Potential carcinogen? LOL.
Pure HCL will dehydrate six carbon sugars and create 5-HMF in a beaker, but I don't believe that this happens in the gut. Pure HCL is excreted from the cells, but it is immediately diluted. There are other solids/liquids that are normally consumed along with the simple sugars, in addition to there being water in your stomach. This dilution must happen quickly because the small intestine is alkaline, so by the time it gets there, it is no longer acid.
But even if hydroxymethylfurfural is formed in the gut, it is in amounts so minute that there is no way for it to be carcinogenic. Of course, if you're a researcher looking for funding, force feeding it to lab mice by the bucket load will certainly create all sorts of horrific results that will strike fear into those who don't know any better.
Glucose, fructose, mannose, and galactose are impossible to avoid in a normal diet. Worrying about the potential for 5-HMF forming as a result of eating a normal diet makes absolutely no sense whatsoever.
Very few real (natural, unprocessed) foods have high amounts of simple sugars occurring naturally, so how are they “unavoidable”? HFCS imparts well over ten times the normal amount of free simple sugars to anything it’s added to.
The precursor to HFCS, all-glucose corn syrup, is itself produced by hydrolyzing corn starch with alpha-amylase (produced from Bacillus subtilis instead of the human pancreas) and HCl, an unnatural process even inside the body. The next step in converting to HFCS is to add enzymes that convert some of the glucose to fructose.
If this is meant to be a defense of crony capitalism’s favorite sweetener, then consider that the Communist Manifesto is the chief historical advocate of “combination of agriculture with manufacturing industries” (ninth plank of Communism per the Manifesto)—something that did not happen in the USA until after WWII, take note.