Iron deficiency is the most common nutritional deficiency worldwide. The problem is that the most prescribed form of iron supplementation — ferrous sulphate — causes constipation, nausea, and stomach cramping in a significant proportion of people, which leads many to stop taking it.
The solution isn't to stop supplementing. It's to switch form.
Why Standard Iron Supplements Cause Constipation
Most iron supplements contain ferrous sulphate — a cheap, effective source of iron that's been used medicinally for decades. It works. The problem is delivery: unabsorbed iron in the gut binds to gut bacteria, disrupts the gut microbiome, generates oxidative stress in the intestinal lining, and slows gut motility. The result is constipation, dark stools, and nausea.
Around 40-50% of people on standard ferrous sulphate supplementation experience GI side effects significant enough to interrupt their regimen. This defeats the purpose.
The Forms That Work Without the Side Effects
Iron Bisglycinate (Ferrous Bisglycinate Chelate)
The best-evidenced gentle alternative. Bisglycinate means the iron is bound to two glycine molecules in a chelated form. This chelate:
- Is absorbed primarily through a different intestinal pathway (peptide transporter, separate from standard iron absorption)
- Produces fewer free iron ions in the gut (the unabsorbed iron that causes GI irritation)
- Has absorption efficiency comparable to ferrous sulphate at lower elemental iron doses
A 2005 study in the Journal of the American College of Nutrition found that iron bisglycinate was absorbed 2-4x more efficiently than ferrous sulphate at equivalent doses, meaning you need less of it to achieve the same iron repletion — which further reduces GI side effects.
A 2014 randomised controlled trial comparing ferrous sulphate to iron bisglycinate in iron-deficient women found equivalent increases in haemoglobin with significantly fewer GI side effects in the bisglycinate group.
Ferrous Gluconate
Less studied than bisglycinate but generally better tolerated than ferrous sulphate. Moderately well absorbed. A reasonable second-choice if bisglycinate is unavailable or cost-prohibitive.
Carbonyl Iron
A form of elemental iron that releases very slowly in the gut, reducing the local concentration of free iron that causes GI irritation. Good tolerability profile. Absorbed more slowly than ionic forms, which can mean slower iron repletion — potentially a consideration when correcting severe deficiency quickly.
Forms to Generally Avoid for Sensitive Stomachs
Ferrous sulphate: Effective but highest GI side effect rate. Fine for people who tolerate it without issues.
Ferric iron forms (ferric sulphate, ferric ammonium citrate): Must be converted to ferrous iron before absorption — less efficient and generally harder on the gut than ferrous forms.
Who Needs Iron Supplementation
Iron deficiency anaemia affects around 1.2 billion people globally, disproportionately women of reproductive age and pregnant women (due to menstrual blood loss and pregnancy demands), vegetarians and vegans (plant-based iron is less bioavailable), distance runners (foot-strike haemolysis breaks down red blood cells), and people with gut absorption issues (coeliac disease, inflammatory bowel disease).
Get tested before supplementing. Iron supplementation without confirmed deficiency is unnecessary and potentially harmful — excess iron causes its own problems (oxidative stress, GI irritation, in serious cases iron overload). Ask your GP for a full blood count plus ferritin (iron stores) test. Ferritin below 12-15 ng/mL indicates depleted stores; ferritin below 30 ng/mL may cause symptoms even without frank anaemia.
Top Iron Supplements for Sensitive Stomachs
1. Best Overall — Thorne Iron Bisglycinate
Elemental iron per capsule: 25mg Form: Iron bisglycinate (Ferrochel — patented chelate form) Third-party tested: NSF Certified for Sport
Thorne is one of the most respected supplement manufacturers for quality and purity. Their iron bisglycinate uses Ferrochel — the specifically patented, best-studied bisglycinate chelate with multiple published bioavailability trials. 25mg elemental iron is a clinically effective dose for daily maintenance. Clean formula with no unnecessary additives. NSF certification provides strong quality assurance.
2. Best Value — Solgar Gentle Iron (Iron Bisglycinate)
Elemental iron per capsule: 25mg Form: Iron bisglycinate Third-party tested: Solgar internal standards (not third-party certified)
Solgar's Gentle Iron is one of the most widely available and best-value bisglycinate options. 25mg elemental iron per capsule, straightforward formula, and their "Gentle Iron" branding accurately reflects its GI tolerability profile. The lack of independent third-party certification is a minor drawback compared to Thorne, but Solgar has a long-standing quality reputation.
3. Best for Pregnancy — Garden of Life Vitamin Code Iron
Elemental iron per serving: 22mg Form: Iron from food culture (saccharomyces cerevisiae) + whole food blend Third-party tested: NSF Certified
A whole-food based iron supplement designed with pregnant women in mind. The iron is cultured in yeast rather than being a synthetic chelate, producing a form that the body recognises alongside natural food cofactors. Includes vitamin C (which significantly enhances iron absorption — the best dietary strategy for improving iron bioavailability) and a probiotic blend for gut support. More expensive but a premium option for pregnancy or for people who prefer food-sourced supplements.
4. Best Liquid Option — Floradix Iron and Herbs
Elemental iron per serving: 10mg Form: Ferrous gluconate in liquid Third-party tested: Independently verified (varies)
Floradix is a well-established liquid iron supplement containing ferrous gluconate alongside fruit juices and herbal extracts. The liquid format allows flexible dosing and some people find liquids easier to tolerate than capsules. Lower elemental iron per dose than bisglycinate capsules, so may require larger serving sizes for significant deficiency correction. Good option for people who can't swallow capsules or want a gentle daily maintenance dose.
Comparison Table
| Product | Form | Elemental Iron | GI Tolerance | 3rd Party | Best For |
|---|---|---|---|---|---|
| Thorne Iron Bisglycinate | Ferrochel bisglycinate | 25mg | Excellent | NSF | Best quality overall |
| Solgar Gentle Iron | Bisglycinate | 25mg | Excellent | Internal | Best value |
| Garden of Life | Food-cultured | 22mg | Very good | NSF | Pregnancy/food preference |
| Floradix | Ferrous gluconate liquid | 10mg | Good | Variable | Can't swallow capsules |
How to Take Iron for Best Results
Take with vitamin C. Vitamin C (ascorbic acid) significantly increases non-haem iron absorption — the addition of 100mg vitamin C (half a bell pepper or a glass of orange juice) alongside iron supplementation increases uptake meaningfully. Most studies on iron bioavailability use this combination.
Avoid taking with calcium, tea, or coffee. Calcium directly competes with iron for absorption. Tea and coffee contain tannins that bind iron and inhibit absorption. Take iron supplements at least 2 hours away from calcium supplements, dairy products, tea, and coffee.
Don't take on an empty stomach if sensitive. Taking iron with a small amount of food reduces GI irritation, with only a modest reduction in absorption. For bisglycinate, GI tolerance is generally good regardless, but a light snack alongside is sensible if you're already sensitive.
Allow 3-4 months for full repletion. Iron stores (ferritin) take 3-4 months of consistent daily supplementation to replenish after deficiency. Don't judge whether a supplement is working based on a few weeks — retest ferritin at 12-16 weeks.