Among polyunsaturated omega-3 fatty acids, ALA (alpha-linolenic acid) provided the first coherent multidisciplinary experimental demonstration of the effect of diet (one of its major macronutrient) on the structure, the biochemistry, the physiology and thus the function of the brain. In fact, DHA (docosahexaenoic acid) is one for the major building structures of membrane phospholipids of brain and absolute necessary of neuronal function.

It was first demonstrated that the differentiation and functioning of cultured brain cells requires not only ALA, but also the very long polyunsaturated omega-3 (DHA) and omega-6 carbon chains. Then, it was found that ALA acid deficiency alters the course of brain development, perturbs the composition of brain cell membranes, neurones, oligodendrocytes and astrocytes, as well as sub cellular particles such as myelin, nerve endings (synaptosomes) and mitochondria. These alterations induce physicochemical modifications in membranes, lead to biochemical and physiological perturbations, and results in neurosensory and behavioural upset.

Consequently, the nature of polyunsaturated fatty acids (in particular omega-3, ALA and DHA) present in formula milks for infants (premature and term) conditions the visual, neurological and cerebral abilities, including intellectual. Dietary omega-3 fatty acids are involved in the prevention of some aspects of ischemic cardiovascular disease (including at the level of cerebral vascularization), and in some neuropsychiatric disorders, particularly depression, as well as in dementia, including Alzheimer's disease and vascular dementia. The implication of omega-3 fatty acids in major depression and bipolar disorder (manic-depressive illness) is under evaluation. Their dietary deficiency (and altered hepatic metabolism) can prevent the renewal of membranes and consequently accelerate cerebral ageing; nonetheless, the respective roles of the vascular component on one hand and the cerebral parenchyma itself on the other have not yet been clearly elucidated. Low fat diet may have adverse effects on mood.

The nature of the amino acid composition of dietary proteins contributes to cerebral function; taking into account that tryptophan plays a special role. In fact, some indispensable amino acids present in dietary proteins participate to elaborate neurotransmitters (and neuromodulators). The regulation of glycaemia (thanks to the ingestion of food with a low glycaemic index ensuring a low insulin level) improves the quality and duration of intellectual performance, if only because at rest the brain consumes more than 50% of dietary carbohydrates, approximately 80% of which are used only for energy purpose. In infants, adults and aged, as well as in diabetes, poorer glycaemic control is associated with lower performances, for instance on tests of memory. At all ages, and more specifically in aged people, some cognitive functions appear sensitive to short term variations in glucose availability. The presence of dietary fibers is associated with higher alertness ratings and ensures less perceived stress.

Although an increasing number of genetic factors that may affect the risk of neurodegenerative disorders are being identified, number of findings show that dietary factors play major roles in determining whether the brain age successfully of experiences neurodegenerative disorders.

Effects of micronutrients have been examined in the accompanying paper.