Dr. Anthony Sclafani's research group just published a study definitively demonstrating that high palatability, or pleasantness of taste, is required for sugar to be fattening in mice (1). Dr. John Glendinning was lead author. Dr. Sclafani's group has done a lot of excellent research over the years. Among other things, he's the person who invented the most fattening rodent diet in the world-- the 'cafeteria diet'-- composed of human junk food.
Mice and rats love sweet food and drinks, just like humans. If you give them a choice between plain water and sugar water, they'll overconsume the sugar water and become obese. I have argued, based on a large body of evidence, that the reward value and palatability* of these solutions are important to this process (2, 3, 4). This is really just common sense honestly, because by definition if the solution weren't rewarding the mice wouldn't go out of their way to drink it instead of water, the same way people wouldn't go out of their way to get soda if it weren't rewarding. But it's always best to confirm common sense with research.
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