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When faced with a cognitively demanding task, older adults tend to disengage and withdraw effort. At the same time, their usual processing preference for positive information disappears. Providing glucose as an energy resource is known to improve cognitive performance and reinstate older adults’ positivity preference. Here, we examined whether glucose can help older adults to exert more effort under high difficulty conditions, and if so, whether such increase is accompanied by a change in positive affect. Fifty-three young and 58 older adults consumed a glucose or a placebo drink and completed a memory-search task at three levels of difficulty. Cognitive engagement was measured through changes in heart rate (HR) and self-reported effort. After each memory-search block, participants completed an implicit emotion-assessment task. In both age groups, glucose produced increased HR (indicating higher task engagement) relative to placebo. In older but not in young adults, glucose also improved cognitive performance and increased positive affect. Subjective effort, in contrast, did not differ between the older-glucose and older-placebo groups. These results suggest that in older adults, glucose improves cognitive performance by promoting higher cognitive engagement while mitigating the subjective costs of effortful exertion.