The scientists had their 14 healthy male volunteers ingest either 80 g of CHO (Control) or 2 g of L-carnitine L-tartrate and 80 g of CHO (Carnitine) twice daily for 24 weeks in a randomised, double blind manner. Other than in previous studies with plain l-carnitine (minus the l-tartrate) there was a significant increase of muscle carnitine content with all the downstream metabolic benefits on exercise performance one would expect:
Muscle TC increased from basal by 21% in Carnitine (P<0.05), and was unchanged in Control. At 50% VO2max, the Carnitine group utilised 55% less muscle glycogen compared to Control (P<0.05) and 31% less pyruvate dehydrogenase complex activation (PDCa) compared to before supplementation (P<0.05). Conversely, at 80% VO2max, muscle PDCa was 38% higher (P<0.05), acetylcarnitine content showed a trend to be 16% greater (P<0.10), muscle lactate content was 44% lower (P<0.05) and the muscle PCr/ATP ratio was better maintained (P<0.05) in Carnitine compared to Control. The Carnitine group increased work output 11% from baseline in the performance trial, while Control showed no change.So, after all, it was not the wrong the substance previous investigations used, but rather the wrong form - l-carnitine l-tartrate - though expensive - is the way to go to burn fat and increase athletic performance. If you do not want to buy it in bulk, I suggest you at least buy a product like MAN Sports Body Octane, where you actually know how much (here 1g) of the carnitine you're getting per serving.