L-carnitine is a quaternary ammonium compound that is synthesised in the body from the amino acids lysine and methionine and that is essential for the transport of long-chain fatty acids into the mitochondrial matrix for beta-oxidation. While L-carnitine is most commonly associated with fat metabolism and with cardiac and skeletal muscle function, it also has critically important functions in the brain — it supports mitochondrial energy production in neurons (which rely primarily on glucose and ketone bodies for their energy needs, but which also oxidise fatty acids under some conditions), it has neuroprotective effects (through the reduction of oxidative stress, the stabilisation of mitochondrial function, and the modulation of the inflammatory response), and it is a precursor of the acetyl-L-carnitine that is used in the synthesis of the neurotransmitter acetylcholine. This dual role of L-carnitine in both fat metabolism and neuronal function makes it one of the most versatile and most evidence-based supplements for cognitive health and for the prevention of age-related cognitive decline.
Acetyl-L-Carnitine and Neurotransmitter Synthesis
Acetyl-L-carnitine (ALCAR) is the acetylated form of L-carnitine, and it is synthesised by the enzyme carnitine acetyltransferase (CRAT), which transfers the acetyl group from acetyl-CoA to L-carnitine. ALCAR is the preferred form of carnitine for neurological applications because it has better CNS penetration than L-carnitine (due to its increased solubility and to the activity of the specific ALCAR transporters that are expressed in the blood-brain barrier). Once in the brain, ALCAR serves two primary functions — it is a precursor of acetylcholine (the neurotransmitter of the cholinergic neurons of the basal forebrain and of the neuromuscular junction), and it is a mitochondrial protector (it supports mitochondrial ATP production, reduces mitochondrial oxidative stress, and modulates the mitochondrial permeability transition pore that is one of the primary triggers of apoptotic cell death in neurons). The importance of ALCAR for cholinergic neurotransmission is underscored by the observation that the cholinergic neurons of the basal forebrain (which are the most vulnerable neurons in Alzheimer disease) have a high rate of ALCAR metabolism and are particularly sensitive to ALCAR deficiency.
Multiple RCTs have demonstrated that ALCAR supplementation (at 1-2g daily for 3-12 months) improves cognitive function in older adults with mild cognitive impairment and with Alzheimer disease, with improvements in memory, attention, and processing speed that are clinically meaningful and that are apparent within the first 3-6 months of treatment. The mechanism of these cognitive benefits is thought to involve the ALCAR-induced improvement in mitochondrial function in neurons (which improves the energy status of neurons and reduces the mitochondrial dysfunction that characterises early Alzheimer disease), the ALCAR-induced reduction in amyloid-beta toxicity (ALCAR has been shown to reduce the neurotoxicity of amyloid-beta in cell culture and in animal models of Alzheimer disease), and the ALCAR-induced enhancement of cholinergic neurotransmission (through the provision of acetyl groups for acetylcholine synthesis).
L-Carnitine and the Metabolic Syndrome
L-carnitine supplementation has also been studied for its effects on the metabolic syndrome — the cluster of insulin resistance, dyslipidaemia, hypertension, and central obesity that is associated with an increased risk of type 2 diabetes and cardiovascular disease. Multiple RCTs have demonstrated that L-carnitine supplementation at 2-4g daily for 8-12 weeks significantly improves insulin sensitivity (as measured by HOMA-IR), reduces fasting blood glucose, reduces fasting insulin, and improves the lipid profile (particularly by reducing triglycerides and increasing HDL cholesterol) in people with the metabolic syndrome and with type 2 diabetes. The mechanism of these metabolic benefits is thought to involve the L-carnitine-induced improvement in the rate of fatty acid oxidation (which reduces the intracellular accumulation of lipid metabolites that interfere with insulin signalling) and the L-carnitine-induced improvement in mitochondrial function (which is impaired in the insulin resistance that characterises type 2 diabetes).
Practical Application
For general L-carnitine and acetyl-L-carnitine supplementation, the evidence-based dose is 1-2g of acetyl-L-carnitine daily (for cognitive support and for Alzheimer disease) or 2-4g of L-carnitine daily (for metabolic support, for cardiovascular support, and for athletic performance). ALCAR is the preferred form for neurological applications (cognitive decline, Alzheimer disease, peripheral neuropathy) because of its superior CNS penetration; L-carnitine is preferred for metabolic and cardiovascular applications (insulin resistance, the metabolic syndrome, heart failure). Both forms are generally well-tolerated with no significant adverse effects at therapeutic doses, though very high doses can produce a fishy body odour and mild GI upset. For comprehensive cognitive support, ALCAR pairs well with phosphatidylserine (which supports neuronal membrane composition and which has independent cognitive benefits), with the omega-3 fatty acids (which are the precursors of the DHA that is essential for neuronal membrane composition), with huperzine A (which prevents the breakdown of acetylcholine and which synergises with ALCAR for the enhancement of cholinergic neurotransmission), and with the Mediterranean dietary pattern (which is associated with slower cognitive decline and with reduced risk of Alzheimer disease).
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