L-carnitine, is a naturally occurring substance found in most cells of the body, particularly the brain, neural tissue, muscle, and heart. Carnitine, whose structure is similar to choline, is widely available in animal foods (meat, poultry, fish and dairy products), whereas plants have very small amounts. Most non-vegetarians consume about 100 to 300 mg of carnitine a day, and the body is able to synthesize this nutrient if dietary intake is inadequate. When ingested as a pill, carnitine is not able to cross the blood-brain barrier as well as its activated form acetyl-L-carnitine. Acetyl l carnitine has a more noticeable mental effect than carnitine. Acetyl l carnitine usually enhances mental clarity and focus, along with slight mood elevation. The effects of carnitine tend to be more physical than mental, in the sense that it provides enhanced physical energy whereas acetyl l carnitine provides more mental energy.