Experience From - Karl King
Scott E. asked about L-Carnitine
L-Carnitine is a non-essential amino acid ( that is, a healthy body can make what it needs from other amino acids ). It is critical in transporting the fat that is burned for energy ( free fatty acids ). Picture a tiny cell associated with muscle fibers - the mitochondrion. These chemical processing factories take in carbohydrate and fat, and release ATP, adenosine tri phosphate, which is the chemical that directly fuels the muscle fibers.
As the free fatty acids approach the cell wall, they are carried by acetyl- coenzyme A, which contains vitamin B5. It cannot get through the cell wall, which is mainly fat. The fat portion is transferred from the Co A to a molecule that is mainly L-Carnitine. In this form, it can go through the cell wall. On the other side, the fat is transferred back to a molecule of Co A, and the chemical reactions which lead to ATP begin.
So, to burn fat, you need vitamin B5 and L-Carnitine. Clearly, since people do not collapse in ultras from lack of these two, it is not a life and death matter. The question is, will a shortage late in the run slow you down? There are clinical situations where lack of these molecules leads to some very serious conditions. Anyone with those is not healthy enough to run much at all.
B5 is very inexpensive and has a very low toxicity. It is also common in food. L-Carnitine also has a low toxicity, but is quite expensive. It is common in meat ( muscle ) but you can't get large amounts unless you pig out on steak every night. Not a good idea.
Based on my sample of 1 or 2 research during long runs and ultras, the following works for me: If running over 25 miles take 250 mg of each, B5 and L-Carnitine at zero, 25, 50, 75, etc.
B5 is available anywhere drugs are sold ( you know what I mean ), and L-Carnitine is available at most health food stores. My choice is Twinlab 250 mg capsules. Since I don't use it often, the bottle goes up in the freezer to wait the next 25+ run.
A few products on the market advertise L-Carnitine as an ingredient. If you look at the amounts, they are generally too low to be of physiological value.
In essence, it amounts to a gimmick. That could be said of many commercial products sold to athletes. The formulas are developed more by the marketing department than the scientists. Often, the best formulations are the simplest, but those offer no distinct possibilities for a sales pitch. So, you seldom see formulations that are based solely on the science.
Some products for weight loss hype a content of L-Carnitine or other "fat burners". It is largely a joke. Very heavy people usually cannot produce energy at a very high rate and are unlikely to run short on either B5 or L-Carnitine. And, piling on extra will not increase the rate of fat burning. Bottom line: you can take these for insurance, but don't expect miracles unless you have some organic shortage.