Iron Supplements: Forms, Absorption and How to Choose

Are you getting enough iron? Most people aren't sure

Iron deficiency is the most common nutritional deficiency worldwide, affecting an estimated 4–6 billion people according to the World Health Organization. In the UK alone, nearly half of teenage girls and a quarter of women of reproductive age fall below recommended intake levels.

Yet choosing an iron supplement is surprisingly confusing. Different forms, different side effects, conflicting advice on when to take it. This guide breaks down the evidence so you can make an informed choice.

What does iron do in the body?

Iron is an essential mineral with several functions authorised as health claims under EU Regulation 432/2012, retained in UK law post-Brexit:

Normal formation of red blood cells and haemoglobin — iron contributes to the normal formation of red blood cells and haemoglobin
Normal oxygen transport — iron contributes to normal oxygen transport in the body
Normal energy-yielding metabolism — iron contributes to normal energy-yielding metabolism
Reduction of tiredness and fatigue — iron contributes to the reduction of tiredness and fatigue
Normal cognitive function — iron contributes to normal cognitive function
Normal immune system function — iron contributes to the normal function of the immune system

These are not marketing claims. They are the specific, evidence-assessed wording permitted by UK and EU food regulators.

How common is iron deficiency in the UK?

The National Diet and Nutrition Survey (NDNS) provides the most comprehensive picture of iron status in the UK population. The Years 9–11 combined report (covering 2016–2019) shows:

Girls aged 11–18: 49% have iron intakes below the Lower Reference Nutrient Intake (LRNI) — the level below which deficiency is likely
Women aged 19–64: 25% have iron intakes below the LRNI
Iron-deficiency anaemia: 9% of older girls and 5% of adult women showed signs of both low haemoglobin and depleted iron stores

Groups at higher risk include women with heavy menstrual periods, vegetarians and vegans (who rely on less bioavailable non-haem iron), pregnant women, and endurance athletes.

How do iron supplement forms compare?

Not all iron supplements are the same. The form determines how much elemental iron you receive per dose, how well it is absorbed, and how likely you are to experience side effects.

Form	Type	Elemental iron	Absorption	GI side effects	Common use
Ferrous sulfate	Ferrous (Fe²⁺)	20% by weight	Good (10–15%)	Highest — OR 2.32 vs placebo	Most prescribed by NHS
Ferrous gluconate	Ferrous (Fe²⁺)	12% by weight	Good (10–15%)	Lower than sulfate	OTC and personalised supplements
Ferrous fumarate	Ferrous (Fe²⁺)	33% by weight	Good (10–15%)	Moderate–high	Common OTC option
Ferric iron polymaltose	Ferric (Fe³⁺)	Varies	3–4× lower than ferrous	Fewer side effects	Alternative when ferrous not tolerated

The three ferrous forms have comparable absorption rates. The key differentiator is tolerability, not bioavailability — a point confirmed by Santiago (2012) in a clinical overview comparing ferrous and ferric formulations. Ferric (Fe³⁺) forms are less well absorbed because they require reduction to ferrous iron (Fe²⁺) before uptake by intestinal cells.

Ferrous gluconate vs ferrous sulfate: why does the form matter?

This is one of the most practical questions for anyone choosing an iron supplement, and the evidence is clear on the trade-offs.

Ferrous sulfate is the most widely prescribed form in the UK — it is cheap, well-studied, and effective at raising iron levels. However, a systematic review and meta-analysis by Tolkien et al. (2015) across 43 trials and 6,831 participants found that ferrous sulfate significantly increased the risk of gastrointestinal side effects compared to both placebo (OR 2.32, 95% CI 1.74–3.08) and intravenous iron (OR 3.05). These side effects — nausea, constipation, abdominal pain, diarrhoea — are the main reason people stop taking iron supplements before their stores recover.

Ferrous gluconate offers a comparable absorption profile to ferrous sulfate but with a meaningfully lower incidence of gastrointestinal side effects. A systematic review by Cancelo-Hidalgo et al. (2013) across 111 studies and 10,695 patients confirmed that formulation choice significantly affects tolerability.

PARTICULAR uses ferrous gluconate because it strikes the best balance between absorption efficiency and tolerability. A supplement only works if you actually take it consistently — and the most common reason people abandon iron supplementation is side effects from the form, not the iron itself.

What affects iron absorption?

Iron absorption is not just about the supplement. Several dietary and timing factors make a significant difference.

Enhancers

Vitamin C is the most potent dietary enhancer of non-haem iron absorption. Hallberg et al. (1989) demonstrated that ascorbic acid prevents the formation of insoluble iron compounds in the gut and reduces ferric iron to the more absorbable ferrous form. Around 50mg of vitamin C per meal provides an optimal effect
Meat and fish contain a "meat factor" that enhances non-haem iron uptake independently of vitamin C

Inhibitors

Calcium directly inhibits iron absorption in a dose-dependent manner. Hallberg et al. (1991) found that 300–600mg calcium reduced iron absorption by 50–60%, affecting both haem and non-haem iron. This is why taking iron and calcium supplements together is counterproductive
Tannins in tea and coffee can reduce iron absorption by up to 60% when consumed with meals
Phytates in wholegrains and legumes bind iron in the gut, reducing bioavailability
Timing with food — iron is better absorbed on an empty stomach, though this increases the likelihood of GI side effects with conventional tablets

Why do iron supplements cause side effects?

The side effects most people associate with iron supplements — nausea, stomach cramps, constipation, dark stools — are largely caused by unabsorbed iron reacting with the stomach lining and gut mucosa.

When a conventional iron tablet dissolves in the stomach, it releases a concentrated dose of free iron ions into an acidic environment. This causes local oxidative stress and irritation. The Tolkien et al. (2015) meta-analysis confirmed that this is a consistent, dose-independent effect of ferrous sulfate — meaning that even lower doses still cause significant GI issues compared to placebo.

This is a delivery problem, not an iron problem. The form and the way it reaches the gut matter enormously.

How do microgranules change iron delivery?

PARTICULAR's microgranule technology addresses the root cause of iron supplement side effects:

Individual enteric coating — each microgranule is coated with a pH-sensitive layer that prevents dissolution in stomach acid. The iron passes through the stomach intact
Intestinal release — the coating dissolves in the higher pH of the upper small intestine, where iron is naturally absorbed. This eliminates the gastric irritation that causes most side effects
Independent release — because each nutrient is in separate granules, iron and calcium are released independently. This prevents the competitive inhibition demonstrated by Hallberg et al. (1991), where calcium taken simultaneously reduced iron absorption by up to 60%
Controlled dose — rather than a single concentrated bolus, microgranules release iron gradually across a wider surface area of the intestine

The result is iron supplementation that works with your body's natural absorption mechanisms rather than overwhelming them.

How does PARTICULAR personalise iron dosing?

Iron is not a one-size-fits-all nutrient. A post-menopausal woman eating red meat regularly has very different requirements from a vegan woman with heavy periods.

PARTICULAR's questionnaire captures the factors that determine your likely iron needs:

Diet type — vegetarian and vegan diets rely entirely on non-haem iron, which is less bioavailable than haem iron from animal sources
Menstrual status — regular menstruation is the single largest cause of iron loss in pre-menopausal women
Existing supplementation — to avoid unnecessary doubling up
Life stage — pregnancy, adolescence, and perimenopause all change iron requirements significantly

Your iron dose is then calibrated as ferrous gluconate within your personalised microgranule blend — the right amount, in a tolerable form, delivered where your body can actually use it.

Key takeaways

Iron deficiency is the most common nutritional deficiency in the UK, disproportionately affecting women and teenage girls
All ferrous iron forms have similar absorption, but tolerability varies significantly — ferrous gluconate causes fewer GI side effects than ferrous sulfate
Vitamin C enhances iron absorption; calcium, tannins, and phytates inhibit it — timing and combination matter
Most side effects from iron supplements are caused by free iron dissolving in the stomach, not by iron itself
Microgranule delivery bypasses the stomach entirely, releasing iron in the intestine where it is absorbed without the gastric irritation
Personalised dosing through the PARTICULAR questionnaire ensures you get the right amount for your diet, life stage, and menstrual status

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