In a Harvard University test, type-2 diabetics' blood sugar levels were found to be more strongly affected by perceived sugar levels of a sweet drink than its actual sugar levels.
From Real Clear Science:
In actuality, the two beverages were the same: a drink with roughly fifteen grams of sugar. Subjects were just told they were different and shown the first two nutrition facts [above] to complete the ruse. After participants drank the beverages (a different one on each day), the researchers monitored their blood glucose levels at twenty-minute intervals over the ensuing hour. They found that participants who thought they were drinking the high-sugar variant had significantly elevated blood sugar levels during the monitoring period compared to those who thought they were drinking the low-sugar variant.