Iron deficiency: testing, supplementation, and when not to supplement

Iron is one of the more important supplements to get right because it's one of the few where excess is genuinely harmful and deficiency is genuinely common. The decision to supplement should follow testing, not assumption.

Who's at higher risk of deficiency#

• Menstruating women, especially with heavy periods
• Pregnant women
• Vegetarians and vegans (non-haem iron is less bioavailable)
• Athletes, particularly endurance runners (foot-strike haemolysis, GI losses)
• Children and adolescents in growth spurts
• People with GI bleeding sources (ulcers, colitis, colon cancer: always investigate)
• People with malabsorption (coeliac, IBD, after gastric surgery)
• Older adults with reduced dietary intake

How to test#

Ferritin is the practical marker. It reflects iron stores. NHS reference ranges vary slightly between labs but typically:

• <15 µg/L: iron deficiency
• 15–30 µg/L: borderline / suggestive of low iron stores
• 30–300+ µg/L: normal range

• 500 µg/L without inflammation: investigate iron overload (haemochromatosis)

Caveats:

• Ferritin is an acute-phase reactant; it rises with inflammation or infection, possibly hiding true iron deficiency. If CRP is elevated, ferritin may underestimate deficiency severity.
• Pregnancy and oral contraceptives affect ferritin levels
• Athletes may have "functional iron deficiency" at higher ferritin levels than the general population thresholds

Full iron panel (ferritin + transferrin saturation + total iron-binding capacity + serum iron) gives more information; reserved for unclear cases or when haemochromatosis is suspected.

Haemoglobin alone misses early iron deficiency. Many people are iron-deficient without being anaemic.

Symptoms#

Classic symptoms of iron deficiency:

• Fatigue, often disproportionate to sleep
• Reduced exercise tolerance
• Cold intolerance
• Brittle nails, hair loss
• Headaches
• Restless legs
• Pica (cravings for ice, dirt, paper)
• Pallor
• Shortness of breath on exertion

Many of these overlap with other conditions; testing matters.

Supplementation#

For confirmed iron deficiency, NHS guidance generally recommends:

• Ferrous sulfate 200 mg once daily (or every other day per recent evidence)
• Ferrous fumarate as alternative
• Both provide ~65 mg elemental iron
• Take on empty stomach for absorption (or with vitamin C)
• Avoid taking with tea, coffee, milk, or calcium supplements (significant absorption inhibition)
• Continue for 3–6 months after ferritin normalises to fully replenish stores

A 2017 trial (Stoffel et al.) in iron-depleted women aged 18–40 found alternate-day dosing gave higher fractional iron absorption than consecutive-day dosing (21.8% vs 16.3%); the trial did not assess GI side effects. Many gastroenterologists now recommend alternate-day as first-line.

Side effects of supplementation#

Up to 30–50% of people experience GI symptoms with standard iron supplementation:

• Constipation (most common)
• Nausea
• Abdominal cramping
• Dark stools (cosmetic only)

Alternatives if intolerant:

• Alternate-day dosing
• Lower-dose preparations
• Heme iron supplements
• IV iron infusion (specialist; for severe deficiency or supplementation failure)

Why not to supplement without testing#

Haemochromatosis is the major reason. The HFE gene mutation causing hereditary haemochromatosis affects roughly 1 in 200 people of Northern European ancestry. Carriers can develop iron overload that damages liver, heart, and pancreas over years.

Routinely supplementing iron in undiagnosed haemochromatosis accelerates this damage.

Other risks of supplementing without deficiency:

• Iron has pro-oxidant effects in excess
• Risk of misdiagnosing the underlying issue (e.g. taking iron when actually fatigued from something else)
• GI side effects without benefit

For most adults, casually adding iron to a multivitamin without testing is acceptable (RNI-level doses are low risk). High-dose iron supplementation as a standalone is not.

What the wellness market overstates#

• "Iron boosts energy." Only if you were deficient. Not in iron-replete adults.
• "Everyone is iron deficient." Not true in well-nourished populations.
• "Liquid iron / heme iron is dramatically better." Modestly better bioavailability and GI tolerance in some products; not transformative.
• "Iron protects against cancer or heart disease." No good evidence; some evidence suggests excess iron may actually increase risk.

What we'd actually say#

If you have symptoms suggesting iron deficiency (particularly disproportionate fatigue, heavy periods, vegetarian/vegan diet, or athletic training load), get tested. A ferritin test on the NHS is a standard part of fatigue work-up via your GP.

If you're confirmed deficient, supplementation is well-evidenced. Standard ferrous sulfate is cheap and effective despite GI side effects.

If you're not deficient, don't supplement iron beyond what's in a standard multivitamin. The downside risk is real.

For menstruating women, women planning pregnancy, and endurance athletes, periodic ferritin checks are a reasonable habit.

When to talk to a clinician#

• Persistent unexplained fatigue
• Heavy menstrual bleeding affecting quality of life
• Iron deficiency that doesn't respond to supplementation (consider malabsorption or ongoing loss)
• Iron deficiency in men or post-menopausal women: always investigate GI source
• Family history of haemochromatosis
• Pregnancy or planning pregnancy
• Vegetarian or vegan diet with ongoing low ferritin