for 30 minutes at 50 percent of maximum oxygen onsumption, 30 minutes at 80 percent, and then took a 30-minute work-output test.
After 24 weeks, while there was no change in the placebo group, men in the carnitine group had 21 percent higher muscle-carnitine levels. During the 50 percent cycle test, compared to placebo, the carnitine group burned 55 percent fewer glycogen reserves, indicating more fat-burning energy. For the 80 percent cycle test, compared to placebo, the carnitine group produced 44 percent less muscle lactate; a byproduct of inefficient energy production. For the final work-output test, while the placebo group did not improve, men in the carnitine group produced 11 percent more output compared to the beginning of the study.
In their summary, researchers said the metabolic effects of carnitine—measured during a scientifically validated exercise performance test—gave men in the carnitine group a sense of reduced effort while producing greater work output. These findings, doctors said, may influence thinking about athletic performance and be useful in conditions where fat metabolism and energy production can be improved.