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whether adequate levels of particular precursors are provided to the CNS by the diet do not exist. Although little information on CNS requirements of specific amine acids is available, results from several lines of related research suggest that the peripheral concentration of particular amine acids can be a factor in the regulation of central neurotransmission, cognitive performance, and mood state. For example, if the amine acid tryptophan is either artificially elevated or lowered, changes in brain function and behavior can occur (Young, 1996). Even in normal humans, acute tryptophan depletion produces transient alterations in mood state (presumably by reducing the CNS concentration of serotonin), in particular increased subjective depression and increased aggression (Young, 1996). In contrast, administration of single doses of pure tryptophan to humans increases sleepiness and may reduce pain sensitivity (Hartmann, 1986; Lieberman et al., 1985). These changes are consistent with the various functions attributed to serotonin in the CNS.

CNS requirements for specific amine acids during periods of undernutrition or when individuals are exposed to highly stressful conditions may be particularly critical. For example, among moderately undernourished, but not highly stressed soldiers participating in a field test of an energy deficient ration, decrements in tryptophan were associated with impaired cognitive performance (Lieberman et al., 1997). Furthermore, a series of studies suggests that supplemental administration of tyrosine increases brain catecholaminergic neurotransmission and has beneficial effects on various behavioral parameters associated with resistance to stress (for a recent review, see Lieberman, 1994). Tyrosine is one of the dietary precursors for the synthesis of the catecholamines, dopamine and norepinephrine. The beneficial neurochemical and behavioral consequences of supplemental tyrosine administration are most readily observed when humans and other animals are exposed to various environmental and psychological stressors (Wurtman et al., 1981).

The Blood-Brain Barrier: A Key Determinant of Brain Nutritional Status

Unlike most other organs, the brain is isolated from the general circulation by the blood-brain barrier (BBB). The nature of the barrier is determined by the special properties of the cerebral vasculature, specifically the epithelial cells of the brain capillaries, which selectively prevent the transport of various substances into the brain (Betz et al., 1994 Pardridge 1977). In general, lipophilic compounds typically can passively cross the BBB, but water soluble compounds, such as amine acids, cannot. The BBB, therefore, must contain special mechanisms for selectively transporting key water soluble compounds, such as essential amine acids, into the brain (Pardridge, 1977; Betz et al., 1994). The special status of the brain with regard to its accessibility to nutritional and other systemic metabolic factors has profound implications for the determination of the nutritional requirements of the CNS. For example, in the

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