Scientists say they have worked out how the gut absorbs iron from meat into the blood - a discovery they hope could lead to new treatments for anaemia.
Meat is the most easily absorbed source of iron
A key protein appears to control the process in mice, the King's College London team told the journal Cell.
Mutations in the protein could affect the ability to absorb iron, they said.
Iron deficiency, which causes tiredness, is the world's most common nutritional problem. In the UK around 20% of women are anaemic.
Iron is the least plentiful nutrient in the typical British diet.
It can come from plant sources or from animal tissues (haem iron).
Haem iron is absorbed around five times more efficiently than inorganic iron - this is why eating red meat is recommended for preventing anaemia.
But until now the way in which haem iron was taken up in the gut had been unknown.
By studying mice with anaemia, the King's team has identified a transporter protein, called HCP1, which appears to solve the riddle.
They found the protein was active in the first part of the intestine, the duodenum, and that it shifted position within the intestinal cells in response to changes in the body's iron stores, allowing cells to take up more, or less, haem as required.
Lead researcher Dr Andrew McKie said: "Currently pregnant women suffering from anaemia are given supplements of inorganic iron, but these are poorly absorbed and poorly tolerated.
"Having identified the haem transporter, we can now generate synthetic analogs of haem iron, which would be more readily absorbed."
The breakthrough also holds out promise of a treatment for haemochromatosis, a hereditary genetic disorder that causes people to absorb too much iron from the diet.
This iron is then deposited in various organs, mainly the liver, leading to severe tissue damage.
Dr McKie said: "Mutations in HCP1 would affect the body's ability to absorb haem, and therefore make it a potential culprit in iron-overload diseases such as haemochromatosis.
"Drugs that inhibit HCP1 could be used to reduce some of the symptoms of this disorder."
Professor Terry Lappin, scientific secretary of the British Society for Haemotology, told the BBC News website the research was very interesting.
He said there was still a question about whether work on mice could be translated into humans - but he suspected it probably would.
"This extends our knowledge of iron metabolism, and paves the way for research into possible new methods of treatment for diseases which have disordered iron regulation as their main feature."