The Two Forms of Dietary Iron and How They Are Absorbed
Dietary iron comes in two distinct forms: heme and non-heme. The body absorbs these two types very differently, which significantly impacts overall bioavailability.
Heme Iron
Heme iron is found exclusively in animal-based foods, primarily in the hemoglobin and myoglobin of meat, poultry, and seafood. This form is readily absorbed by the body, with absorption rates ranging from 15% to 35%, and it is less affected by other dietary components. For individuals who eat meat, this accounts for a significant portion of their absorbed iron.
Non-Heme Iron
Non-heme iron is present in plant-based foods such as grains, beans, fruits, and vegetables, as well as in iron-fortified products. This form is less efficiently absorbed, with bioavailability estimated at around 2% to 10%. Non-heme iron absorption is highly sensitive to both inhibiting and enhancing dietary factors present in a meal.
Dietary Factors That Limit Iron Absorption
Certain compounds found in everyday foods can act as powerful inhibitors, binding to non-heme iron and preventing its uptake in the small intestine.
List of Common Inhibitors
- Phytates: Found in whole grains, nuts, legumes, and seeds. Phytates, or phytic acid, can decrease iron absorption dramatically, even in small amounts. Soaking, sprouting, and fermentation can help reduce phytate content.
- Polyphenols: These antioxidant compounds are present in tea, coffee, wine, cocoa, and certain fruits and vegetables. The tannins in tea and coffee, in particular, are strong inhibitors of non-heme iron absorption. It is often recommended to consume these beverages a couple of hours before or after an iron-rich meal.
- Calcium: Unlike other inhibitors, calcium affects the absorption of both heme and non-heme iron. It is found in dairy products, fortified soy products, and leafy greens. To minimize its inhibitory effect, calcium supplements and calcium-rich foods should ideally be taken at different times than iron supplements or meals high in iron.
- Oxalates: Present in foods like spinach, chard, beans, and nuts, oxalates can bind to non-heme iron and inhibit its absorption.
- Certain Proteins: Animal proteins like casein (in milk) and egg whites, as well as soy protein, have also been shown to inhibit iron absorption.
Physiological Conditions Affecting Iron Absorption
Beyond diet, an individual’s internal bodily state plays a critical role in regulating iron absorption.
Body Iron Stores
The body has a sophisticated system to maintain iron balance. When iron stores are low, absorption increases, and when stores are high, absorption is suppressed. The peptide hormone hepcidin is the master regulator of this process, binding to a protein called ferroportin to limit iron release into the bloodstream.
Gastrointestinal Disorders
Conditions that damage the lining of the small intestine, where most iron absorption occurs, can significantly impair uptake. These include:
- Celiac Disease: An autoimmune disorder where gluten consumption damages the intestinal lining.
- Inflammatory Bowel Disease (IBD): Conditions like Crohn’s disease and ulcerative colitis cause chronic inflammation of the digestive tract.
- Atrophic Gastritis and H. Pylori Infection: These can reduce stomach acid, which is necessary to convert ferric ($Fe^{3+}$) iron into the more absorbable ferrous ($Fe^{2+}$) state.
- Bariatric Surgery: Procedures that bypass or remove part of the small intestine can decrease the surface area for absorption.
Chronic Disease and Inflammation
Chronic diseases like kidney disease, heart failure, and certain cancers can trigger an inflammatory response in the body. This inflammation increases hepcidin levels, which in turn reduces iron absorption and sequesters iron in storage, leading to a functional iron deficiency known as anemia of chronic disease.
Genetics
Rare genetic disorders can disrupt the body’s iron regulation. Hemochromatosis, for instance, is an inherited condition that causes the body to absorb too much iron. Conversely, iron-refractory iron deficiency anemia (IRIDA) is caused by a genetic mutation that leads to inappropriately high hepcidin levels, blocking absorption.
Comparison of Heme and Non-Heme Iron
| Feature | Heme Iron | Non-Heme Iron |
|---|---|---|
| Source | Animal products (meat, poultry, seafood) | Plant-based foods (grains, legumes, nuts, veggies) and fortified foods |
| Absorption Rate | High (15–35%), more readily absorbed | Low (2–10%), less efficiently absorbed |
| Inhibitor Sensitivity | Mostly unaffected by dietary inhibitors | Highly sensitive to inhibitors like phytates, polyphenols, and calcium |
| Enhancer Interaction | Less dependent on enhancers for absorption | Absorption is greatly improved by enhancers, especially vitamin C |
Strategies to Improve Iron Absorption
Since many factors limit iron absorption, strategic dietary choices and cooking methods can make a significant difference. Combining iron-rich foods with enhancers can counteract the effects of inhibitors.
List of Absorption Enhancers
- Vitamin C (Ascorbic Acid): This is the most effective enhancer of non-heme iron absorption. It forms a chelate with iron, keeping it soluble for absorption. Pairing foods like bell peppers, citrus fruits, or broccoli with plant-based iron sources is highly beneficial.
- The “Meat Factor”: Meat, fish, and poultry not only provide high-quality heme iron but also enhance the absorption of non-heme iron from other foods eaten in the same meal.
- Vitamin A and Beta-Carotene: Found in carrots, sweet potatoes, spinach, and kale, these nutrients can help release stored iron and counteract the effects of inhibitors like polyphenols and phytates.
- Cooking in Cast Iron: Using a cast-iron skillet can increase the iron content of your food, particularly for acidic dishes.
Conclusion
While many dietary and physiological elements can limit iron absorption, maintaining optimal levels is achievable by understanding these factors. The type of iron consumed, the presence of inhibitors and enhancers, and an individual’s health status all play a role. For those with high iron needs or certain health conditions, strategic meal planning—such as pairing non-heme iron with vitamin C or eating a balanced, varied diet—is essential. By being mindful of these influences, individuals can take proactive steps to support their body’s iron utilization and overall health. For further reading, an excellent resource on dietary iron is available on the NIH Office of Dietary Supplements website.
