Inhibitors and Promoters of Iron Absorption

As discussed in this blog post, there are two types of iron in our diet: haem iron and non-haem iron. In short, haem iron is well absorbed, while non-haem iron is not so well absorbed. 

Some of the reasons that non-haem iron is not so well absorbed is that it easily interacts with a number of food components when in the intestine. Many times, these food components bind to the non-haem iron, form a complex which cannot be absorbed into the body, and are passed straight through the gastrointestinal tract. Fortunately, there are a number of factors that promote non-haem iron absorption as well. 

In this post, we discuss some of the major inhibitors and promoters of iron absorption, as well as some general tips on improving dietary iron intake. 

Factors which inhibit iron absorption 

Phytate

Phytate is the major food component found in plants which inhibit iron absorption. Although related structurally to the vitamin inositol, our bodies lack the enzyme necessary to degrade phytate, which means that any phytate that we ingest can participate directly in inhibiting iron absorption. When ingested alongside iron, phytate binds to iron, forming a complex that cannot be absorbed. This means that the iron which complexes with phytate simply passes through our gastrointestinal tract. Phytate does not only inhibit iron, but also calcium, zinc and copper. Phytate is found mainly in grains and legumes. Fortunately, cooking, soaking and fermenting foods are all ways that we can naturally decrease the phytate content.


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Oxalate

Like phytate, oxalate is another compound that binds to a variety of minerals and inhibits their absorption. Oxalate is found in a variety of sources, including spinach, tea and berries. As with phytate, the way in which food is prepared can influence the oxalate content. For instance, boiling and steaming vegetables has been found to decrease their oxalate content.

Polyphenols 

Polyphenols are a very large and diverse group of compounds found across all plant foods. Some of these polyphenols have a significant inhibitory effect on iron absorption, while others do not. Two polyphenol-rich beverages to be mindful of are black tea and coffee. Herbal teas also contain inhbitory polyphenols, but less so than black tea. 

Calcium

Although haem iron is immune to almost all inhibitory factors, unfortunately it is not immune to calcium, and neither is non-haem iron. One of the biggest culprits for calcium content is milk and dairy products. One study has found that 165mg of calcium, which is about as much calcium as that found in ½ cup of milk, was enough to have inhibitory effect on iron absorption. 

Others 

The list doesn’t end here: a broad array of other compounds and nutritional deficiencies are also believed to inhibit iron absorption, including animal proteins (except animal muscle tissue), soy protein and vitamin B2 deficiency.

Factors which promote iron absorption 

Vitamin C 

Probably the most widely known factor that improves iron absorption is vitamin C. To understand how vitamin C is working, we need to take a closer look at how non-haem iron is absorbed. 

The first organs that digests iron is the stomach. In the stomach, iron is released from its food components, and is present in a state known as ‘ferric iron’. While it is fine for iron to be in its ferric state while in the acidic environment of the stomach, when ferric iron passes into the alkaline environment of the small intestine, it can turn into a molecule that cannot be absorbed, and which in turn passes straight through the gastrointestinal tract. So, to prevent this, ferric iron needs to be converted into its other state ‘ferrous iron’. Ferrous iron doesn’t react in this same way in the alkaline environment of the small intestine. Vitamin C is important for this conversion of ferric iron to ferrous iron. Alternatively, it is believed that vitamin C can also directly complex with non-haem iron, helping iron into the body through another pathway. 

Vitamin C can be destroyed during cooking, so raw fruits and vegetables (when safe to eat) are great accompaniments to non-haem iron-rich meals. 

Animal muscle tissue 

Interestingly, while we have already discussed how haem iron, from animal muscle tissue, is well absorbed, animal muscle tissues also enhances the absorption of non-haem iron. This means that the iron in your spinach is likely to be better absorbed when you are having some pork or beef at the same meal than if you were eating spinach on its own. Importantly, this effect isn’t found for any animal protein, it is only found for animal muscle tissue. So, for instance, eggs don’t help, but chicken breast does. 

Other compounds 

As with non-haem iron, the list doesn’t stop here, other compounds, such as dietary fibre, are also believed to improve iron absorption. 


Do I have to avoid inhibiting factors, and include promoting factors? 

So, there are a number of factors which promote iron absorption, and a number of factors that inhibit iron absorption. What is the takeaway? What do you need to do with this information? Fortunately not a great deal. 

Observe the nutrient reference value for Iron 

In the blog post on the nutrient reference values for iron, we discuss that there are three factors which influence the amount of iron recommended per day: 

  1. Gender 

  2. Age 

  3. Vegan/vegetarian eating pattern

As we have discussed above, eating a largely plant based diet means that you are going to be consuming many inhibitors of iron absorption. Fortunately, the nutrient reference value for iron takes this into account. Roughly, international guidelines suggest that about 18% of the iron consumed as part of a mixed Western diet is absorbed. When you eat a vegan/vegetarian diet, however, that percentage drops to about 10%. This means that only 10% of the iron you consume is absorbed. Thus, anyone consuming a plant-based diet is recommended to consume 1.8 times more iron than someone consuming a mixed meat and plant-based diet. 

So, if you simply consume the nutrient reference value, you are likely to overcome any inhibitors present in your diet. 

Of course, in reality it is not quite as easy as that—one extremely important factor that these guidelines don’t take into account is the iron status of the individual. These values were set assuming the population were iron deficient. When you are iron deficient, your ability to absorb iron increases, so an individual who is iron sufficient is likely to have lower iron absorption, and therefore possibly different iron requirements. 

Eat a varied diet 

Even if you are consuming the recommended quantity of iron per day, it is important to be aware of one thing: different foods, cooked in different ways, are going to inhibit iron absorption to different extents. The iron in some foods is extremely poorly absorbed, while the iron in others is well absorbed. The 18% and 10% average iron absorption percentages given above are just that, averages. You need to be mixing up your diet to maximise your chances that you aren’t only consuming those foods which have very poor iron absorption, while maximising your chances that you are consuming some iron promoters as well. 

Don’t drink tea or coffee with meals

Tea and coffee are amazing. They are not amazing, however, with meals. Beyond just iron, tea and coffee are also known to inhibit other minerals, so in general, try not to drink tea or coffee with any nutrient dense meals. Aim to drink tea and coffee about an hour away from nutrient-dense meals.

Include some vitamin C-rich foods with your meals 

Vitamin C is a really easy addition to your meals to boost iron absorption. Around 75mg of ascorbic acid has been found to be the optimal amount, so aim to include some fresh fruit and vegetables with your meals. Fresh capsicum, kiwifruit, paw paw and orange are all good candidates here.


References 

  1. Gropper SS, Smith JL, Groff JL, ‘Advanced Nutrition and Human Metabolism’, 2009, 5th edn., Wadsworth, California. 

  2. Hurrell R, Egli I. Iron bioavailability and dietary reference values. Am J Clin Nutr. 2010 May;91(5):1461S-1467S. doi: 10.3945/ajcn.2010.28674F. 

  3. Fairweather-Tait SJ, Jennings A, Harvey LJ, Berry R, Walton J, Dainty JR. Modeling tool for calculating dietary iron bioavailability in iron-sufficient adults. Am J Clin Nutr. 2017 Jun;105(6):1408-1414. doi: 10.3945/ajcn.116.147389. 

  4. Reddy MB, Hurrell RF, Cook JD. Estimation of nonheme-iron bioavailability from meal composition. Am J Clin Nutr. 2000 Apr;71(4):937-43. 

  5. Frazer DM, Anderson GJ. Is there a better way to set population iron recommendations? Am J Clin Nutr. 2017 Jun;105(6):1255-1256. doi: 10.3945/ajcn.117.158188.