Endurance athletes looking to boost their endurance – legally of course – may be interested in the study that nutritionists at the Korean Chonbuk National University published in 2001 in the Journal of Nutritional Science and Vitaminology. The researchers tested experimental sports drinks containing large amounts of caffeine and L-carnitine on rugby players. The results were promising.
The five subjects that the Koreans used for their study had all trained for at least five years. They used no drugs or doping and refrained from consuming products containing caffeine during the experiment. The athletes were given 250 ml of a sports drink that contained different active ingredients on four separate occasions on different days.
On one occasion the drink contained nothing extra [CON]. On a second occasion the drink contained 5 mg caffeine per kg bodyweight for each athlete [CAF]. On the third occasion the drink contained 15 g L-carnitine per kg bodyweight [CAR], and on the fourth occasion the drink contained both 5 mg caffeine and 15 g L-carnitine per kg bodyweight [CAF+CAR].
Caffeine prevents adenosine from working in the body. In its free form, adenosine attaches itself to the adenosine receptors and reduces the metabolic rate of cells. By inhibiting that process caffeine has a stimulatory effect. Caffeine also boosts the concentration of the signal molecule cAMP, which makes cells more alert to natural pep-hormones like adrenalin. As a result of these processes, muscles are able to work harder and longer, and fat cells also release more fatty acids into the bloodstream.
L-Carnitine has a less intrusive effect. It helps the mitochondria, the cells' energy powerhouses, to absorb more fatty acids and convert them into energy.
An hour after intake the test subjects were made to cycle: first they rode for 45 minutes at 60 percent of their VO2max. That's a speed at which you can just keep up a conversation. After that the athletes upped their speed to 80 percent of their VO2 max, a speed at which it's no longer possible to keep talking. According to the researchers' measurements, the athletes in the CAF+CAR group kept up the high speed for twice as long as the athletes in the CON group.
Analyses of the athletes' blood suggested that the ****tail works in the way the theory would lead one to imagine. Caffeine chases fatty acids out of the fat cells, and L-carnitine induces the muscles to burn more fatty acids.
"This study showed that carnitine ingestion could promote fat oxidation and endurance performance in athletes, and especially these ergogenic effects of carnitine coingested with caffeine more than a single bout of carnitine", the Koreans conclude.
The doses used were high. If you are sensitive to caffeine, a dose of 5 mg per kg bodyweight can cause psychological side effects. Some people also report mild gastro-intestinal complaints after taking 15 g L-carnitine.
J Nutr Sci Vitaminol (Tokyo). 2001 Dec;47(6):378-84.