When the researchers conducted spatial learning and memory tests using the Barnes maze, the aspartame mice at four months consistently moved more slowly and covered less distance during training than animals in the control group. They also took nearly twice as long on average to locate the target escape hole, showing impaired memory recall (however, this was inconsistent and not seen as statistically meaningful). By eight months, performance gaps widened even further, with two out of six aspartame-treated mice failing to complete the task at all.

It makes you dumb, unfit and fat (around the organs).