Advanced Glycation End-Products (AGEs)
Sugar literally browns your collagen like a steak. That's why skin stiffens and yellows.
Grounded in the research on Advanced glycation end-product (Wikipedia)
What they actually are
AGEs are proteins or lipids that got chemically glued to a sugar with no enzyme involved. Glucose latches onto an amino group, forms a Schiff base, rearranges into an Amadori product, then cooks into a stable end-product. Same non-enzymatic Maillard reaction that browns a steak or a brioche crust, running slowly inside your body at 37C. The acronym is honestly perfect: A.G.E.
The mechanism: crosslinking
The damage isn't the sugar itself, it's the bridge. AGEs like glucosepane crosslink long-lived structural proteins, locking adjacent collagen and elastin fibers together. Crosslinked collagen stops sliding and flexing. Result: tissue gets stiff, brittle, and turns yellow-brown. Collagen has a half-life measured in years to decades, so the damage accumulates instead of clearing.