Iron. It’s one of our most important essential minerals and sits at the heart of oxygen transport and energy metabolism. In this post, I put the spotlight on iron, outline its key functions and explore the food factors that control how much of it we absorb. Because iron deficiency ranks as one of the world’s most common nutritional disorders, I go into detail of the consequences of this, how deficiency is tested for and importantly: how it can be managed with diet and supplements.
Iron’s best-known role is its part in oxygen transport by the protein haemoglobin found in red blood cells. Haemoglobin is partly made from iron and accounts for about two-thirds of the body’s total iron stores.
Then there is the special protein myoglobin that helps store oxygen in muscle cells. Myoglobin contains iron and is responsible for the red colour of muscle. Muscles are big users of oxygen during exercise, so myoglobin helps meet those oxygen demands. Many enzymes in the body feature iron as part of them including those involved in energy production. And finally, a well-functioning immune system relies, in part, on sufficient iron to fight off infection.
Many of the symptoms of iron deficiency can be seen to be directly related to its function in oxygen transport, energy production and immune function.
Different forms of iron
Iron is quite unique as unlike many nutrients where there is an easy way for the body to excrete excesses of it, iron is held onto very tightly by the body. This means more iron is absorbed when iron stores are low and less is absorbed when stores are full. This all helps keep stores at a balanced level.
So, iron status in the body hinges on how much of it is absorbed and for that, dietary sources can alter this. Iron occurs in two forms in foods: the first is haem iron, which is found only in foods derived from the flesh of animals such as meats, poultry and fish. The second form is non-haem iron, which is found in both plants and animal-derived foods.
On average, haem iron represents about 10 percent of the iron a person consumes in a day. Even though haem iron accounts for only a small proportion of what is consumed, it is so well absorbed that it contributes to up to a third of the iron absorbed in a typical omnivore diet.
Iron inhibitors and enhancers
Overall, about 18 percent of dietary iron is absorbed from a mixed omnivore diet. For vegetarian diets, this figure drops to about 10 percent. This is because the vegetarian diet lacks the more absorbable haem form of iron while also being high in food substances that can reduce the absorption of iron.
In addition to dietary influences, iron absorption also depends on a person’s health, stage in the life cycle, and iron status as well as individual genetic variations. Our bodies adapt very well though to changing needs. For example, in pregnant women, who need the most iron, absorption can increase by 60 percent relative to pre-pregnancy levels.
Outside of animal-based haem iron, which is absorbed very well, vitamin C also enhances non-haem iron absorption from foods eaten in the same meal. It does this by capturing the iron and keeping it in the more soluble and absorbable ferrous form.
On the other side of the coin, there are some dietary factors that bind with non-haem iron, reducing its absorption. These factors include phytate which is found in legumes and whole grains. Phytate is the main storage form of phosphorus in many plant tissues, especially bran and seeds. The vegetable proteins in soybeans and other legumes and nuts can also reduce iron absorption. Polyphenols found in tea and coffee can also reduce iron absorption. While some studies have found that oxalic acid (present in spinach, silverbeet and beetroot leaves) may reduce iron absorption, recent studies find these effects are relatively insignificant. Calcium has also been considered an inhibitor of both haem and non-haem iron absorption. But more recent research suggests that, over a long period, calcium has a limited effect on iron absorption.
But let’s put all this in context. If you ate to only optimise absorption of nutrients, then you would starve. All nutrients can interact with each other to an extent to either increase or decrease absorption of other nutrients. So, treat this as a case of swings-and-roundabouts where inhibitors and enhancers may cancel each other out, particularly in a diet that includes a wide variety of foods. Knowing about iron inhibitors is more useful if you have been diagnosed with iron deficiency. In which case you may pay a bit more attention to them. For everyone else, best to carry on and ignore most of this and just enjoy food.
Iron deficiency rates as the most common nutritional disorder in the world. As well as affecting many women and children in developing countries, it is the only nutrient deficiency that is also significantly prevalent in developed industrialised countries too. The numbers are huge: almost two billion people – that’s over 30 per cent of the world’s population – have anaemia. Most of those cases of anaemia are due to iron deficiency.
Depletion of iron stores and iron deficiency can occur in all age groups. But the groups most at risk include children, women after the onset of menstruation, the elderly, people following a vegetarian diet, and in disadvantaged populations such as indigenous groups, refugees, recent migrants and institutionalised people. Preventing and correcting iron deficiency are high public health priority areas.
Iron deficiency though is not always caused by inadequate dietary iron but may result from various medical conditions. Dialysis treatment in people with chronic renal failure can lead to loss of iron. Gastrointestinal inflammatory diseases such as Crohn’s disease or coeliac disease may impair iron absorption. Then there is iron deficiency arising from gastrointestinal blood loss. Excessive intake of zinc supplements may also impair iron absorption.
The vegetarian diet
Just to focus a bit more on the vegetarian diet and iron as that gets a lot of attention, a review paper from 2018 looked at the iron status in people following this way of eating. Vegetarians on average do have a higher risk of having depleted iron stores. And at least globally, a higher proportion of vegetarians, compared to nonvegetarians, have iron deficiency anaemia. This is especially true for premenopausal women who follow a vegetarian diet.
But in an interesting turn, if you look more at western countries like Australia, where we enjoy a varied food supply, vegetarians are no more likely to suffer from iron deficiency anaemia than non-vegetarians. And this was the topic of a review paper in the Medical Journal of Australia. Having low iron stores, without iron deficiency anaemia, has not been shown to adversely affect normal body function to any great extent. Iron deficiency clearly only impairs function when haemoglobin concentrations are measurably decreased.
Stages of iron deficiency
Iron deficiency is not a black and white thing. It develops in stages with anaemia being the final result. See Table 1 for a summary of these stages. For anyone that has had blood tests done to check for iron deficiency, you may be familiar with some of the following from pathology test results.
In the first stage of iron deficiency, iron stores diminish. This can be seen by a decrease in ferritin in the blood. Ferritin reflects iron stores and is a valuable initial indicator that stores are dropping.
The second stage of iron deficiency is characterised by a decrease in the transport of iron. Here, blood iron levels fall while levels of the iron-carrying protein transferrin increase. Increasing transferrin is one way the body compensates for low levels of iron. So together, measuring both iron and transferrin can determine the severity of the deficiency – the more transferrin and the less iron in the blood, the more advanced the deficiency is.
The third stage of iron deficiency occurs when the lack of iron limits haemoglobin production. This is the stage that is called iron-deficiency anaemia. So, remember, you can be low in iron, but not have anaemia. Whereas iron-deficiency anaemia is seen with low haemoglobin and a low haematocrit (haematocrit is the percentage of the blood volume taken up by red blood cells).
Table 1. Stages of iron deficiency
|Stage 1||Stage 2||Stage 3|
In iron-deficiency anaemia, haemoglobin synthesis decreases, resulting in red blood cells that are pale and small. These cells can’t carry enough oxygen from the lungs to the tissues. Without adequate iron, energy metabolism falters. The result is fatigue, weakness, headaches, apathy, pallor and poor resistance to cold temperatures. Because haemoglobin is the bright red pigment of the blood, the skin of a fair person who has anaemia may become noticeably pale. In a dark-skinned person, the tongue and eye lining, normally pink, is very pale.
A battery of tests requested by a doctor that looks at iron, ferritin, transferrin and haemoglobin allows the homing in on the likely cause of anaemia. Iron deficiency is not always the cause of anaemia though as other nutrient deficiencies such as folate and B12 deficiency can also end up presenting as anaemia. Hence why it is important to get suspected anaemia thoroughly investigated rather than self-diagnose it and start popping iron supplements.
Iron needs and food sources
How much iron do you need each day? The recommended daily intake in Australia for men is 8 milligrams. For women in their reproductive years, it goes up to 18 milligrams a day. This higher requirement for women in their reproductive years is because of the blood loss of iron from menstruation. Remember, most of your iron stores are found in the blood bound up in haemoglobin in red blood cells.
Vegetarians require 80 percent more iron in their diet than meat eaters to make up for the lower bioavailability of iron in their diets. Good plant-based sources of iron include green leafy vegetables, peas and wholegrains, enriched cereals, dried fruit (such as apricots and sultanas) and legumes. Combining these foods with foods high in vitamin C helps the body absorb the iron even better.
For someone who is iron deficient or at high risk of developing it, then there can be the need for supplements on top of an iron-rich, absorption-enhancing diet. Iron from supplements is less well absorbed than that from food, so the doses usually need to be higher. The absorption of iron taken in the form of ferrous sulphate is better than that from other iron forms so that is one to look out for. Absorption also improves when supplements are taken between meals, at bedtime on an empty stomach and with liquids other than milk, tea or coffee, which can reduce absorption.
If you’ve been advised to take iron supplements, keep in mind that the most common side effect of iron supplements is dark-coloured or black stools, so don’t be alarmed by this change to your bowel habits. Other common side effects include nausea, vomiting, constipation and diarrhoea.
Supplements should be taken exactly as advised by your doctor. The human body is not particularly good at excreting iron and you could poison yourself if you take more than the recommended dose. The body stores iron very efficiently and too much iron can be toxic. Symptoms of toxicity include nausea, vomiting, diarrhoea, a rapid heartbeat, a weak pulse, dizziness, shock and confusion. As little as 200 milligrams of iron has caused the deaths of many young children and remains a leading cause of accidental poisoning in children. So, if you are taking iron supplements, keep them well out of reach of little hands and in a child-proof bottle.