What Is Iron Deficiency? Causes, Stages & Who's at Risk

The short answer. Iron deficiency is a state in which the body does not have enough iron to meet its needs. Iron is the mineral that lets red blood cells carry oxygen, so when stores run low the whole body works harder for less. It happens for three broad reasons: not enough iron coming in from the diet, too much being lost through bleeding, or too little being absorbed by the gut. It develops in stages, starting with depleted stores that a routine blood test cannot see, and ending, if unaddressed, in iron deficiency anaemia.

What iron actually does

Iron sits at the centre of haemoglobin, the protein inside red blood cells that binds oxygen in the lungs and releases it around the body. It also supports the enzymes that turn food into energy and the signalling that keeps muscles and the brain working normally. When iron falls, oxygen delivery falls with it, which is why the earliest signs are tiredness, breathlessness, and difficulty concentrating rather than anything obviously blood-related.

The body holds a reserve of iron, mostly as ferritin, and it spends that reserve first. This is the reason deficiency is quiet at the start: the account is being drained before anything visibly changes.

The three ways iron runs low

Almost every case of iron deficiency traces back to one or a combination of three mechanisms.

• Not enough coming in. A diet low in iron, or one that relies on plant sources. Plant iron is non-heme iron and absorbs at a fraction of the rate of the heme iron found in meat and fish, so a person can eat what looks like enough and still fall short.
• Too much going out. Blood loss is the fastest route to deficiency because blood is where most of the body's iron lives. Heavy periods are the single largest cause in Canada. Pregnancy and childbirth, regular blood donation, and slow bleeding in the gut all draw the reserve down.
• Not being absorbed. Even with enough iron in the diet, the gut has to let it through. Coeliac disease, inflammatory bowel disease, H. pylori infection, bariatric surgery, and long-term acid-reducing medication all lower how much iron crosses the gut wall.

The stages, and why the timing is confusing

Iron deficiency is a slope, not a step. It moves through three stages, and only the last one reliably shows on a standard blood count. This is the source of the most common frustration people report: feeling unwell while being told their results are fine.

• Storage depletion. Ferritin falls while haemoglobin holds steady. A complete blood count looks normal. Symptoms can already be present. Most people spend months here without a diagnosis.
• Iron-deficient production. Stores are empty and the body starts running short of the iron it needs to build red cells. Haemoglobin begins to slip, sometimes still inside the normal range.
• Iron deficiency anaemia. Haemoglobin drops below normal and the routine blood count finally flags it. This is where most diagnoses happen, long after the symptoms began.

The takeaway for anyone asking for a test: request ferritin by name, not just a complete blood count. Ferritin is what sees the early stages, and it is the number worth knowing.

Who is most at risk

Iron need is not spread evenly across people. The highest-need groups are set by biology and life stage rather than by age alone.

• Women with heavy or frequent periods, who lose iron on a monthly cycle that intake struggles to match.
• People who are pregnant or in the first postpartum year, when requirement roughly doubles and then stays elevated through recovery and breastfeeding.
• Vegetarians and vegans, relying on non-heme iron and therefore needing more of it and better absorption conditions.
• Endurance athletes, who lose iron through sweat, gut, and the impact of foot strike, and absorb less after hard sessions.
• Infants, toddlers, and teenagers, growing faster than a typical diet keeps up with.
• Anyone with a gut condition or on long-term acid reducers, where absorption is the bottleneck.

A frequent misconception: iron deficiency is not a condition of older age. Menopause lowers iron need. The genuine high-risk window is perimenopause, when cycles often become heavier before they stop.

How it is confirmed and corrected

Diagnosis is a blood test: a complete blood count for haemoglobin, and ferritin for stores. Correction depends on the cause. Where the cause is dietary or menstrual, iron is usually addressed with diet and a supplement, and the form matters, because the standard high-dose salts that upset some stomachs are a common reason people stop before their stores recover. Where the cause is bleeding of unknown origin, especially after menopause, the priority is finding the source rather than topping up the iron.

Frequently asked questions

What is iron deficiency in simple terms?

It is having too little iron for the body's needs, which limits how well red blood cells carry oxygen and leaves you tired, breathless, and foggy.

What is the main cause of iron deficiency?

Blood loss, most often heavy periods, is the leading cause in Canada. Low dietary intake and poor absorption are the other two routes.

Is iron deficiency the same as anaemia?

No. Iron deficiency can exist with normal haemoglobin. Anaemia is the later stage, once haemoglobin has dropped below normal.

What test confirms iron deficiency?

A ferritin test shows iron stores and catches the early stages. A complete blood count checks haemoglobin and catches anaemia. Ask for both.