People preferably take zero or low-calorie beverages and foods with artificial sweeteners even though it has been recently suggested that long-term artificial sweetener use affects physiological functions. In addition, a lower body weight was considered to be more healthful, but an abnormally low body weight caused by an excessive diet has been reported to cause health problems. Acesulfame potassium (AceK) is one of the most commonly used for foods and beverages because of its resistance to thermal degradation and marked sweetness. However, the combined effect of AceK and a low body weight on the physiological functions remains unknown. Here, we investigated the effect of long-term AceK fluid intake on the cognitive function under dietary restriction. We administered AceK to mice fed a low carbohydrate (LC) diet for 4 weeks, and behavioral assays were then performed for a week. The mice fed the LC diet with AceK treatment for 4 weeks showed an increase in water intake and a decrease in short-term and object cognitive memories in the Y-maze and novel object recognition tests, respectively. Mice were sacrificed after behavioral tests to measure glucose levels. The glucose levels in the frontal cortex were significantly decreased in mice fed the LC diet with AceK treatment in comparison with mice fed the LC diet alone, although there was no significant difference in the plasma glucose levels. These results suggest that the combination of long-term AceK intake and the LC diet affects the cognitive function through the reduction of cortical glucose levels.