Things that stop absorption of iron, calcium and zinc

Things that stop absorption of iron, calcium and zinc

What are the food components that prevent the absorption of iron, calcium & zinc? Mineral absorption refers to the amount of dietary minerals that the body obtains and uses from food; for instance, healthy adults absorb about 10% to 15% of dietary iron.

Individual absorption is influenced by several factors, including the amount of the mineral already stored in the body: the higher the levels in storage, the less is absorbed – and vice versa. Though one can nonetheless suffer the effects from too much absorption, in our society we are more likely to be suffering a deficiency. The deficiency can be as a result of eating too little of the mineral bearing foods in the first place and by other factors that actually inhibit absorption or increase excretion of the minerals in question.

In summary, the factors affecting bioavailability of iron, calcium and zinc are the presence of:

  • Phytates (found, eg, in foods with fibre)
    Spinach can block absorption of some minerals
    Spinach can block absorption of some minerals
  • Polyphenols (found, eg, in tea, coffee and nuts)
  • Oxalic acid (found, eg, in tea, chocolate, spinach and aubergine)
  • Other minerals (such as iron, calcium and zinc)

Vitamin C and cadmium: Lack vitamin C can mean a decrease in absorption. Vitamin C may not be available for a number of reasons including not being in the foods eaten, degeneration, denaturing and the presence of cadmium; the latter catalyses the transformation of vitamin C into oxalic acid. (This can be a problem for people exposed to high levels of cadmium in the diet, in the workplace, or through smoking.)

Oxalic acid binds and forms insoluble oxalate salts with iron, calcium* and zinc. This binding can occur in the less acid regions of the gut and restricts the absorption of iron, calcium, zinc and more. Even if there is a variety of foods on the plate, foods with a high oxalate to mineral ratio can bind some minerals from foods consumed at the same time, so reducing mineral uptake; this, however, does not appear to be the case with calcium. Foods with high levels of oxalic acid include spinach, collard greens, sweet potatoes, rhubarb, and beans. Oxalates can also increase excretion of calcium

The oxalates are excreted as minute crystals in the urine and they can also form kidney stones. Those with kidney disorders, gout, rheumatoid arthritis, or certain forms of chronic vulvar pain (vulvodynia) are often advised to avoid foods high in oxalic acid. Cooking foods breaks down oxalic acids and eating nutrients such as vitamin C can help increase mineral uptake from these plants.

Phytates are the principle form of storage of phosphorous in plant tissues and thus are ubiquitous in many diets. Because we don’t have the enzyme phytase, we cannot digest phytates. This is not a problem in itself, but the phytates stop the absorption of iron, zinc and calcium by chelating them (that is, the phytates bind the metal ions).  Foods high in phytic acid include whole-grain products and wheat bran, beans, seeds, nuts, and soy isolates.

(Some phytates are water soluble, so can be removed by soaking foods prior to cooking and the presence of animal proteins in the same meal appears to also help. In addition, vitamins A, beta carotene and vitamin C are thought to help prevent the chelation of these minerals, so eating foods containing these vitamins at the same time as grains may help absorption of minerals. Spelt has a lower phytate content than modern strains of wheat, so bread eaters could be advised to eat spelt-based bread.)

Polyphenols are seen as health boosting nutrients, but some studies have indicated some types (such as those found in grape seed extract, tea, coffee and, to a lesser extent, wine) can decrease uptake of zinc and inhibit absorption of iron. If this is the case, then polyphenols may therefore also indirectly inhibit iron-dependent calcium uptake.

Fibre is not digested (or absorbed) by humans. It can bind iron, thus making plant-based (non-haem) iron harder to absorb; meat-based haem iron uptake is not affected in this instance and can even help the absorption of the non-haem iron.

Age and life also have an effect: The efficiency of calcium absorption decreases as people age, especially for those over 70.

Vitamin D, present in some foods and produced in the body when skin is exposed to sunlight, increases calcium absorption.

Elimination, while not the same as inhibition, has the same effect in that a proportion of minerals are rendered available. For instance, some absorbed calcium is eliminated from the body in urine, faeces, and sweat; this amount is affected by factors such as high sodium, caffeine, alcohol, cereal grains and protein intake.

Photo by Daniella Segura

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