Benefits of Combining Supplements: L Carnitine & Methyl B 12: The Center for Internal and Integrative Medicine: Integrative Medicine Doctors L Carnitine L Tartrate: 8 Key Benefits Dr. Lam Acetyl L Carnitine Superior Supplement Products Science First Designs for Health Ultra L Carnitine NutriRestore L Carnitine 1000mg Energy, Metabolism & Fat Burning L carnitine is a naturally occurring amino acid structure that is made in the human brain, liver, and kidneys. L carnitine plays a critical role in energy production, as it converts fat into energy.