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Iron (Bisglycinate)
Iron bisglycinate is the right form if you have confirmed iron deficiency and cannot tolerate ferrous sulfate.
- Evidence
- Strong Evidence
- Category
- Vitamins & Minerals
- Best form
- ferrous bisglycinate chelate
- Effective dose
- 18mg elemental iron daily for maintenance (RDA for premenopausal women)
- Lab tested
- 7 of 8 products
- Category
- Vitamins & Minerals
- Best form
- ferrous bisglycinate chelate
- Effective dose
- 18mg elemental iron daily for maintenance (RDA for premenopausal women)
- Lab tested
- 7 of 8 products
Key takeaways
- →Works for confirmed deficiency, full stop. Bisglycinate matches ferrous sulfate's hemoglobin gains with meaningfully fewer GI side effects.
- →Maintenance is 18mg elemental; anemia treatment runs 50-65mg under medical guidance. Take fasted with vitamin C, away from coffee, dairy, and calcium.
- →Thorne ($0.33/day, NSF Certified for Sport) is the top pick; NOW Foods Iron Bisglycinate 36mg ($0.12/day) is the value option.
- →Never supplement without a ferritin test - iron has no excretion route, and excess causes oxidative damage. Separate from levothyroxine by 4 hours.
What Is Iron (Bisglycinate)?
Iron bisglycinate is the right form if you have confirmed iron deficiency and cannot tolerate ferrous sulfate. In confirmed deficiency the case for iron is unambiguous, it restores hemoglobin and ferritin and resolves the associated fatigue and cognitive symptoms, and iron deficiency affects an estimated 2 billion people, concentrated in pregnant women, menstruating women, and anyone with low ferritin. Head-to-head trials show bisglycinate matches ferrous sulfate on iron status while causing significantly less nausea, constipation, and stomach pain. The one absolute rule: do not supplement without a blood test. The body cannot excrete excess iron, and men and postmenopausal women should only take it when ferritin confirms they need it.
The form matters mostly for tolerability. Ferrous sulfate is the standard clinical form, but it causes nausea, constipation, and stomach pain in many people. Head-to-head trials show that iron bisglycinate achieves similar improvements in iron status with significantly fewer GI side effects. This is the central argument for bisglycinate: comparable efficacy, much better tolerability.
Bisglycinate is a chelated form where iron is bound to glycine molecules. This protects the iron from binding with compounds in food (phytates, tannins, calcium) that block absorption, and it uses a different absorption pathway that causes less gut irritation. It absorbs nearly as well as ferrous sulfate when fasting and better when taken with food.
One critical point: iron supplementation should not be taken without confirmed deficiency. The body has no way to excrete excess iron, so it accumulates and causes oxidative damage. Men and postmenopausal women rarely need iron and should only supplement if blood work confirms low levels. A serum ferritin test - not just a standard blood count - is the most sensitive marker and should be checked before and during supplementation.
Does It Work? The Evidence
How A-F grades workTreatment of iron-deficiency anemia (confirmed by blood test)
Pasricha et al. Cochrane Review 2013; WHO iron supplementation guidelines; multiple large RCTs in women and children
Bisglycinate form: fewer GI side effects vs. ferrous sulfate
Tolkien ZJ, et al. PLOS ONE 2015 (n=90 RCT); Name JJ, et al. Int J Vitam Nutr Res 2018 (systematic review)
Reducing fatigue in non-anemic iron deficiency (low ferritin, normal hemoglobin)
Vaucher P, et al. CMAJ 2012 RCT (n=198); Favrat B, et al. Blood 2011 - fatigue improved with ferritin normalization
Cognitive function in iron-deficient adolescents and women
Murray-Kolb LE, Beard JL. Am J Clin Nutr 2007; Bruner AB, et al. Lancet 1996 - improved attention and memory in iron-deficient adolescent girls
Athletic performance improvement in iron-deficient athletes
Burden RJ, et al. Br J Sports Med 2015 meta-analysis; improvements in VO2 max and endurance in iron-deficient (not just anemic) athletes
Hair loss due to iron deficiency
Trost LB, et al. J Am Acad Dermatol 2006 review - association between low ferritin and telogen effluvium; causality not firmly established in RCTs
Iron supplementation in replete individuals (no deficiency)
No RCTs support iron supplementation in iron-sufficient individuals; risk of harm from excess iron is established
| Grade | Claimed Benefit | Key Studies | Our Verdict |
|---|---|---|---|
| A | Treatment of iron-deficiency anemia (confirmed by blood test) | Pasricha et al. Cochrane Review 2013; WHO iron supplementation guidelines; multiple large RCTs in women and children | Supported |
| B | Bisglycinate form: fewer GI side effects vs. ferrous sulfate | Tolkien ZJ, et al. PLOS ONE 2015 (n=90 RCT); Name JJ, et al. Int J Vitam Nutr Res 2018 (systematic review) | Supported |
| B | Reducing fatigue in non-anemic iron deficiency (low ferritin, normal hemoglobin) | Vaucher P, et al. CMAJ 2012 RCT (n=198); Favrat B, et al. Blood 2011 - fatigue improved with ferritin normalization | Early Signal |
| B | Cognitive function in iron-deficient adolescents and women | Murray-Kolb LE, Beard JL. Am J Clin Nutr 2007; Bruner AB, et al. Lancet 1996 - improved attention and memory in iron-deficient adolescent girls | Early Signal |
| B | Athletic performance improvement in iron-deficient athletes | Burden RJ, et al. Br J Sports Med 2015 meta-analysis; improvements in VO2 max and endurance in iron-deficient (not just anemic) athletes | Early Signal |
| C | Hair loss due to iron deficiency | Trost LB, et al. J Am Acad Dermatol 2006 review - association between low ferritin and telogen effluvium; causality not firmly established in RCTs | Conflicted |
| F | Iron supplementation in replete individuals (no deficiency) | No RCTs support iron supplementation in iron-sufficient individuals; risk of harm from excess iron is established | Ineffective |
How to Choose: Forms, Doses & What Matters
Clinical dose: 18mg elemental iron daily for maintenance (RDA for premenopausal women); 50-65mg elemental iron daily for treating documented iron-deficiency anemia. Dose must be confirmed by blood test.
Best forms: ferrous bisglycinate chelate, iron bisglycinate, ferrous sulfate (standard clinical form, lower GI tolerance), ferrous fumarate, carbonyl iron
For best absorption, take iron on an empty stomach (30-60 minutes before a meal or 2 hours after). However, iron taken on an empty stomach causes more GI discomfort in most people - this is less of an issue with bisglycinate than ferrous sulfate, but still a factor. Taking it with a small amount of food is a reasonable tradeoff for those who cannot tolerate it fasted. Vitamin C (ascorbic acid, 100-200mg) taken simultaneously enhances non-heme iron absorption by maintaining iron in the ferrous (Fe2+) state, which is more readily absorbed. This is well-established and recommended by the NIH. Orange juice or a vitamin C supplement works equally well. Avoid taking iron within 2 hours of: dairy products and calcium supplements (calcium directly competes with iron for absorption), coffee and tea (tannins and polyphenols reduce absorption by 40-90%), phytate-rich foods like whole grains and legumes, and the medications listed above. Alternate-day dosing (every other day rather than daily) has emerging evidence suggesting it may be as effective as daily dosing for some people while causing fewer side effects and potentially being more cost-effective. A 2017 study in Blood by Stoffel et al. found that fractional iron absorption was significantly higher on alternating days due to reduced hepcidin upregulation. Do not exceed prescribed doses or self-escalate. The typical course for treating iron-deficiency anemia is 3-6 months of supplementation, with recheck of ferritin and hemoglobin at 4-8 weeks and at treatment completion. Continue supplementation for at least 3 months after hemoglobin normalizes to replenish iron stores.
Who Should Take Iron (Bisglycinate)?
Iron supplementation is appropriate only when deficiency is confirmed by blood testing. The relevant tests are serum ferritin (most sensitive indicator of iron stores), hemoglobin and hematocrit (detect anemia), and optionally transferrin saturation and total iron binding capacity (TIBC) for a complete picture. Populations with a legitimate need for iron supplementation include: Premenopausal women with heavy menstrual bleeding - the most common cause of iron deficiency in developed countries. Women lose 20-80ml of blood per cycle on average; heavy bleeding (more than 80ml) can deplete iron stores even with an otherwise adequate diet. Pregnant women - iron requirements nearly double during pregnancy due to expanded red blood cell mass, placental needs, and fetal development. The NIH recommends 27mg elemental iron daily during pregnancy, and supplementation is routinely recommended. Infants (especially premature) and toddlers - rapid growth combined with low iron stores at birth in premature infants creates high deficiency risk. People with gastrointestinal conditions causing malabsorption - celiac disease, Crohn's disease, gastric bypass surgery, and H. pylori infection all impair iron absorption. Frequent blood donors - regular whole-blood donation depletes iron stores; the American Red Cross recommends monitoring ferritin in frequent donors. Athletes, particularly endurance athletes and female athletes - high iron losses through sweat, hemolysis from foot-strike, and GI microbleeding combine with often-marginal dietary intakes to create deficiency risk. Vegans and vegetarians consuming only non-heme iron (plant-based iron), which has substantially lower bioavailability than heme iron from meat.
Who Should Avoid It?
Not for everyone
Side Effects & Safety
Product Scores
8 products scored on dosing accuracy, third-party testing, cost per effective dose, and label transparency.
The Scorecard: 8 Products Compared
Nature Made Iron 65mg (Ferrous Sulfate)
Nature Made$8.99 ÷ 450 days at ~53mg/day (0.8 servings × 65mg)
Included as a comparison reference point. USP Verified and extremely cheap. The catch: ferrous sulfate causes GI side effects (nausea, constipation) in 30-70% of users. If you tolerate it, it is equally effective to bisglycinate at a fraction of the cost. If you do not tolerate it, bisglycinate is the answer.
Prices checked 2026-03-01. Cost shown is per clinically effective daily dose, not per pill.
Thorne Iron Bisglycinate
Thorne$19.80 ÷ 60 days at 25mg/day (1 serving × 25mg)
NSF Certified for Sport makes this the top choice for competitive athletes and anyone prioritizing certification rigor. Clean capsule with minimal excipients.
Prices checked 2026-03-01. Cost shown is per clinically effective daily dose, not per pill.
Pure Encapsulations Iron-C
Pure Encapsulations$26.40 ÷ 60 days at 25mg/day (1 serving × 25mg)
Built-in vitamin C is a meaningful practical advantage. The go-to option for people with food allergies or multiple sensitivities. Practitioner-grade quality.
Prices checked 2026-03-01. Cost shown is per clinically effective daily dose, not per pill.
NOW Foods Iron Bisglycinate 36mg
NOW Foods$10.90 ÷ 91 days at 36mg/day (1 serving × 36mg)
The most cost-effective bisglycinate option in this comparison. Good choice for budget-conscious buyers who do not require NSF or USP certification.
Prices checked 2026-03-01. Cost shown is per clinically effective daily dose, not per pill.
Solgar Gentle Iron 25mg
Solgar$18.75 ÷ 89 days at 25mg/day (1 serving × 25mg)
Uses Albion FERROCHEL - the most clinically studied iron bisglycinate chelate form. A meaningful distinction for those who want the exact form used in trials.
Prices checked 2026-03-01. Cost shown is per clinically effective daily dose, not per pill.
MegaFood Blood Builder
MegaFood
$27.99 ÷ 60 days at 26mg/day (1 serving × 26mg)
The only product in this comparison with a brand-specific clinical trial. NSF Content Certified. The beet root and whole-food matrix are marketing differentiators without established clinical superiority over plain bisglycinate.
Prices checked 2026-03-01. Cost shown is per clinically effective daily dose, not per pill.
Garden of Life mykind Organics Iron
Garden of Life$9.99 ÷ 30 days at 18mg/day (1 serving × 18mg)
Limited to the RDA dose (18mg) and uses a proprietary blend that obscures the iron form. Adequate for maintenance in non-deficient people but not appropriate for treating iron-deficiency anemia.
Prices checked 2026-03-01. Cost shown is per clinically effective daily dose, not per pill.
Flora Floradix Iron + Herbs Liquid Formula
Flora
$27.99 ÷ 50 days at 10mg/day (1 serving × 10mg)
Popular with pregnant women and those who cannot swallow capsules. The 10mg per serving dose is too low for deficiency treatment - would require multiple servings daily to reach therapeutic range, making cost very high. Better suited for maintenance than repletion.
Prices checked 2026-03-01. Cost shown is per clinically effective daily dose, not per pill.
Full Comparison
| Category | Nature Made Iron 65mg (Ferrous Sulfate) Nature Made | Thorne Iron Bisglycinate Thorne | Pure Encapsulations Iron-C Pure Encapsulations | NOW Foods Iron Bisglycinate 36mg NOW Foods | Solgar Gentle Iron 25mg Solgar | MegaFood Blood Builder MegaFood | Garden of Life mykind Organics Iron Garden of Life | Flora Floradix Iron + Herbs Liquid Formula Flora |
|---|---|---|---|---|---|---|---|---|
| Brand Score | 94/100Winner | 92/100 | 90/100 | 88/100 | 86/100 | 77/100 | 76/100 | 61/100 |
| Dosing & Form | 25/25Winner | 25/25 | 25/25 | 25/25 | 25/25 | 25/25 | 25/25 | 18/25 |
| Purity | 22/25 | 25/25Winner | 23/25 | 20/25 | 20/25 | 20/25 | 19/25 | 17/25 |
| Value | 25/25Winner | 19/25 | 17/25 | 23/25 | 19/25 | 13/25 | 15/25 | 11/25 |
| Transparency | 22/25 | 23/25 | 25/25Winner | 20/25 | 22/25 | 19/25 | 17/25 | 15/25 |
| Cost/Day | $0.02Winner | $0.33 | $0.44 | $0.12 | $0.21 | $0.47 | $0.33 | $0.56 |
| Dose/Serving | 65mg | 25mg | 25mg | 36mg | 25mg | 26mg | 18mg | 10mg |
| Form | ferrous sulfate tablet | ferrous bisglycinate chelate capsule | iron bisglycinate + 100mg ascorbic acid, hypoallergenic capsule | iron bisglycinate chelate vegetable capsule | iron bisglycinate chelate (Albion FERROCHEL), vegetable capsule | ferrous bisglycinate chelate + whole-food cofactors, tablet | iron from organic plant blend (form unspecified), tablet | ferrous gluconate liquid with herbal extracts |
| Third-Party Tested | ✓ Yes | ✓ Yes | ✓ Yes | ✓ Yes | ✓ Yes | ✓ Yes | ✓ Yes | No |
| Proprietary Blend | No | No | No | No | No | No | Yes | Yes |
Frequently Asked Questions
What is the difference between iron bisglycinate and ferrous sulfate?
Both supply elemental iron to treat deficiency, but they differ in tolerability and how they are absorbed. Ferrous sulfate is the classic clinical form - cheap, well-studied, and effective, but it releases ionic iron that directly irritates intestinal lining, causing nausea and constipation in 30-70% of users. Iron bisglycinate chelate binds each iron atom to two glycine amino acids. This protects the iron from binding to inhibitors in food (phytates, tannins, calcium) and allows it to be absorbed through a different intestinal pathway that causes far less irritation. A 2015 RCT by Tolkien et al. found bisglycinate produced equivalent increases in ferritin and hemoglobin to ferrous sulfate but with significantly fewer GI adverse effects. The tradeoff is cost - bisglycinate products typically cost 2-5x more per milligram of elemental iron.
Do I really need a blood test before taking iron supplements?
Yes - this is one of the most important safety points about iron. Unlike water-soluble vitamins that your body can simply excrete if you get too much, iron accumulates in the body. Excess iron generates oxidative damage to the liver, heart, and other organs. Iron overload disorders (including hereditary hemochromatosis, which affects 1 in 250 people of Northern European descent) can be worsened or unmasked by supplementation. Additionally, the correct dose depends entirely on your current iron status - a person with ferritin of 8 needs a very different approach than someone with ferritin of 12. A serum ferritin test costs under $30 and gives you the information you need. Do not guess.
How long does it take for iron supplements to work?
Hemoglobin typically begins rising within 2-4 weeks of adequate iron supplementation, with a target of increasing by at least 1 g/dL in 4 weeks in those with true iron-deficiency anemia. Symptoms like fatigue and weakness often improve within 4-8 weeks as hemoglobin normalizes. However, replenishing iron stores (restoring ferritin to a healthy level) takes considerably longer - typically 3-6 months of continued supplementation after hemoglobin normalizes. Stopping supplementation as soon as you feel better is a common mistake that leads to relapse. Follow your doctor's guidance on duration and recheck ferritin at treatment completion.
Why do iron supplements cause constipation and how can I prevent it?
Constipation from iron is mainly caused by free ionic iron irritating the intestinal lining and altering gut motility. Several strategies reduce it: choose iron bisglycinate or carbonyl iron over ferrous sulfate; take with food (sacrifices some absorption but greatly improves tolerance); try alternate-day dosing (which also allows better absorption per dose according to 2017 research in Blood by Stoffel et al.); increase dietary fiber and fluid intake; and ensure adequate magnesium intake, which supports bowel motility. If constipation is severe and unresponsive to these measures, IV iron (given in a clinical setting) bypasses the GI tract entirely and is used in patients with severe intolerance or malabsorption.
Should I take vitamin C with my iron supplement?
Yes, and this is a well-supported strategy. Vitamin C (ascorbic acid) keeps iron in the ferrous (Fe2+) state, which is more readily absorbed by intestinal cells than the ferric (Fe3+) state. Taking 100-200mg of vitamin C simultaneously with your iron supplement increases non-heme iron absorption. This works with any iron form. A small glass of orange juice with your iron supplement is a practical way to do this. Note that this benefit matters more for ferrous sulfate than for bisglycinate, since bisglycinate's chelation already partially protects against absorption inhibitors.
Can vegans and vegetarians get enough iron from diet alone?
It is possible but requires deliberate effort. Plant foods contain only non-heme iron, which has 2-5 times lower bioavailability than the heme iron found in meat, poultry, and fish. Additionally, plants high in iron (legumes, whole grains, spinach) often contain phytates and oxalates that further reduce iron absorption. The NIH recommends that vegetarians consume 1.8 times the standard iron RDA due to lower bioavailability. Good plant iron sources include lentils, tofu, tempeh, pumpkin seeds, quinoa, fortified cereals, and dark leafy greens. Pairing them with vitamin C-rich foods and avoiding coffee and tea at meals significantly improves absorption. Vegans and vegetarians should monitor ferritin levels, particularly if female or athletically active.
Related Supplements
Related Reading
Related Articles
Sources
- NIH Office of Dietary Supplements. Iron Fact Sheet for Health Professionals. Updated 2023.
- Tolkien ZJ, Stecher L, Mander AP, Pereira DI, Powell JJ. Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis. PLoS ONE. 2015;10(2):e0117383.
- Name JJ, Vasconcelos AR, Maluf-Souza AC, Lages JF. Iron bisglycinate chelate and polymaltose iron for the treatment of iron deficiency anemia: a pilot randomized trial. Int J Vitam Nutr Res. 2018;88(3-4):182-190.
- Pasricha SR, Flecknoe-Brown SC, Allen KJ, et al. Diagnosis and management of iron deficiency anaemia: a clinical update. Med J Aust. 2010;193(9):525-532.
- Stoffel NU, Cercamondi CI, Brittenham G, et al. Iron absorption from oral iron supplements given on consecutive versus alternate days and as multiple versus single doses in iron-depleted women: two open-label, randomised controlled trials. Lancet Haematol. 2017;4(11):e524-e533.
- World Health Organization. Guideline: Daily iron supplementation in adult women and adolescent girls. Geneva: WHO; 2016.
- Vaucher P, Druais PL, Waldvogel S, Favrat B. Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial. CMAJ. 2012;184(11):1247-1254.
- Burden RJ, Morton K, Richards T, Whyte GP, Pedlar CR. Is iron treatment beneficial in, iron-deficient but non-anaemic (IDNA) endurance athletes? A meta-analysis. Br J Sports Med. 2015;49(21):1389-1397.
FDA Disclaimer: These statements have not been evaluated by the Food and Drug Administration. The products discussed on this page are not intended to diagnose, treat, cure, or prevent any disease. Always consult with a qualified healthcare professional before starting any supplement regimen.