measurement is not easily undertaken in humans, although magnetic resonance spectroscopy does makes it possible (Cohen et al., 1995).
Plasma choline concentration varies in response to diet and is found in the water-soluble fraction as free choline (Buchman et al., 1993; Burt et al., 1980; Chawla et al., 1989; Sheard et al., 1986; Zeisel et al., 1991). It decreases approximately 30 percent in subjects fed a choline-deficient diet for 3 weeks (Zeisel et al., 1991). Plasma choline concentration can increase twofold after a meal high in choline content and three- or fourfold after a supplemental choline dose (Zeisel et al., 1980b). Fasting plasma choline concentrations vary from 7 to 20 µmol/L, with most subjects having concentrations of 10 µmol/L. The disadvantage of using plasma choline as a functional indicator is that these concentrations do not appear to decline below approximately 50 percent of normal, even when subjects fast for more than 1 week (Savendahl et al., 1997). Perhaps this is because membrane phospholipids, which are a large storage pool for choline, are hydrolyzed to maintain plasma choline concentration above this minimal level. Fasting plasma phosphatidylcholine concentrations (mostly as part of plasma lipoproteins) are approximately 1 to 1.5 mmol/L (Aquilonius et al., 1975; Zeisel et al., 1980b, 1991). Plasma phosphatidylcholine concentration also decreases in choline deficiency (Zeisel et al., 1991) but is also influenced by factors that change plasma lipoprotein levels.
Studies in rodents suggest that dietary intake of choline early in life can diminish the severity of memory deficits in aged animals (Bartus et al., 1980; Meek and Williams, 1997a, b, c). Most available human studies have used choline-containing compounds to treat rather than prevent the symptoms of dementia and therefore did not address whether dementias could be prevented. In the absence of food composition data, epidemiological studies on the association of choline intake with dementia are not available. More human studies are needed to determine whether dietary choline intake is useful in the prevention of dementia.