Meal Timing

Does the timing of a meal in the day make a difference to any effects on behavior? In other words, do any behavioral effects differ between breakfast, midday, and evening meals or between mid-morning and afternoon snacks?

Breakfast The potential effects of breakfast on performance and well-being continue to attract much interest, not least from industry, especially concerning the performance of schoolchildren. Pollitt and colleagues have argued that children are likely to be more susceptible than adults to the effects of fasting, owing to their greater brain metabolic demands relative to their glycogenic and gluconeogenic capacity. The numerous studies in this area have produced inconsistent results, which is partly attributable to variation in the populations studied, their nutritional status, and the designs used. There is a consensus that breakfast is more likely than not to benefit schoolchildren's performance, particularly if the children are already nutritionally vulnerable and have mental abilities with room for improvement.

In all of us, there is a tendency for levels of arousal and alertness to rise during the morning, reaching a peak near midday. Some evidence suggests that breakfast may help to control this arousal, so that attention can be successfully focused on the task in hand. Conversely, omitting breakfast may increase autonomic reactivity, leading to less-focused attention. This effect could explain the finding that children without breakfast showed better recall of objects to which they had not been asked to attend; such attention to irrelevant stimuli is also known to occur with increased anxiety. Furthermore, increasing hunger is likely to be distracting.

Less attention has been paid to the effects of breakfast in adults. However, there are several studies of the effects of giving breakfast to students that show a benefit in spatial and verbal-recall tasks 1-2 h later, compared with missing breakfast. Interestingly, attention-based and reaction-time tasks were not improved by breakfast, and a logical-reasoning task was even slightly impaired. Perhaps performance in those tests benefits more from mild arousal, which could be acutely reduced by some breakfasts. These studies did not determine whether performance later in the morning is affected by breakfast. Differential effects of breakfast content and size will be discussed below.

Midday meal Several studies have demonstrated a drop in performance after the midday meal, particularly in vigilance tasks requiring sustained attention. However, this 'post-lunch dip' may not simply be an effect of eating, because vigilance has also been found to decline from late morning to early afternoon in subjects not eating lunch. That is, there is an underlying circadian rhythm in performance that is confounded by the effect of a midday meal. In fact, using noise stress to arouse subjects during a midday meal prevented any decline in performance due to the meal. It has also been shown that the more anxious one is feeling prior to lunch, the less one will experience a post-lunch dip in performance. In support of this, another study found that subjects scoring highly on a personality measure of extraversion and low on neuroticism were more likely to be affected by a post-lunch dip. These are examples of the importance of individual differences and context for meal effects.

Evening meal There are few studies of the effects of eating later in the day, although there has been some interest in the effects of meals during night shifts. Accuracy of performance declines with eating during a night shift, but, unlike the effects of lunch, pre-meal anxiety levels had no effect. One study in students on the effects of eating a large freely chosen evening meal found little evidence for consistent changes in performance relative to missing the meal. Despite this, the students who omitted the meal reported feeling more feeble and incompetent and less outgoing than those who had eaten.

Snacks One study specifically addressed whether an afternoon snack (approximately 1-1.2 MJ (240-290 kcal) of yoghurt or confectionery) eaten 3h after lunch (or no lunch) affected task performance. A beneficial effect of the snack was found on memory, arithmetic reasoning, and reaction time 15-60 min later. The comparison was with performance after a 'placebo' zero-energy drink (participants were unaware of the energy content). This rather different placebo does not preclude effects due to differences in sensory experience and expectations. Moreover, whether or not lunch had been eaten beforehand had little effect on the outcome, suggesting that any nutritional effects must be isolated to the acute impact of the snack. It is known that snacks of this size eaten after a meal have only a small effect on blood glucose, although insulin rises sufficiently to inhibit lipolysis and suppress the release of plasma free fatty acids later in the postprandial period.

The authors reported that these performance benefits from an afternoon snack were not found with a snack eaten in the late morning. The most likely reason is that the beneficial effect depends on the decline in alertness that normally occurs during the afternoon.

Other studies have found differential effects of the macronutrient content of snacks; these are discussed below.

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