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Food · Animal Foods · Red Meat
food source · 3
→Iron
Most of the iron in beef is heme iron, which is absorbed several times more efficiently than the non-heme iron in plants. Phytate and tea polyphenols in the same meal barely hold it back; only calcium may lower it somewhat.
Mechanism · Two kinds of iron, two doors
The iron in plants is an unframed ion that has to be reduced to ferrous iron before it can enter a gut cell, and on the way phytate and tea polyphenols can lock it up. The iron in beef is framed inside a molecule called a porphyrin ring; gut cells take it in ring and all, then open the ring inside the cell and add the iron to the same pool. So vitamin C helps the plant iron on the same plate get absorbed, not the iron in this meat.
Numbers · How much is absorbed, and when less is
Estimated across a whole diet, about 14–18% of iron is absorbed from a mixed diet with meat, seafood and vitamin C, and about 5–12% from a vegetarian diet (Hurrell and Egli 2010). Both figures were estimated for people whose iron stores are empty and are used to set dietary recommendations; they are not the absorption from any single meal. Efficient absorption does not mean more is better: the body has almost no active way to get rid of iron, and in someone whose iron is already sufficient, the gut absorbs less on its own.
iron-bioavailability-hurrell-egli-2010B12 comes almost only from animal foods. Cattle cannot make it themselves; the microbes in their rumen do, and the cow absorbs it and stores it in liver and muscle, so one serving of beef covers a good part of a day's need.
Mechanism · The cow concentrates it
Only bacteria and archaea make B12. The microbes in the rumen make it, and the cow absorbs it and stores it in liver and muscle, so beef does not produce B12; it concentrates it. The lower human gut also has bacteria that make B12, but they sit downstream of where it is absorbed, so what they make arrives too late. From plate to blood there are three steps: stomach acid frees B12 from food protein, intrinsic factor (an escort protein made by the stomach) picks it up, and the end of the small intestine takes it in.
Clinical · Eating meat does not always prevent it
If any of the three steps fails, the amount on the plate does not count. People with less stomach acid in older age, people on long-term acid-suppressing drugs, and people whose stomach lining can no longer make intrinsic factor can run short of B12 even while eating meat. The liver holds a store of B12, so symptoms appear long after intake stops; sometimes numbness in the hands and feet or unsteady walking comes first, while the blood count is still normal.
→Zinc
Red meat is one of the best dietary sources of zinc. Meat contains no phytate or similar compounds that lock zinc up, so the zinc in beef is well absorbed.
Mechanism · Zinc holds proteins in shape
Zinc is a cofactor for hundreds of enzymes, and it holds a class of proteins called zinc fingers in the shape they need to grip DNA. In plants, zinc is often locked up by phytate; meat has none, so for meat the amount in a food-composition table and the amount actually absorbed are not far apart.
Myth · Zinc from food does not push out copper
Saying that the zinc in red meat is well absorbed means little of it is lost, not that the amount is remarkably high. Zinc and copper compete for the same absorption route, but that competition becomes a problem only with long-term high-dose zinc supplements (the adult tolerable upper intake level for zinc is 40 mg a day); the zinc you get from meat does not push copper out.
contrast · 1
Processed meat and fresh beef are often lumped together as red meat, but IARC graded them differently: processed meat is Group 1 (sufficient evidence in people), unprocessed red meat is Group 2A (limited evidence in people). By mechanism, fresh red meat carries mainly the heme iron line, while processed meat stacks several lines together.