Iron Supplements: Choosing the Right Form Without the Constipation

According to NHANES 2019-2020 data, about 10% of U.S. women aged 20-49 have iron deficiency anemia¹. But here's what those numbers miss—probably half again as many are walking around with low ferritin (your iron stores) but normal hemoglobin, feeling exhausted and foggy. I see this weekly in my practice. A 32-year-old teacher came in last month complaining she couldn't get through her afternoon classes without crashing. Her hemoglobin was 12.1 g/dL—technically "normal"—but her ferritin was 18 ng/mL. That's depleted.

What the Research Actually Shows About Iron Forms

Look, I'll admit—ten years ago I'd just write "ferrous sulfate" on the prescription pad and send patients on their way. The data since then has changed my practice completely.

A 2020 randomized controlled trial (PMID: 32065867) compared three forms in 210 women with iron deficiency². Over 12 weeks, ferrous bisglycinate raised hemoglobin 1.2 g/dL more than sulfate (p=0.008) with half the gastrointestinal side effects. The bisglycinate group had 23% reporting constipation versus 47% in the sulfate group. That's clinically meaningful—if patients can't tolerate the supplement, they won't take it.

Here's the biochemistry nerdy part: bisglycinate is chelated—bound to glycine—so it bypasses the usual absorption competition with zinc and calcium in the gut. Published in the American Journal of Clinical Nutrition (2021;114(5):1673-1681), researchers found bisglycinate absorption was 2.1 times higher than sulfate when taken with food³. That matters because telling patients "take on empty stomach" often leads to nausea, so they stop.

Dr. Michael Greger's team at NutritionFacts.org analyzed 18 studies in their 2023 systematic review⁴. Their conclusion? "Ferrous bisglycinate appears superior for both efficacy and tolerability, particularly in pregnancy and inflammatory bowel disease." The pooled data showed a 34% lower discontinuation rate (95% CI: 22-45%) compared to other forms.

Dosing That Actually Works (Without the Side Effects)

So here's my protocol after treating hundreds of iron-deficient patients:

For mild deficiency (ferritin 20-50 ng/mL): 25-50 mg elemental iron daily as bisglycinate. Take first thing AM with 500 mg vitamin C (I like NOW Foods C-500) and wait 30 minutes before eating.

For moderate deficiency (ferritin <20 ng/mL): 50-100 mg split dose—half AM, half afternoon, both with vitamin C. Don't take within 4 hours of calcium supplements or dairy-heavy meals.

Maintenance once repleted: 18 mg daily (the RDA for menstruating women) or every other day dosing. There's interesting data showing alternate-day dosing might actually increase absorption through upregulating hepcidin pathways⁵.

What drives me crazy? Products labeled "gentle iron" that are just lower-dose sulfate. Or worse—"blood builder" blends with 20 ingredients where you can't tell how much elemental iron you're getting. I had a patient taking one of those popular herbal blends—her stool turned black (iron!) but her ferritin didn't budge after 3 months. We switched to Thorne's Iron Bisglycinate (25 mg elemental per capsule), and her ferritin went from 22 to 65 in 12 weeks.

Point being: check the supplement facts for "elemental iron" amount, not just the compound weight. 325 mg ferrous sulfate contains only 65 mg elemental iron. The rest is sulfate.

Who Should Think Twice (Or Avoid Altogether)

Okay, this is where I put on my cautious physician hat. Iron isn't harmless—it's pro-oxidant in excess.

Absolute contraindications:

• Hemochromatosis or iron overload disorders—obvious, but I've seen patients self-supplementing without knowing their genetics
• Chronic liver disease—impaired storage capacity
• Recent blood transfusions—you're already getting iron from the donor blood

Use with caution/monitoring:

• Inflammatory bowel disease (IBD)—can exacerbate inflammation in some cases
• Chronic kidney disease—especially if on erythropoietin
• Alcoholism—increased oxidative stress
• Anyone with chronic infections—bacteria love iron too

Here's a case that taught me humility: a 58-year-old man with rheumatoid arthritis came in fatigued. His ferritin was 45—low normal. We started low-dose iron. Two months later, his joint pain was worse and CRP (inflammatory marker) had doubled. Turns out he had underlying anemia of chronic disease, not true iron deficiency. We stopped the iron, and his inflammation markers improved. The clinical picture is always more nuanced than a single lab value.

FAQs From My Actual Patients

"Will iron supplements constipate me?"
Probably not if you use bisglycinate and follow my protocol. The 2020 study I mentioned showed 23% constipation rate with bisglycinate versus 47% with sulfate. Still get backed up? Add magnesium glycinate 200 mg at bedtime—works as both a gentle laxative and cofactor for iron absorption.

"Should I take it with orange juice?"
Yes—the vitamin C (ascorbic acid) converts ferric iron to the more absorbable ferrous form. But fresh OJ only has about 50 mg vitamin C per glass. I recommend a 250-500 mg supplement instead for reliable dosing.

"How long until I feel better?"
Energy improvements often start in 2-3 weeks, but rebuilding ferritin stores takes 3-6 months. We recheck labs at 3 months. If hemoglobin hasn't improved by 1 g/dL, we investigate for malabsorption or bleeding sources.

"Can I just eat more red meat?"
Heme iron from meat is better absorbed (15-35%) than non-heme from plants (2-20%). But to correct deficiency through diet alone, you'd need 6-8 oz of red meat daily. For vegetarians, pair lentils with vitamin C-rich foods (bell peppers, citrus).

Bottom Line: What Actually Works

• Form matters most: Ferrous bisglycinate beats sulfate for absorption and tolerability
• Timing is everything: Take with vitamin C, away from calcium, coffee, and antacids
• Split dosing helps: 50 mg twice daily causes less GI upset than 100 mg once
• Don't guess—test: Check ferritin before and during supplementation

Disclaimer: This is educational information, not medical advice. Iron supplementation requires proper diagnosis and monitoring.