Mammalian brain requires many nutrients from blood for proper functioning (Rapoport, 1976; Bradbury, 1979). For example, deprivation of glucose will cause unconsciousness within minutes. Nutrients for brain may be divided into macronutrients, which are required in relatively large amounts for proper brain functioning (Table 1)(Pardridge, 1986), and micronutrients, which are required in only small amounts for proper brain functioning, eg, vitamins and certain nucleosides, including uridine (Spector, 1986~). Macronutrients, eg, glucose and, to a lesser extent, monocarboxylic acids and amino acids, are catabolized in brain (Bradbury, 1979; Pardridge, 1986) to a much greater extent than micronutrients (see below). In this discussion, we will focus on micronutrients. By definition, for the purposes of this review, micronutrients for the brain are defined as organic substances (a) present in plasma at concentrations of< 50 pA4,(b) essential for the brain’s proper functioning, and (c) not readily synthesized in brain. Implicit in this definition is the fact that these micronutrients come exclusively, or in large part, into brain from outside, ie, from blood. Most micronutrients are not degraded in brain but enter the compartment from blood plasma and reside in brain for varying periods, although they may be anabolized, eg, into enzyme cofactors. Frequently, they are released from brain back into the blood in the form in which they circulate in the plasma. In other words, these micronutrients turn over in brain without being destroyed. Some micronutrients are true vitamins, which are not synthesized in adult mammalian brain, and some function like vitamins.(Certain hormones, although necessary for the brain’s proper functioning, are not generally considered micronutrients.) A list of the micronutrients discussed in this review is shown in Table 2. There are, of course, several other crucial micronutrients not covered in this review, eg, vitamins BIZ, A, and D, because of insufficient information.

A certain ambiguity exists about the word vitamin in brain. In general mammalian nutrition, the term vitamin is applied to several organic substances that mammals cannot synthesize and that appear in small amounts in foods. These vitamins are generally cofactors for enzymatic reactions. In brain, however, the term vitamin is often used in a slightly different sense (Spector, 198 1). For example, in certain mammals, with the notable exceptions of primates and the guinea pig, vitamin C (ascorbic acid) can be synthesized from glucose in the liver but not in the brain (Spector, 198 1). Hence, vitamin C would not be considered a vitamin for these species. Yet, in these species, vitamin C for the brain must be obtained from the blood, from vitamin C that either entered blood from the diet or was synthesized in the liver. In this sense, vitamin C might be considered a vitamin-for-the-brain, because it must be obtained from outside the brain (Spector, 1981). In the rest of this review, the term vitamin will be used in the classical sense, ie, an essential organic substance that primates cannot synthesize. A vitamin-for-brain, ie, a vitamin-like substance (Table 2), is defined as an essential organic substance that the brain is unable to synthesize in adequate amounts. This distinction is shown in Table 2 in the listing of true vitamins and vitamin-like micronutrients. For many years, it has been recognized that micronutrient homeostasis in brain is almost always better than in most other tissues (see, for example, Lowry, 1952; Burch et al., 1956). This is not due to a lack of turnover of micronutrients in brain. Until recently, the