You sleep enough, you train as always — and yet every session feels harder than it used to. The last repetitions no longer happen, your pulse rises faster, your concentration fades. Among women and in endurance sport, one suspicion quickly comes up: iron deficiency. The suspicion is justified, but it does not justify self-treatment — iron is one of the few nutrients where too much is just as harmful as too little. Here you will find what iron does, the typical symptoms, the risk groups, the difference between haem and non-haem iron and the factors that promote or inhibit absorption.
What iron does in the body
Iron is a trace element — a mineral of which the body needs less than 100 mg per day. Small does not mean unimportant here, though: iron plays its central role in haemoglobin, the transport protein of the red blood cells. Without haemoglobin there is no oxygen transport, and without oxygen transport there is no aerobic energy supply.
On top of that: red blood cells live for around 120 days on average. For oxygen transport capacity to remain constant, the body has to keep producing new ones — and for that it needs a continuous supply. Iron is not a store you fill up once, but an ongoing expenditure.
The reference value for daily intake is 10 mg for men and 15 mg for women. In pregnancy the requirement doubles — one more reason why the supply is medically supervised at this stage of life rather than being handled by gut feeling.
Symptoms: non-specific, and that is precisely the problem
Iron deficiency rarely announces itself clearly. Typical signs are:
- persistent tiredness and exhaustion that sleep does not resolve
- a drop in training performance, especially in endurance exercise
- shortness of breath and a higher pulse at the usual workload
- pallor, difficulty concentrating, headaches
- increased susceptibility to infection, brittle nails, hair loss
The problem: these very signs also fit lack of sleep, an excessive training volume or too low an energy intake. Before you think of iron, it is worth taking an honest look at your sleep quality and your energy balance. Low energy availability — defined in the course material as less than 30 kcal per kg of fat-free mass — brings with it an increased risk of fatigue and overtraining symptoms, weakens the immune system and can lead to menstrual cycle disturbances and reduced bone density in women. This connection is known as the female athlete triad, or as RED-S, because it also affects men. Anyone training in a calorie deficit should be aware of this threshold.
Who is particularly affected
Menstruating women
The monthly blood loss explains why the reference value for women, at 15 mg, is considerably higher than the one for men. With heavy or prolonged bleeding, the loss rises further. How training can sensibly be aligned with the cycle is covered in the article on strength training and the menstrual cycle.
Endurance athletes
According to the course material, iron requirements in sport can be somewhat increased — «somewhat» does not mean «double». Anyone who runs a lot and for long distances also has losses via sweat and the gastrointestinal tract. If intake does not grow along with a high training volume, too little of everything arrives, not just iron. Especially when preparing for a half marathon, it is therefore worth letting your nutrition grow with it.
People living vegetarian and vegan lifestyles
Two factors interlock here: the somewhat increased requirement in sport and the poorer usability of plant-based iron. The course material therefore notes that athletes on a vegetarian diet have to pay particular attention to iron-rich foods and make targeted use of the promoting and inhibiting factors. More on this in the articles on the vegetarian diet and on vegan sports nutrition.
Haem and non-haem iron: the decisive difference
There are two forms of iron in our food — and they differ enormously in how much of it actually arrives in the body:
- Haem iron from animal products such as red meat, poultry, liver and eggs is absorbed at a rate of 15 to 35 %.
- Non-haem iron from plant-based foods such as breakfast cereals, bread, pulses, nuts or green leafy vegetables is only absorbed at a rate of 5 to 12 %.
So what counts is not what is written on the label. According to the course, the best plant-based sources are fortified breakfast cereals, bread, pulses, dried beans, gluten-based meat alternatives, nuts, dried fruit and green leafy vegetables.
What promotes absorption — and what inhibits it
This is exactly where the practical lever lies, and it costs nothing. The course provides an overview of this, which can be summarised as a list as follows:
Inhibiting for iron absorption:
- Phytic acid — grain products, pulses, soya
- Oxalic acid — spinach, rhubarb, nuts, chocolate
- Polyphenols — tea, coffee, red wine, grapes
- Minerals such as zinc, copper and calcium — but only in the form of supplements
Promoting for iron absorption:
- Vitamin C — citrus fruits, fruit in general, juices, fresh vegetables or a vitamin C supplement
- A low pH value — acidic foods improve absorption
The textbook example from the course material: fortified breakfast cereals together with orange juice. And conversely the most common mistake: coffee or black tea right alongside the iron-rich meal. Separating the two in time is one of the most effective measures there is — and one that requires no capsule.
The spinach myth
The belief that spinach contains a particularly large amount of iron goes back to a piece of misinformation from the past. Fresh spinach supplies around 2.7 mg of iron per 100 g — a respectable amount compared with other vegetables, but not enough to cover 10 or 15 mg per day. On top of that, spinach also contains oxalic acid, that is, an inhibitor. A good example of why nutritional tables alone say little — more such cases in the article on superfoods.
Diagnosis belongs with the doctor — and supplements are not for guesswork
Whether there really is an iron deficiency cannot be read off a symptom, only established through a blood test. The most important value is ferritin, which reflects the iron stores; depending on the situation, additional parameters are assessed as well. This assessment and the decision about any treatment belong in medical hands — not least because other causes that need to be investigated can lie behind tiredness and a drop in performance.
The course material states clearly why self-supplementation is delicate: with minerals there is a danger that too high an intake will cause toxic effects. There is a safe range between the minimum required and the maximum tolerable dose. Iron is also excreted only to a limited extent — excess iron is deposited in the body. That is why the rule is: never supplement iron on suspicion. Have it measured first, then decide. Incidentally, this is not the same for all micronutrients — the article on magnesium shows where the rules are different.
In principle, the course assumes that even in sport a healthy and varied diet supplies enough vitamins and minerals, as long as energy requirements are covered. So the order is: eat enough, combine cleverly, have it measured — and only then talk about supplements.
Course tip from update Akademie: How to realistically assess micronutrient requirements in sport, where the line runs between counselling and medical investigation, and how to support vegetarian and vegan athletes competently — you learn all of this in the Sporternährungscoach programme at update Akademie, with dedicated modules on nutrients, ways of eating and evidence-based practice. Part-time and practice-oriented.
Frequently asked questions about iron deficiency
How much iron do I need per day?
The reference value is 10 mg for men and 15 mg for women per day. In sport the requirement can be somewhat increased, and in pregnancy it doubles. These values apply to intake through food, not to supplements.
Why do I absorb plant-based iron less well?
Because it is present as non-haem iron, which is only absorbed at a rate of 5 to 12 %. Haem iron from animal products reaches 15 to 35 %. Vitamin C at the same meal considerably improves the absorption of plant-based iron.
May I drink coffee with my meal?
Better not with an iron-rich meal. Polyphenols from coffee, tea, red wine and grapes inhibit iron absorption. Leaving a gap in time between the meal and the coffee solves the problem in most cases.
Can I simply take iron supplements as a precaution?
No. With minerals, too high an intake can have toxic effects, and excess iron is stored in the body. Whether there is a deficiency is shown by a blood test with a determination of ferritin — diagnosis and treatment belong in medical hands.
How do I tell whether my tiredness comes from training?
That cannot be distinguished with certainty without investigation. Chronically low energy availability produces complaints similar to iron deficiency, including menstrual cycle disturbances and an increased susceptibility to infection. That is why training volume, energy intake and blood values are looked at together.
Conclusion
Iron is unspectacular and yet decisive for performance: without haemoglobin no oxygen reaches the muscles, and because red blood cells are constantly replaced, the body needs a continuous supply. Menstruating women, endurance athletes and people on a vegetarian or vegan diet are the most likely to be affected — but the biggest lever lies not in the tin, rather on the plate: haem iron is absorbed at a rate of 15 to 35 %, non-haem iron at only 5 to 12 %, and vitamin C, or going without coffee at the meal, shifts this balance noticeably. Everything beyond that starts with a blood test, not with a supplement.
