Haem iron (also spelled “heme iron”) is the form of dietary iron found in animal tissue, where the iron atom sits inside a ring-shaped molecule called a porphyrin. This structure is the same one that gives blood its red color and allows haemoglobin to carry oxygen through your body. What makes haem iron nutritionally important is that your body absorbs it far more efficiently than the iron found in plants: roughly 15 to 35% of haem iron is absorbed, compared to less than 10% for non-haem iron.
How Haem Iron Differs From Non-Haem Iron
All dietary iron comes in one of two forms. Haem iron is bound inside a porphyrin ring, a flat structure made of four linked nitrogen-containing rings with a single iron atom at the center. This is the iron in haemoglobin (in blood) and myoglobin (in muscle). Non-haem iron, by contrast, is “free” iron found in both plant and animal foods. It exists in grains, beans, leafy greens, and fortified cereals, and also makes up a portion of the iron in meat.
The porphyrin ring is what gives haem iron its absorption advantage. Your intestinal cells have a dedicated transport protein that pulls the entire haem molecule across the gut lining intact. Once inside the cell, an enzyme breaks the ring apart and releases the iron for use. Non-haem iron, on the other hand, must be converted to a specific chemical form before it can cross the intestinal wall, a process that is easily disrupted by other compounds in your meal.
Why Absorption Rates Matter So Much
Haem iron makes up only about 10 to 15% of total iron intake in a typical Western diet. But because of its high absorption rate, it can account for more than 40% of the iron your body actually takes in. This is why a relatively small serving of red meat or shellfish can have an outsized effect on your iron status.
Non-haem iron absorption is heavily influenced by what you eat alongside it. Vitamin C boosts it; compounds in tea, coffee, whole grains, and legumes can reduce it substantially. Haem iron largely sidesteps these interactions because it enters the body through a different pathway. The one notable exception is calcium. At doses of 800 mg or higher (roughly the amount in three cups of milk), calcium has been shown to reduce haem iron absorption by about 38%. Below that threshold, the effect is minimal.
Best Food Sources
Haem iron is found exclusively in animal foods, specifically in muscle tissue, organ meats, and seafood. The richest sources per standard serving include:
- Organ meats: 1.8 to 19 mg per 3-ounce serving, depending on the type (liver is at the high end)
- Oysters: 6.9 mg per 3 oysters
- Beef: 2.5 mg per 3 ounces
- Sardines (canned): 2.5 mg per 3 ounces
- Clams: 2.4 mg per 3 ounces
For context, adult men need about 8 mg of iron per day, while premenopausal women need 18 mg and pregnant women need 27 mg. Vegetarians are advised to aim for nearly double these amounts because they rely entirely on the less absorbable non-haem form. Darker cuts of meat generally contain more haem iron than lighter ones, since myoglobin concentration is higher in muscles that do more sustained work.
How Cooking Changes Haem Iron
Heat breaks down the porphyrin ring structure, converting some haem iron into non-haem iron. This happens quickly: in beef cooked to just 60°C (140°F), the proportion of soluble haem iron dropped from 65% in raw meat to 22%. Further cooking continued to reduce it, though more gradually. The iron doesn’t disappear. It simply shifts into the less absorbable non-haem form.
This means rare and medium-rare meat retains more haem iron than well-done meat. Surface portions, which reach higher temperatures first, lose haem iron faster than the interior. If maximizing iron absorption is a goal, cooking to lower internal temperatures preserves more of the haem form. That said, even well-done meat still provides meaningful amounts of iron overall.
Haem Iron Supplements
Haem iron is available as a supplement, typically marketed as “heme iron polypeptide” under brand names like Proferrin. The selling point is that it uses a different absorption pathway and should cause fewer gut side effects like constipation and nausea. In practice, the evidence for these supplements has been mixed.
A review in Canadian Family Physician found that traditional iron salts (the standard, inexpensive supplements) improved haemoglobin levels by 10 to 20 g/L more than newer formulations, and resolved iron deficiency anaemia in roughly one in five more patients at the three-month mark. In one small study of post-gastric-bypass patients, standard iron supplements improved haemoglobin levels after eight weeks while haem iron supplements did not. Evidence that haem iron supplements cause fewer side effects has been inconsistent across studies, though one trial in blood donors found significantly less constipation with a haem-containing formulation (14% versus 35%).
Potential Risks of High Intake
The same chemical property that makes haem iron easy to absorb also raises concerns at high intakes. The iron in haem is in a form that readily generates free radicals, reactive molecules that can damage DNA. In the colon specifically, haem iron also promotes the formation of compounds called nitrosamines, which are known carcinogens.
A large European study (the EPIC cohort) and four separate meta-analyses have examined the link between haem iron and colorectal cancer. The meta-analyses consistently found a modest positive association: roughly 11% higher risk for each additional milligram of daily haem iron intake. In the EPIC study, the association was strongest in men, particularly for rectal cancer (29% higher risk comparing the highest to the lowest intake group) and cancers of the upper colon. In women, no significant association was found overall.
Substitution modelling from the same study suggested that if men replaced 1 mg of daily haem iron with 1 mg of non-haem iron, colorectal cancer risk dropped by about 6%, with a 10% reduction specifically for upper colon cancers. This doesn’t mean haem iron is dangerous in normal amounts, but it does support the general dietary guidance to moderate red and processed meat intake rather than relying on it as your primary iron source.