There has been, over the years, an increasing body of evidence suggesting that vitamin and mineral status is significantly related to both brain development in childhood and the degree of cognitive decline experienced as we age. Indeed, it is certainly the case that deficiencies of some vitamins are associated with negative neurological symptoms such as neural-tube defects. The work examining vitamin and mineral supplementation comprises both cohort studies and nutritional interventions and has generated much confusing and contradictory data. To a large degree, this confusion and contradiction result from variation in a number of factors such as the methodological rigour of each study, the measures of cognitive function used, and the precise nutrient being studied.
Early work in this area concentrated on the notion that supplementing the diet of schoolchildren with multivitamin supplements would improve both their IQ scores and their academic achievement. This work was controversial and marked by a number of deficiencies, such as any clear indication of whether the participants were nutritionally compromised prior to treatment, difficulties in determining which, if any, of the vitamins in the cocktail were producing effects, and the lack of any clear hypotheses regarding the mechanisms responsible. The consensus now is that supplementation will benefit cognitive development and IQ (especially non-verbal) in a minority of children who are not otherwise adequately nourished. In Britain and the USA at least, there is particular concern that a significant proportion of adolescents, especially girls, are deficient in iron. There is good evidence that iron deficiency contributes to poor cognitive ability, perhaps in association with low vitamin C status, which has also been linked to reduced cognitive function. Iron is known to be essential for the synthesis and function of neurotransmitters, such as dopamine, noradrenaline, and serotonin (5-HT). Selenium is another mineral that may be important for brain function, and low levels of selenium have been associated with cognitive decline and depressed mood in the European population.
Much recent work, however, has concentrated on the use of vitamins in the treatment of age-related cognitive decline and dementia and, to varying degrees, is more scientifically rigorous than the earlier work. The overwhelming majority of the experimental work has targeted the action of two groups of micronutrients: antioxidants and B-com-plex vitamins. Research into the effects of antioxi-dant vitamins, whilst showing some promise in that correlational studies show that levels of these vitamins (vitamin E most consistently) are associated with function in a range of cognitive domains, is more contradictory when one considers the clinical intervention trials. The work on B-complex vitamins is, however, more consistent and supported by a strong hypothetical basis. This relies on the roles of vitamins B12 and folate in methylation of membrane phospholipids (see below) and neurotransmitters and in breaking down the toxic sulfur amino-acid homocysteine. High levels of homocysteine are now considered by some to be a far greater risk factor for the development of coronary and vascular problems than are high levels of cholesterol. Elevated levels of homocysteine may be a cause of minor ischemic events, which, cumulatively, lead to a degradation of cognitive function due to either subclinical deficiencies of or problems with the absorption of B-complex vitamins. Indeed, a large number of studies have consistently demonstrated relationships between homocysteine levels, B-complex vitamin levels, and neuropsychological task performance. The number of direct intervention trials that have supplemented the diets of the elderly with B-complex vitamins is, however, small. Whilst some studies have shown no net benefit of supplementation on the cognitive function of the elderly, a larger number of studies have shown a stabilization of cognitive function and a reduction in homo-cysteine levels resulting from B-complex vitamin supplementation. As with the studies of antioxidant vitamins, however, these studies must be interpreted with a degree of caution since they use differing dosages, periods of supplementation, and measures of neuropsychological function.
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