Acetyl-L-carnitine is a form of carnitine. Carnitine (also known as L-carnitine or levocarnitine) is naturally produced by our bodies. It is also an important component of a healthy diet, particularly in young children, people under stress, and in people with many metabolic disorders and mitochondrial dysfunction. Carnitine ferries long-chain fatty acids into mitochondria, and thus is essential for fat metabolism. Carnitine also plays a detoxifying role by ferrying incompletely oxidized fatty acids, organic acids, and other metabolic intermediates out of the mitochondria, so that they can be excreted. Carnitine deficiency can present in many ways, but fatigue and muscle weakness are common. Since carnitine supplementation has few side effects, many experts offer carnitine supplementation to their patients with conditions related to metabolic or mitochondrial dysfunction. Acetyl-L-carnitine can more easily cross membranes than regular carnitine. Because of an increased ability to cross the blood-brain barrier, acetyl-L-carnitine is sometimes used for a variety of brain disorders, including ADD/ADHD and autism. Side effects of carnitine or acetyl-L-carnitine are few, including nausea, diarrhea and/or a fish-like odor, generally at higher dosing. Laboratory testing can in many cases be helpful in terms of optimizing carnitine supplementation, as levels of free carnitine in the blood over 30 to 40 mcmole/L can be associated with improved outcomes.