Glucose and mental performance

Abstract

An abundant literature suggests that intellectual perform-ance can be improved following the intake of a glucose load, or of some carbohydrate-rich food. Improved mental performance generally correlates with the subjects' blood glucose level at the time of tests. Inconclusive studies also exist and the overall picture of glucose effects on cognition remains enigmatic. Cognition represents a complex and multidimensional set of abilities. Validated tests assess specific dimensions of 'intelligence'. Memory, reasoning, attention and psycho – motor coordination are only a few of the functions that contribute to mental ability. Memory itself is a complex set of processes (visual, spatial, verbal, short-term, long-term, etc.) that can be investigated using very different tools. In addition to the fundamental complexity of mental functions, intellectual performance under specific test conditions depends on multiple factors, amongst which are individual ability, motivation, general arousal, previous learning, fatigue, time of day, etc. Nutritional factors play a demonstrated role in mental test performance under particular conditions: for example, vitamin-deficient sub-jects, such as undernourished children, display poor performance in mental tests such as non-verbal intelligence quotient and supplementation is likely to improve their scores. The impact of nutritional factors on mental performance in well-nourished individuals is more difficult to demonstrate. Given the large number of factors that potentially affect mental responses, however, it is not surprising that the beneficial effect attributed to glucose ingestion is neither consistently nor universally reported. Typically, studies of glucose effects investigate several aspects of post-ingestive mental performance and use a battery of tests. Most report significant effects on some measures, and non-significant effects on other measures, in the same subjects. Obviously, cognitive processes are not equally sensitive to enhancement following a given load of glucose. Positive effects on short-term memory have been reported often (for example, Manning et al. 1997; Korol & Gold, 1998) and other benefits have been demonstrated on rapid information processing, performance on the Stroop test, word recall, focused and sustained attention, maze learning, arithmetic ability (Benton et al. 1987, 1994), long-term free and cued recall tasks (Foster et al. 1998), etc. Among negative results, glucose produced no effect on digit span measures (Gonder-Frederick et al. 1987), digit recall or long-term non-verbal memory (Foster et al. 1998). The rise in blood glucose that follows a glucose drink correlates with improved decision time in a reaction time task, faster information processing, better word recall, and improvement on a cognitive conflict task (Benton & Owens, 1993). This effect is not due to the correction of hypoglycaemia, since it is reproduced across a range of baseline blood glucose levels. Hypoglycaemia, however, is associated with slower reaction times. A study carried out in eighty adults confirmed that a glucose drink improved performance on a memory test and that performance was positively associated with the blood glucose level (Martin & Benton, 1999). Glucose also improves memory in patients with Down's syndrome (Manning et al. 1998) and Alzheimer's disease (Craft et al. 1992, 1996; Messier & Gagnon, 1996). In type 2 diabetes mellitus, cognitive impairments are sometimes reported, including selective attention deficits, verbal and visuo – spatial memory deficits. Cognitive deficits in patients with poor glucose tolerance are not necessarily permanent correlates of the condition, but can be reversed by short-term (1 –2 months) treatment with oral hypoglycaemic agents (Gradman et al. 1993). These observations suggest that at least some cognitive impair-ment is associated with ongoing glucose regulatory status rather than permanent anatomical functional alterations produced by diabetes. Impaired glucose tolerance has been identified as a predictor of cognitive impairment with age (Vanhanen et al. 1998). In healthy subjects, variations in normal glucose regulation capacity could also affect mental functions. This has been investigated in a sample of elderly individuals on the basis of a glucose recovery index. Several cognitive tasks were affected in subjects with poor glucose control relative to subjects with good glucose control (Messier et al. 1997). In young healthy volunteers, glucose regulation capacity again appeared a good predictor of memory performance (Messier et al. 1999). Individuals with poor glycaemic control had impaired immediate and delayed memory for words as compared with controls, but this difference disappeared after the subjects were given a 50 g glucose load. The mechanisms mediating the memory-enhancing properties of glucose are not elucidated at the present time. They could include both peripheral and central processes. In particular, glucose could act as a cholinergic agonist and as an opioid antagonist (Rodriguez et al. 1999) or else via its effects on insulin (Craft et al. 1994; Hoyer, 1997). The study by Green et al. (2001) in the present issue of the British Journal of Nutrition presents changes in mental performance that are not correlated with the level of glycaemia, but that appear affected to some extent by expectancy effects after the subjects had ingested glucose. Although the classic correlation between blood glucose and cognitive performance was not reproduced in this study, improved responses occurred after the intake of glucose, but