Substances and compounds required by living tissues for metabolism and energy production, growth, maintenance and reproduction, remodelling, and communication.

'Food for thought' is not merely a cliché. The discussion of the association of satisfactory nutrition with efficient cognition has transcended long ago the domains of folk psychology. Ribot, one of the forefathers of modern neurology (see "amnesia), postulated that '... the basis of memory is... nutrition; that is to say, the vital process per excellence' (Ribot 1882). It is now established that even without too much mental effort, our brain consumes an impressive amount of energy. The adult human brain, while weighing only 2% of the total body weight, consumes 15-20% of the total resting oxygen consumption (Guyton 1991; Ganong 1993); in a 6-year-old child the value is about 50% (Kennedy and Sokoloff 1957). In the resting brain, energy is generated almost exclusively by the oxidation of glucose (Siesjo 1978). A substantial proportion of this energy is used up by molecular pumps that maintain proper concentration gradient of ions, such as "calcium, across the neuronal membrane, thus ensuring neuronal excitability and responsiveness (Guyton 1991; "ion channel). Glucose is derived continuously from the capillary blood, as only a tiny supply of glucose is normally stored as glycogen in the neurons at any given time (ibid.). In periods of activity, local metabolic rate and blood flow increase dramatically in the activated area (Roy and Sherrington 1890; Fox et al. 1988; Sokoloff 1989; Malonek and Grinvald 1996). These fast activity dependent processes provide the biological basis for a variety of "functional neuroimaging techniques (Raichle 1994). They are also reflected in the intense molecular turnover that is detected in neurons immediately after a physiological or behavioural experience ("consolidation, "homeostasis, "immediate early genes, "protein synthesis). Which metabolic pathways supply the instantaneous surge of energy in activated neural circuits, is still a subject of controversy. Whereas some investigators favour the view that hard-pressed neurons resort to anaerobic metabolism, similarly to exercising muscles (Fox et al. 1988), others report that active neurons, similar to resting ones, stick to oxidative metabolism (Malonek and Grinvald 1996).

The fact that a working mind consumes much energy, has led to the idea that the lack of proper foodstuff might impair intelligence. Many types of nutrients in addition to energy sources are also very important for brain and cognition. Vitamins or minerals are examples. Deficiency in these may cause severe neurological and cognitive disorders. For example, thiamine deficiency in chronic alcoholism results in "dementia (Korsakoff's syndrome, Butters 1985). Many other vitamins as well as minerals (e.g. iodine, iron) are also essential for a healthy brain (Ganong 1993; Scrimshaw 1998). Improper intake of vitamins, minerals, and essential amino acids is a pressing problem in undernourished societies, where it may adversely affect child "development and cognitive achievements (Scrimshaw 1998). Lipids, which are required among others in intercellular and "intracellular signal trans-duction and in regulation of membrane fluidity and excitability, are also important for proper cognitive function. Some lipid preparations were claimed to act as cognitive boosters ("nootropics) and memory enhancers (Yehuda and Carasso 1993, Yehuda et al. 1996).

The composition of the diet should also not be ignored by investigators who experiment on laboratory animals. A change of a diet may affect "performance on learning tasks, and under certain conditions, an imbal-anced or poor diet may ultimately result in amnesia (e.g. Guo et al. 1996).

Even in healthy individuals on a balanced diet, what is placed into the mouth, and especially when it is placed there, may make a lot of difference as far as cognition is considered. This, at least, is the conclusion that emerges from a number of reports in recent years. For example, drinking a glucose-rich beverage was found to improve learning and recall in healthy adults (Manning et al. 1998). Similarly, in healthy students, failure to eat breakfast impaired learning and memory throughout the day; the problem was easily amended by drinking a glucose-supplemented drink (Benton and Parker 1998). The improvement in memory performance correlated with blood glucose. It has yet to be determined whether the beneficial effect of sugar on cognition is a consequence of a boost to the energy of the brain, or of augmentation of neurotransmitter synthesis (e.g. "acetylcholine), or other metabolic processes. Nevertheless, all in all, the data imply that children should not be sent to school without breakfast; that politicians should not decide on state matters without ensuring that their blood glucose has reached a sufficiently high level; and that elderly people should pay proper attention to what they eat and when they do that. It also argues against diets that reduce calorie intake to absurdity—unless definitely deemed a must by an expert physician.

Selected associations: Dementia, Development, Homeostasis, Nootropics

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