Inverse Dose-Response Effect of Breakfast on Psychomotor Performance

Implicit in the mantra ‘breakfast is the most important meal of the day’ is the notion of a requirement to match energy intake with energy expenditure over the short term, so as to avoid actual and anticipated energy deficits. However, the energy content of a single meal is trivial compared with the amount of energy stored in the liver and adipose tissue (and readily available to maintain supply of glucose to the brain), and food ingestion represents a physiological challenge that might be expected to acutely impair performance. In that respect, small meals may be better ‘tolerated’ than a large ones (cf Woods, 1991, Psychol Rev 98:488-505). Perhaps this is why breakfast is also typically the smallest meal of the day despite coming after the longest ‘fast.’ Healthy adult participants (n=71, 35 female) consumed either no breakfast, or 300 kcal or 600 kcal of a blended yogurt, cream and fruit meal (0.75 kcal/g). As a cover story, the study was described as an investigation into circadian effects on performance, with the no meal/meal being a control for nutritional state. Performance was measured before (to control for individual difference in performance) and starting at 15 and 95 minutes after the no meal/meal. Choice reaction time was unaffected by the amount eaten, but tapping performance (motor speed) and delayed recall for a word list varied inversely with the amount eaten. Avoidance of adverse effects of ingestion may be an important determinant of meal size – a large meal is better eaten in anticipation of rest (e.g., in the evening) than in anticipation of mentally and/or physically demanding activity.