Mechanistically, how can high dietary iron intake reduce copper absorption in the intestine?

Iron increases gut motility so dramatically that copper-containing foods pass too quickly through the intestine to be digested and absorbed, and this is the primary reason copper decreases.

High iron intake stimulates bile synthesis that binds copper and sends it back to the liver for storage, thereby increasing apparent copper absorption while actually decreasing functional copper status.

Iron and copper share some intestinal transport pathways and competitive uptake mechanisms; when luminal iron is high, it competes for absorption and can reduce copper uptake through shared transporters.

High iron intake chemically converts copper into an insoluble polymer in the stomach which is then excreted and cannot be absorbed, a process unique to iron that does not involve transporters.