There's only one way to find out: Edward Cussler and Brian Gettelfinger of the University of Minnesota, Minneapolis filled a swimming pool with syrup and tested how fast 16 volunteers swam.
The most troublesome part of the experiment was getting permission to do it in the first place. Cussler and Gettelfinger had to obtain 22 separate kinds of approval, including persuading the local authorities that it was okay to put their syrup down the drain afterwards.
But it was worth the hassle, Cussler says, not least because his quest for an answer made him something of a celebrity on campus. "The whole university was arguing about it," he recalls. "It was absolutely hilarious."
The result? No - humans don't swim slower in syrup.