Choline is an essential nutrient that is the precursor of acetylcholine (the primary neurotransmitter of the parasympathetic nervous system and of the neuromuscular junction) and of phosphatidylcholine (the most abundant phospholipid in cell membranes and the precursor of the neurotransmitter phosphatidylcholine and of the key methyl donor betaine). Choline deficiency is associated with liver dysfunction (fatty liver, elevated liver enzymes), with cognitive impairment (particularly in the frontal lobe and hippocampal-dependent memory tasks), and with neural tube defects in the developing fetus (when maternal choline intake is inadequate during early pregnancy). The clinical importance of choline is underscored by the observation that the majority of the adult population does not achieve the adequate intake (AI) of 425-550mg daily from diet alone, and that the risk of choline deficiency is particularly high in vegans and vegetarians (because choline is most abundant in animal products including egg yolks, meat, fish, and dairy), in pregnant and lactating women (who have an increased choline demand), and in people with the genetic polymorphisms that reduce the efficiency of choline metabolism.
Choline and the phosphatidylethanolamine Methyltransferase Pathway
Choline is oxidised in the mitochondria of the liver and kidney to betaine via the two-step choline dehydrogenase (CHDH) and betaine aldehyde dehydrogenase (BADH) pathway, and betaine is then used as a methyl donor for the betaine-homocysteine methyltransferase (BHMT) reaction, which remethylates homocysteine to methionine. This choline-to-betaine pathway is the primary source of betaine in the body and is the primary alternative to the folate-dependent remethylation pathway for the maintenance of normal homocysteine levels. When choline intake is low (as in the majority of the adult population), the betaine-homocysteine methyltransferase pathway is impaired, homocysteine accumulates, and the risk of hyperhomocysteinemia and of the cardiovascular disease that is associated with it is increased. The clinical importance of this choline-betaine-homocysteine connection is underscored by the observation that choline supplementation (at 500-1,000mg daily) has been shown to lower homocysteine levels in people with hyperhomocysteinemia, possibly through the betaine-dependent activation of the BHMT pathway.
The phosphatidylethanolamine N-methyltransferase (PEMT) pathway is the other major choline-dependent pathway in the body. In this pathway, phosphatidylethanolamine (PE) is methylated three times by phosphatidylethanolamine N-methyltransferase (using SAM as the methyl donor), generating phosphatidylcholine (PC). This PEMT pathway is particularly important in the liver, where it accounts for approximately 30% of hepatic phosphatidylcholine synthesis (the remainder coming from the CDP-choline pathway, which uses dietary choline directly). The phosphatidylcholine that is synthesised by the PEMT pathway is required for the assembly and secretion of very-low-density lipoprotein (VLDL), and when the PEMT pathway is impaired (as in choline deficiency or in genetic deficiency of the Pemt gene in mice), hepatic phosphatidylcholine levels fall, VLDL assembly is impaired, and hepatic fat accumulates (producing the fatty liver phenotype that is the hallmark of choline deficiency). The PEMT pathway is also important in the brain, where it is one of the primary sources of the phosphatidylcholine that is required for myelin synthesis and for the maintenance of neuronal membrane integrity.
Choline and Brain Function
Choline is the precursor of acetylcholine, the primary neurotransmitter of the parasympathetic nervous system (the rest-and-digest system), of the cholinergic neurons of the basal forebrain (which are the primary source of cortical acetylcholine and which are critical for attention, memory, and executive function), and of the neuromuscular junction (where acetylcholine is the neurotransmitter that activates skeletal muscle contraction). The acetylcholine produced from choline is essential for the normal function of all cholinergic synapses in the body, and its deficiency is associated with cognitive impairment (particularly of the hippocampal-dependent memory systems), with the autonomic dysfunction that characterises conditions of parasympathetic insufficiency, and with the muscle weakness that characterises the neuromuscular disorders that affect the neuromuscular junction. Choline supplementation (at 500-1,000mg daily of CDP-choline or alpha-GPC, the two most bioavailable forms of choline) has been shown in multiple RCTs to improve 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.
Practical Application
For general choline supplementation, the evidence-based dose is 500-1,000mg of choline daily from CDP-choline (citicoline) or alpha-GPC, the two most bioavailable forms. The standard choline forms (choline bitartrate, phosphatidylcholine) have lower bioavailability and are less effective at increasing brain choline levels than CDP-choline or alpha-GPC. Choline is generally well-tolerated with no significant adverse effects at therapeutic doses, though very high doses (above 3g daily) can produce a fishy body odour, GI upset, and in some cases, hypotension. For comprehensive choline support, CDP-choline or alpha-GPC pairs well with the omega-3 fatty acids (which are the precursors of the DHA that is essential for neuronal membrane composition and which have independent cognitive benefits), with the B-complex vitamins (which are required for the function of the choline metabolism enzymes, including the folate-dependent remethylation enzymes), with huperzine A (an acetylcholinesterase inhibitor that prevents the breakdown of acetylcholine and that synergises with choline for the enhancement of cholinergic neurotransmission), and with the Mediterranean dietary pattern (which is associated with better cognitive function and with reduced risk of cognitive decline).
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