Glycation
Sugar literally hardens your collagen. It's called glycation and it's why fast food ages your face.
Grounded in the research on Glycation (https://en.wikipedia.org/wiki/Glycation)
What it actually is
Glycation is a sugar molecule bonding to a protein or fat with NO enzyme controlling it. Non-enzymatic = random, uncontrolled, no off switch. First described by Louis-Camille Maillard in 1912 (yes, the same Maillard reaction that browns your steak is happening inside you, just slower). Different from glycosylation, which is the controlled, enzyme-driven version your body actually wants.
The mechanism, step by step
A glucose or fructose hooks onto an amino group on a protein and forms a Schiff base in minutes. That rearranges into a more stable Amadori product over hours to days. Over weeks to months those degrade and cross-link into AGEs: Advanced Glycation End-products. AGEs are the end stage and they don't reverse easily. Fructose glycates roughly 10x faster than glucose, which is why high-fructose intake hits harder.