Iron Supplements: Why Testing Comes First, Food Sources and Safe Use

Iron is the most common nutritional deficiency worldwide and the supplement most likely to be taken for the wrong reason. Fatigue is not a diagnosis, iron overload is a real condition, and iron tablets are a leading cause of accidental poisoning in young children. This is the one where the order matters: test first, supplement second.

Key points

• UK reference nutrient intakes: 8.7 mg a day for men and for women over 50; 14.8 mg a day for women aged 19–50 because of menstrual losses.
• Iron contributes to normal formation of red blood cells and haemoglobin, normal oxygen transport, normal energy-yielding metabolism, normal cognitive function, normal immune function and a reduction of tiredness and fatigue.
• Do not self-treat suspected iron deficiency. A GP can arrange a full blood count and ferritin, and identify why iron is low — which sometimes matters more than the iron itself.
• The NHS advises that supplemental doses above 17 mg a day may cause constipation, nausea, abdominal pain and black stools.
• Keep iron supplements away from children. Overdose can be fatal.

What iron is and the two dietary forms

Iron exists in food in two forms with very different absorption:

• Haem iron, from meat, fish and poultry. Absorbed at roughly 15–35%, and relatively unaffected by other dietary components.
• Non-haem iron, from plants, eggs and fortified foods. Absorbed at roughly 2–20%, and strongly influenced by what else is on the plate.

Enhancers of non-haem absorption: vitamin CClinical (an authorised claim), and meat, fish or poultry eaten in the same meal.

Inhibitors: tannins in tea and coffee, phytates in wholegrains and pulses, calcium, and polyphenols. This is why the traditional advice to avoid tea with an iron-rich meal has a genuine basis.

Supplement forms include ferrous sulphate, ferrous fumarate and ferrous gluconate — the standard prescribed options — plus gentler, lower-dose forms such as iron bisglycinate and iron polysaccharide complexes.

What it does in the body

Iron is the functional centre of haemoglobin, which carries oxygen in blood, and of myoglobin in muscle. It is also required by enzymes involved in energy production and immune function. When iron stores fall, the body depletes ferritin first, then reduces haemoglobin production — which is why ferritin drops before anaemia appears, and why testing ferritin is informative.

Iron deficiency anaemia causes fatigue, breathlessness on exertion, pallor, palpitations, headaches, brittle nails, hair shedding and sometimes unusual cravings for ice or non-food substances (pica), and restless legs.

Food sources and UK examples

Haem iron:

• Red meat — beef, lamb
• Liver and kidney (avoid liver in pregnancy)
• Sardines, mackerel, mussels, cockles
• Poultry, particularly darker meat

Non-haem iron:

• Lentils, chickpeas, kidney beans, baked beans
• Tofu and tempeh
• Fortified breakfast cereals — a major UK contributor
• Wholemeal bread and fortified white flour
• Dark green vegetables such as kale and spring greens (spinach contains oxalate, which reduces absorption)
• Dried apricots, prunes and figs
• Pumpkin seeds, cashews

A practical vegetarian tactic: pair the iron source with a vitamin CClinical source — lentils with tomato and peppers, fortified cereal with a glass of orange juice — and keep tea or coffee to between meals rather than with them.

Who may be at higher risk

• Women with heavy menstrual bleeding — the most common cause in the UK
• Pregnant women, with substantially increased requirements
• Vegetarians and vegans, because of non-haem absorption
• Frequent blood donors
• Infants and toddlers, especially with prolonged cows' milk intake
• Adolescents, through growth and diet
• Older adults, where gastrointestinal blood loss must be excluded — new iron deficiency in an older adult, especially a man or postmenopausal woman, is a recognised red flag requiring investigation
• People with coeliac disease, Crohn's disease, H. pylori infection, or after gastric surgery
• People taking long-term proton pump inhibitors or aspirin

Why testing comes first

This is the central message. Iron deficiency is a finding, not a diagnosis — the important question is often why. Unexplained iron deficiency can be the first sign of gastrointestinal bleeding, coeliac disease or gynaecological pathology, all of which need identifying.

Equally, the reverse risk exists. Haemochromatosis, a hereditary iron overload disorder, is relatively common in people of northern European ancestry. Taking iron supplements with this condition causes harm. Since the symptom people self-treat — fatigue — occurs in both deficiency and overload, guessing is a poor strategy.

Tests a GP may use: full blood count, ferritin, transferrin saturation, and in some cases coeliac serology, faecal immunochemical testing or endoscopy. Ferritin is an acute phase protein and rises with inflammation, which can complicate interpretation.

See a GP if you have persistent fatigue, breathlessness, heavy periods, blood in your stool, unexplained weight loss, or a change in bowel habit.

What research says about supplements

• Treating diagnosed iron deficiency anaemia with oral iron is effective and well established. NICE guidance covers dosing, duration and follow-up.
• Alternate-day dosing. Research on hepcidin, the hormone regulating iron absorption, has shown that a single daily dose raises hepcidin and blunts absorption of subsequent doses. Studies indicate that alternate-day, single-dose regimens can achieve comparable or better absorption with fewer side effects. Some UK practice has moved in this direction — discuss with your GP or pharmacist rather than changing a prescribed regimen yourself.
• Iron in non-anaemic fatigue. Some trials in women with low ferritin but no anaemia have reported improvements in fatigue. The evidence is mixed, and the appropriate route is still a test and a clinical decision.
• Pregnancy. Routine universal iron supplementation is not standard UK practice; iron is given where anaemia is identified through antenatal screening.

Safety, side effects and interactions

• Common side effects: constipation, nausea, abdominal pain, dark or black stools (harmless and expected), and metallic taste. Lower doses or alternate-day dosing usually improve tolerance.
• Overdose is dangerous. Iron poisoning in children can be fatal. Store supplements in child-resistant containers, out of sight and reach. In suspected overdose, contact emergency services immediately.
• Haemochromatosis and other iron overload states — avoid iron supplements entirely unless a specialist directs otherwise.
• Interactions: iron reduces absorption of levothyroxine, tetracycline and quinolone antibiotics, bisphosphonates, levodopa and methyldopa. Separate doses by at least two to four hours. Calcium supplements, antacids and proton pump inhibitors reduce iron absorption.
• Take with water or a vitamin CClinical source; avoid taking with tea, coffee, milk or calcium supplements.

Choosing and using iron responsibly

• Get tested before starting. This is the single most important step.
• Follow the prescribed dose and duration. Treatment usually continues for around three months after haemoglobin normalises, to refill stores.
• Consider a lower-dose or gentler form if side effects are limiting — bisglycinate is often better tolerated — after discussing with a pharmacist or GP.
• Take it with vitamin CClinical, away from tea, coffee, dairy and calcium supplements.
• Do not add extra iron to a multivitamin without checking totals.
• Repeat testing as advised, to confirm response and stores.

Frequently asked questions

Should I take iron for tiredness?

Not without testing. Fatigue has many causes, and taking iron when you are not deficient can be harmful — particularly if you have an undiagnosed iron overload condition.

How much iron do I need a day?

UK reference nutrient intakes are 8.7 mg for men and women over 50, and 14.8 mg for women aged 19–50.

Why do iron tablets cause constipation?

Unabsorbed iron irritates the gut. Lower doses, alternate-day dosing, adequate fluid and fibre, and gentler forms such as bisglycinate can help. Discuss changes with your GP or pharmacist.

Is it normal for stools to turn black on iron?

Yes, that is expected. However, black tarry stools with a strong odour can indicate gastrointestinal bleeding — if you are unsure, seek medical advice.

Can I get enough iron on a vegan diet?

Yes, with attention. Combine pulses, tofu, fortified cereals, seeds and dried fruit with vitamin C sources, and keep tea and coffee between rather than with meals.

Can you take too much iron?

Yes. The NHS notes side effects above 17 mg a day from supplements, and acute overdose is a medical emergency, especially in children. People with haemochromatosis must avoid iron supplements.

Conclusion

Iron is essential, deficiency is common and treatable, and it is the clearest case in supplementation for seeing a healthcare professional first. A blood test costs little and answers two questions at once: whether you are deficient, and whether something is causing it. If you have been prescribed iron, take it as directed, use vitamin C to help absorption, and keep the bottle well away from children.