Nitrite, high-heat products, and heme all land on the same stretch of colon.
Full mechanism, in text
How it raises cancer risk
1 · What is dangerous is often not the molecule you ate
The N-nitroso compounds formed during curing cannot touch DNA on their own; liver enzymes oxidize them into an unstable intermediate, and that is what makes them dangerous.Nitrite in stomach acid or heat makes N-nitroso compounds with amines. They are not reactive when they first enter; they cannot touch DNA on their own. Liver enzymes oxidize them first into an unstable intermediate — what is dangerous is often what the body makes while trying to process it.
2 · One grows out of the meat, one drifts back from the fire
Once the surface dries and heats up, the meat makes heterocyclic amines itself, while fat dripping onto coals sends polycyclic aromatic hydrocarbons back onto the meat in the smoke.Heterocyclic amines are made by the meat itself once the surface dries and heat shoots up. Polycyclic aromatic hydrocarbons come from outside: fat drips onto coals, and the smoke settles back. Processed meat often adds a smoking step, so both lines are present.
3 · Three lines land on the same stretch of colon
Heme iron follows residue into the large intestine and catalyzes oxidation there, and the products of all three lines reach the colon with food, where contact lasts longest.Heme iron does not go into the liver; it follows residue to the large intestine and catalyzes oxidation there, damaging the wall and forcing the mucosa to keep renewing. All three products travel with the food, and the colon is the last stretch — longest residence, tightest contact.
4 · What accumulates is the number of times
The intermediate forms DNA adducts; most are repaired, and the few that slip through become mutations, so frequency matters more than the size of a single serving.The intermediate hangs a small group onto DNA — an adduct. Repair clears most of them before replication. What becomes a mutation is the small fraction that slips through. Carcinogenesis is never a single event. It is a string of unwiped errors — frequency matters more than the size of a single serving.
Most links in these mechanisms come from animal experiments and lab measurements: they can explain the link between processed meat and colorectal cancer seen in population studies, but how much each step weighs in people has not been measured directly.