Coffee with a donut is a classic pairing, and recent research from Cornell University offers interesting insight into why they seem to go so well together. Findings on how caffeine might temporarily reduce your perception of sweetness were published in the Journal of Food Science. 

Volunteers drank decaffeinated coffee with sugar added. Some cups were dosed with 200 mg of caffeine to simulate a strong cup of coffee. Asked to describe their level of alertness after drinking coffee, subjects could not tell if their cup contained caffeine or not. But the subjects who got the caffeinated brew rated it as tasting less sweet than subjects who had the regular decaffeinated coffee.