Although, an assessment of the literature does indicate that the serotonin theory is flawed which was highlighted in a review by Benton and Donohoe (1999). More than thirty studies of blood amino acid profiles taken after subjects consumed meals varying in their carbohydrate content were examined. It was found that only when the protein content of the meal was less than two per cent did the resulting amino acid profile favour a rise in serotonin levels in the brain. It was found that even foods considered being high in carbohydrate such as bread, fifteen per cent of calories were from protein. This means that there are very few instances when the level of carbohydrate in a meal is high enough to have a direct impact on serotonin levels.
Moreover, research findings indicate that there is a positive correlation between blood sugar levels and mood. Benton and Owens (1993) found a relationship between increased blood sugar and feelings of energy with reduced tension. Also, a recent study by Martino and Morris (2003) showed that raising blood sugar levels improved mood. Although, not only was energy increased and anxiety reduced, but also there was furthermore an improvement in one’s internal state. In addition, Benton (1996) found that eating breakfast was associated with improved mood, particularly when it provided high glucose levels in the morning. Therefore, the overall implication is that raising blood glucose enhances mood. In contrast, other studies have found no specific effects of carbohydrate on mood. Consider the following example, when carbohydrate is administered by it self without any other nutrients it fails to illicit any substantial effect on mood. In a study conducted by Reid and Hammersley (1995) females experienced a slight increase in energy after intake of a sucrose drink, while males did not experience any significant increase in energy. Both females and males did not have any significant mood elevation.
Furthermore, higher blood glucose levels have also shown to improve performance on cognitive tasks, possibly a consequence of mood and arousal enhancement. Lapp (1981) found that the memory of sixteen year olds for lists of both high and low imagery nouns were greater when blood glucose levels were high. Also, Benton et al. (1987) showed that administration of a glucose drink to children one and a half hours after lunch improved reaction time, relative to the administration of a saccharine sweetened placebo drink. In general the pattern of results from these studies suggests that glucose administration selectively improves short-term memory or improves performance on vigilance tasks that require a large component of working memory for their successful completion. Morris and Sarll (2001) showed that a glucose drink improved listening span in students who had missed breakfast, but not in those who did not usually eat breakfast. Similarly Benton and Parker (1998) found that a glucose drink improved memory by reversing the effects of missing breakfast. What these findings suggest is that increasing already moderate blood sugar levels is unlikely to influence cognitive performance; rather glucose can be taken to ameliorate negative effects of low blood sugar on mood. Although, the present findings conflict with those of Azari (1991), who found no significant effect of glucose administration upon memory performance by young adults.
In conclusion, it is generally agreed that carbohydrate consumption does result in an elevation in mood. The pathway through which carbohydrate acts on the central nervous system to produce the effect is not clear. The dominant hypothesis suggested by Wurtman and Wurtman (1986) has been the serotonin hypothesis, which states that carbohydrate increases plasma tryptophan ratio and thereby increases brain serotonin level. Serotonin the neurotransmitter, which affects mood, has been found to be the main contributing factor to improved mood. Furthermore, there is increasing evidence that, providing the brain with a good supply of glucose helps increase mental performance. The literature examined suggests that this improvement occurs because blood sugar levels are elevated and thus glucose is available as brain fuel. On the whole, this explanation helps to explain how and why carbohydrate consumption affects mood and cognition.
References
Azari, N.P. (1991) Effects of glucose on memory processing in young adults. Psychopharmacology, 105, 521-524.
Benton, D. (1996) Food for Thought. Harmondsworth: Penguin.
Benton, D. and Brett, V. Brain PF (1987) Glucose improves attention and reaction to frustration in children. Biological Psychology, 24, 95-100.
Benton, D. and Donohoe, R.T. (1999) The effects of nutrients on mood. Public health nutrition. 2, 403-409.
Benton, D. and Owens, D. (1993) Is raised blood glucose associated with the relief of tension? Journal of Psychosomatic Research, 37, 723-735.
Benton, D. and Parker, P.Y. (1998) Breakfast, blood glucose and cognition. American Journal of Clinical Nutrition, 67, 772S-778S.
Fernstrom, J.D. and Wurtman, R.J. (1971) Brain serotonin content: increase following ingestion of carbohydrate diet. Science, 13, 1023-1025.
Lapp, J.E. (1981) Effects of glycaemic alterations and noun imagery on the learning of paired associates’. Journal of Learning Disorders, 14, 35-38.
Martino, O.I. and Morris, N. (2003) Drinking glucose improves mood at the beginning of the working day. In P. McCabe (Ed.), Contemporary Ergonomics 2003, 226-231. London: Taylor & Francis.
Morris, N. and Sarll, P. (2001) Drinking glucose improves listening span in students who miss breakfast. Educational Research, 43, 201-207.
Reid, M. and Hammersley, R. (1995) Effects of carbohydrate intake on subsequent food intake and mood state. Physiology and Behaviour 58(3): 421-427.
Wurtman, R.J. and Wurtman, J.J. (1986) Nutrition and the brain: Food constituents affecting normal and abnormal behaviours. New York: Ravens, 7.
