The old school thinking of "if a little is good for you, then a lot must be better" has been proved wrong by Stanley Hazen, whose research last year established a link between bacteria in your gut, l-carnitine, and heart risk.
Yes, it's well understood that l-carnitine is crucial. It helps produce all your body's energy by transporting fatty acids to your mitochondria. However, humans produce all the l-carnitine they need. Supplementing with it (and eating a diet rich in red meat) does no good, and in fact, does harm.
Your gut bacteria can be "trained" to digest l-carnitine by eating a lot of it. The more you ingest, the more your gut fauna evolves to digest it. A byproduct of this digestion is a substance that promotes plaque build-up in your veins.
Instead of accelerated energy, you end up with accelerated Atherosclerosis!
Want to hear it straight from the horse's mouth? Then give this a listen! http://www.sciencefriday.com/guests/stanley-hazen.html#page/full-width-list/1