Carbohydrates and mental function: feeding or impeding the brain?

Summary  Despite over 25 years of research into carbohydrates and mental function, findings have, if anything, become more divergent in recent years, and no clear mechanistic model that satisfactorily explains such variability has been produced. At best, there is some consensus that individuals with poor regulation of glucose, and who find the tests of cognitive performance particularly challenging, may benefit from some improvement in mental function after a specific dose of carbohydrate. Even then, this is mainly limited to hippocampally mediated verbal memory. Recent developments in understanding the regulation of energy supply (principally glucose) for neuronal function suggest that it is too simplistic to assume that ingesting carbohydrates will inevitably improve cognitive function. There is a damping or buffering seen between glucose changes in blood and brain extracellular fluid, and even evidence for an acute dissociation of these glucose changes during task performance in specific brain regions. This may be mainly owing to the role of astrocytes, which are large, star‐shaped cells within the central nervous system that can store glucose as glycogen, and when needed, release energy to neurones ‘on demand’. Variables such as age, effort, personality, glucoregulation, time of day, task demand, task domain, and nature and amount of the carbohydrate, can all affect the outcome. One factor that could be involved in all of these variables is the limbic–hypothalamic–pituitary–adrenal axis, which controls release of glucocorticoid hormones such as cortisol, and is believed to be a major route by which the brain regulates its energy supply. Moreover, rapidly absorbed carbohydrate provokes a greater cortisol release during stressful tasks and, like glucose, cortisol is known to have dose‐dependent, and bidirectional, effects on cognitive function, especially memory. This process might help explain recent findings that carbohydrate‐rich foods with lower glycaemic indices provide more benefit to mental function than high‐glycaemic foods. The challenge now is to identify characteristics of both susceptible individuals and the carbohydrate manipulations that can best predict improvements in selective cognitive functions. This should be aided by a better understanding of the regulation of energy supply to the brain, together with knowledge of the relevant nutritional, neuronal, physiological and psychological parameters that would consistently promote improvements in cognitive function by carbohydrates.