Pushing her meal cart into the hospital room, a research assistant hands out tall glasses of reddish-pink liquid, along with a gentle warning: "Remember, you guys have to finish all your Kool-Aid."

One by one, young volunteers chug down their drinks, each carefully calibrated to contain a mix of water, flavoring and a precisely calibrated solution of high fructose corn syrup: 55% fructose, 45% glucose.

The participants are part of an ongoing study run by Kimber Stanhope, a nutritional biologist at the University of California, Davis. Volunteers agree to spend several weeks as lab rats: their food carefully measured, their bodies subjected to a steady dose of scans and blood tests. At first, each volunteer receives meals with no added sugars. But then, the sweetened drinks start showing up.