The Possible Essentiality of Choline

In many animals, dietary deprivation of choline leads to liver dysfunction and growth retardation, and some patients maintained on choline-free total parenteral nutrition develop liver damage that resolves when choline is provided, suggesting that endogenous synthesis may be inadequate to meet requirements (Zeisel, 2000). There is inadequate information to permit the setting of reference intakes, but the Acceptable Intake for adults is 550 mg (for men) or 425 mg (for women) per day (Institute of Medicine, 1998). In experimental animals choline deficiency is exacerbated by deficiency of methionine, folic acid, or vitamin B12, which impairs the capacity for de novo synthesis.

There is some evidence that the availability of choline may be limiting for the synthesis of acetylcholine in the central nervous system under some conditions, and supplements of phosphatidylcholine increase the rate of acetylcholine turnover. One systematic review concludes that phosphatidylcholine supplements result in some improvement in cognitive function in patients with dementia, especially when this is secondary to cerebrovascular disorder (Fioravanti and Yanagi, 2000), but another concludes that there is no evidence to support its use in the treatment of dementia (Higgins and Flicker, 2000). Although phosphatidylcholine has been used to treat tardive dyskinesia associated with neuroleptic medication, there is little evidence to support its use (McGrath and Soares, 2000).

14.3 CREATINE

Creatine functions as a phosphagen in muscle. Neither the small amount of ATP in muscle nor the speed with which metabolic activity can be increased, and hence ADP be rephosphorylated, matches the demand for ATP for rapid or sustained muscle contraction. Muscle contains a relatively large amount

Figure 14.5. Synthesis of creatine. Glycine guanidotransferase (amidinotransferase),EC 2.1.4.1; guanidinoacetate methyltransferase, EC 2.1.1.2; and creatine kinase, EC 2.7.3.2.

of creatine phosphate (aboutfour-foldhigherthan ATP). This acts as a reservoir or buffer to maintain a supply of ATP for muscle contraction until metabolic activity increases.

Creatine is not a dietary essential; as shown in Figure 14.5, it is synthesized from the amino acids glycine, arginine, and methionine. However, a single serving of meat will provide about 1 g of preformed creatine, whereas the average daily rate of de novo synthesis is 1 to 2 g, and endogenous synthesis is inhibited by a dietary intake.

Both creatine and creatine phosphate undergo a nonenzymic reaction to yield creatinine, which is metabolically useless and is excreted in the urine. Because the formation of creatinine is a nonenzymic reaction, the rate at which it is formed, and the amount excreted each day, depends mainly on muscle mass, and is therefore relatively constant from day to day in any one individual. This is commonly exploited in clinical chemistry; urinary metabolites are commonly expressedper mole of creatinine, and the excretion of creatinine is measured to assess the completeness of a 24-hour urine collection. There is normally little or no excretion of creatine in urine; significant amounts are only excreted when there is breakdown of muscle tissue.

Creatine supplements are often used as a so-called ergogenic aid to enhance athletic performance. Supplements of 3 to 20 g of creatine per day increase muscle creatine and creatine phosphate by 10% to 15% in people whose muscle creatine is initially relatively low, and have some effect onmus-cle work output and athletic performance, with little evidence of adverse effects (Casey and Greenhaff, 2000; Poortmans and Francaux, 2000; Hespel et al., 2001).

14.4 INOSITOL

Inositol is a hexahydric sugar alcohol. Its main function is in phospholipids; phosphatidylinositol constitutes some 5% to 10% of the total membrane phospholipids. In addition to its structural role in membranes, phosphatidylinositol has a major function in the intracellular responses to peptide hormones and neurotransmitters, yielding two intracellular second messengers: inositol trisphosphate and diacylglycerol.

Plant foods contain relatively large amounts of inositol phosphates, including the hexaphosphate, phytic acid. Phytate chelates minerals, such as calcium, zinc, and magnesium, forming insoluble complexes that are not absorbed. However, both intestinal phosphatases and endogenous phospha-tases (phytase) in many foods dephosphorylate a significant proportion of dietary phytate. The inositol released can be absorbed and utilized for phos-phatidylinositol synthesis.

Inositol can also be synthesized endogenously; inositol 1-phosphate is formed by isomerization of glucose 6-phosphate, catalyzed by an NAD-dependent enzyme, although overall there is no change in redox state. Phos-phatidylinositol is formed by a reaction between CDP-inositol and diacylglyc-erol. Most inositol is catabolized by oxidation to glucuronic acid.

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