Iron Deficiency Symptoms: Causes, Risk Factors & Sources

What Is Iron?

Iron is a mineral essential for making haemoglobin, the protein in red blood cells that carries oxygen around the body. It’s one of the most common nutrient deficiencies worldwide, and iron-deficiency anaemia is the most common form of anaemia in the UK — particularly among women of reproductive age.

There are two dietary forms: haem iron (from animal sources, well absorbed) and non-haem iron (from plant sources, less well absorbed but still useful, especially combined with vitamin C).

Why Your Body Needs It

EFSA-authorised health claims for iron include:

• Contributes to the reduction of tiredness and fatigue
• Contributes to normal cognitive function
• Contributes to normal formation of red blood cells and haemoglobin
• Contributes to normal oxygen transport in the body
• Contributes to normal energy-yielding metabolism
• Contributes to normal function of the immune system
• Has a role in the process of cell division

Signs of Iron Deficiency

Iron deficiency develops in stages — reduced stores first, then reduced red blood cell production, and eventually iron-deficiency anaemia if unaddressed. Common signs include:

• Persistent tiredness and low energy
• Pale skin, especially inside the lower eyelid
• Shortness of breath during normal activity
• Heart palpitations
• Headaches and dizziness
• Brittle nails, hair thinning
• Restless legs
• Unusual cravings for non-food items (a specific sign called pica, though uncommon)

Because these overlap heavily with B12 and general fatigue, a GP blood test (ferritin, full blood count) is the only reliable way to confirm iron deficiency rather than assuming from symptoms alone.

Who’s Most at Risk

• Women with heavy periods — the single most common cause of iron deficiency in the UK
• Pregnant women — iron requirements roughly double during pregnancy
• Vegetarians and vegans — non-haem iron from plants is less bioavailable than haem iron from meat
• Endurance athletes — increased red blood cell turnover and iron loss through sweat and foot-strike haemolysis
• People with gastrointestinal conditions — coeliac disease, IBD, or previous bariatric surgery affecting absorption
• Frequent blood donors

Food Sources

• Red meat, particularly liver — the richest source of well-absorbed haem iron
• Poultry and fish
• Beans, lentils, and chickpeas
• Fortified breakfast cereals
• Dark leafy greens (spinach, kale) — non-haem iron, absorption improved when eaten with vitamin C-rich foods

Tip: Tea and coffee consumed close to meals can significantly reduce non-haem iron absorption — worth spacing out if you’re relying on plant sources.

Gentle Iron vs Ferrous Sulfate: Which Causes Less Constipation?

Iron bisglycinate (“gentle iron”) generally causes fewer digestive side effects than ferrous sulfate, but part of that difference comes from the dose, not only the form.

Ferrous sulfate is the form GPs most commonly prescribe in the UK. It’s cheap, effective and well studied, and it’s what most clinical guidance is based on. It also has a well-known downside: constipation, nausea, stomach cramps and dark stools are common, and they’re the main reason people stop taking it before their iron stores recover.

Iron bisglycinate is iron bound to the amino acid glycine. It’s absorbed through a slightly different route and tends to be gentler on the gut. Several small trials have found fewer gastrointestinal complaints with bisglycinate, though the evidence base is much smaller than for ferrous sulfate.

The dose matters as much as the form. A standard 200mg ferrous sulfate tablet provides around 65mg of elemental iron. Most gentle iron supplements provide around 20–25mg. Lower doses cause fewer side effects whatever the form, so some of the “gentleness” is simply a smaller amount of iron.

What this means in practice:

• If your GP has prescribed ferrous sulfate to treat iron-deficiency anaemia, don’t switch to a lower-dose supplement without discussing it first. The prescribed dose is chosen to rebuild your stores within a set timeframe.
• If ferrous sulfate is causing side effects you can’t tolerate, tell your GP. Options include a different form, a lower dose, or taking it on alternate days (see below).
• If you’re maintaining iron levels after treatment, or have a mild shortfall confirmed by a blood test, a gentle bisglycinate supplement is a reasonable choice.

• Bisglycinate (gentle iron) forms are generally better tolerated with less constipation than standard ferrous sulfate
• Pair with vitamin C to improve absorption
• Don’t supplement iron without a confirmed deficiency — iron overload is a real risk, particularly for people with the genetic condition haemochromatosis, and unnecessary supplementation isn’t harmless

Our pick: Solgar Gentle Iron Bisglycinate — a well-tolerated form suited to people who find standard iron supplements upset their stomach.

Best Time to Take an Iron Supplement

Take iron in the morning, ideally on an empty stomach or with a light breakfast, together with vitamin C, and away from tea, coffee, calcium and zinc.

Morning is better than evening. Absorption is regulated by a hormone called hepcidin. Hepcidin levels follow a daily rhythm and tend to be lower in the morning, so you absorb more iron from a morning dose.

Empty stomach vs with food. Iron is best absorbed on an empty stomach, about an hour before breakfast. If that makes you feel sick, taking it with a light meal is a reasonable compromise. A dose you keep taking beats a better-absorbed one you abandon.

Pair it with vitamin C. A glass of orange juice or a vitamin C tablet taken at the same time improves absorption of non-haem iron. “Vitamin C increases iron absorption” is an EFSA-authorised health claim. → Vitamin C guide

Keep these at least two hours away from your iron dose:

Reduces iron absorption	Fine with iron
Tea and coffee	Vitamin C
Calcium supplements, milk, dairy	Vitamin D
Zinc supplements	Vitamin B12
Antacids and some indigestion medicines

Alternate-day dosing. Each iron dose raises hepcidin for around a day, which reduces absorption of the next dose. Research suggests that taking iron every other day can improve the proportion absorbed and may cause fewer side effects. Some NHS services now offer this approach. Discuss it with your GP rather than changing a prescribed regimen yourself.

How much iron do you need? The UK Reference Nutrient Intake is 8.7mg a day for men and women over 50, and 14.8mg a day for women aged 19–50. Treatment doses for deficiency are much higher and should come from your GP.

Iron Deficiency Without Anaemia: Symptoms and Why It’s Missed

You can be low in iron without being anaemic. Your body uses up its iron stores first and keeps haemoglobin normal for as long as it can. Anaemia only appears once those stores are exhausted. The stage before that is called iron deficiency without anaemia, and it can cause symptoms of its own.

Commonly reported symptoms include:

• Tiredness and low energy, often the main complaint
• Difficulty concentrating
• Hair shedding
• Restless legs, particularly at night
• Reduced exercise tolerance
• Brittle nails

These overlap with many other causes, so they can’t confirm iron deficiency on their own.

Why it gets missed: a full blood count measures haemoglobin, and haemoglobin can still be normal at this stage. The test that shows depleted stores is serum ferritin. NICE guidance treats a ferritin below 30 micrograms per litre as confirming iron deficiency. Ferritin also rises during infection and inflammation, so a “normal” result can sometimes hide low stores.

If you’re tired and your blood test came back “normal”, ask your GP whether ferritin was included and what the actual number was. Women with heavy periods, vegetarians and vegans, endurance athletes, regular blood donors and pregnant women are the groups most likely to be in this situation.

→ Vitamin B12 vs Iron Deficiency: How to Tell the Difference · Why Am I Always Tired?

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Frequently Asked Questions

This page is for general educational purposes only and is not medical advice. Persistent fatigue, breathlessness, or palpitations should be checked by a GP with a blood test before starting iron supplementation — taking iron without a confirmed deficiency can cause harm.

Sources & Further Reading

• NHS — Iron deficiency anaemia
• NIH Office of Dietary Supplements — Iron Fact Sheet
• EFSA — Health claims on iron
• Linus Pauling Institute — Iron Micronutrient Information
• Examine.com — Iron

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