Tom, this is how Science Daily explained it:
High-fructose corn syrup and sucrose are both compounds that contain the simple sugars fructose and glucose, but there at least two clear differences between them. First, sucrose is composed of equal amounts of the two simple sugars -- it is 50 percent fructose and 50 percent glucose -- but the typical high-fructose corn syrup used in this study features a slightly imbalanced ratio, containing 55 percent fructose and 42 percent glucose. Larger sugar molecules called higher saccharides make up the remaining 3 percent of the sweetener. Second, as a result of the manufacturing process for high-fructose corn syrup, the fructose molecules in the sweetener are free and unbound, ready for absorption and utilization. In contrast, every fructose molecule in sucrose that comes from cane sugar or beet sugar is bound to a corresponding glucose molecule and must go through an extra metabolic step before it can be utilized.
I'm guessing the "free and unbound" bit makes more of a difference than that extra 5%.
In happier food science news, dark roast coffee really is better for your stomach:
Some coffees are labeled "stomach friendly," because they're steam treated to drive out caffeine and other chemicals thought to cause gastrointestinal distress. But food chemists [Veronika Somoza et al] wanted to know exactly which chemicals were behind the heartburn. So they took extracts of two coffee blends—one light, one dark—and their steam-treated counterparts. Each extract proved to be a unique chemical mix, with different amounts of caffeine and other compounds.
When the researchers served these coffee extracts to cultured human stomach cells, the cells jacked up acid production. Except in the case of one extract, high in a compound called N-methylpyridinium—a chemical produced in the roasting process that's not found in raw green coffee beans. And the darker the roast, the more there is. Now the researchers are test roasting a stomach-soothing N-methylpyridinium blend. Human trials will determine if it has all the boldness with none of the reflux.