The effect of meal size on behavior has been little studied, perhaps because there are a number of methodological difficulties and an absence of theory. For example, what counts as a large or small meal? Should the difference be measured in terms of absorbed energy, or weight or volume eaten, or even consumption time? If absorbed energy is used as the measure, then behavioral outcomes would need to be measured with a sufficient delay for differences in energy absorption to be discriminable. Moreover, the influence of expectations and habit might confound experimental nutritional differences.
Two studies in adults found that eating large lunches (at least 4MJ (1000 kcal)) impaired vigilance relative to eating small or medium-sized lunches. There was also evidence that this effect depended on the meal size being different from that habitually consumed. In adolescents, a larger breakfast (2.6 MJ (634 kcal), on average) resulted in poorer vigilance but better short-term memory 3 h later, compared with a smaller breakfast (1.6 MJ (389kcal), on average). Thus, there is some evidence that vigilance is adversely affected by a large meal.
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