Iron (ferritin-gated)
Bisglycinate, alternate-day
Strong
Several good studies agree
Strong — conditional on a ferritin test. Two RCTs (CMAJ, BMJ) plus IV-iron trials agree; hemochromatosis (~1 in 250) makes blind supplementation unsafe.
In women with low iron stores, tiredness fell by about half, against about a third on a placebo. Low iron also lowers your fitness and raises your resting heart rate, so fixing it shows up clearly on a watch. Only worth it if a blood test says you are low.
In menstruating women with low ferritin (under 50 µg/L) but no anemia, iron cut fatigue by almost half versus a third on placebo. Iron deficiency also lowers VO₂ max and raises resting heart rate — so correcting a real deficiency is one of the most wearable-visible changes possible. But iron is only helpful when you are actually low, and harmful when you're not: test first.
Iron is needed for hemoglobin and mitochondrial enzymes; deficiency limits oxygen delivery and forces the heart to beat faster for the same output.
Menstruating women, endurance athletes, vegetarians — after ferritin + CBC. Alternate-day dosing improves absorption; take away from coffee, tea and calcium.
What to buy
- ✓Ferrous bisglycinate (gentle) 25–30 mg elemental, or ferrous sulfate 65 mg
- ✓Take every OTHER day with vitamin C
- ✕Any iron before a ferritin + CBC test
- ✕Taking with coffee, tea, calcium or dairy
- ⚑ Ferritin + CBC
- · NEVER without ferritin + CBC — hemochromatosis affects ~1/250; excess iron is pro-oxidant.
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Chelated iron with fewer GI side effects.
Buy on AmazonUmbral does not pin a product here on purpose. The instant ferritin strips sold on Amazon rate between 1.9 and 3.4 stars and are not accurate enough to decide whether to take iron — and taking iron you do not need causes real harm. Ask your doctor for a ferritin plus CBC panel drawn at a lab.
Prefer to compare yourself? This opens Amazon’s full selection for this supplement, filtered to the form we recommend.
See all on AmazonEvery result, next to the line that means no effect
Each diamond is a study's best estimate. The thin line shows how sure the study was: the shorter the line, the more certain the result.
- Fatigue score (iron)% · Vaucher 2012−47.7%[not given]
- Fatigue score (placebo)% · Vaucher 2012−28.8%[not given]
Each unit gets its own scale, so numbers are never mixed on one line. When a study gave no range, we leave it blank instead of inventing one.
3 studies over 14 years
The studies
- 1.Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trialCMAJ · 2012 · rct · n = 198
80 mg/day for 12 weeks: fatigue −47.7% vs −28.8% placebo in women with ferritin <50 µg/L.
- 2.Iron supplementation for unexplained fatigue in non-anaemic women: double blind randomised placebo controlled trialBMJ · 2003 · rct · n = 144
Fatigue improved with iron, mainly in women with ferritin ≤50 µg/L.
- 3.Iron absorption from oral iron supplements given on consecutive versus alternate daysThe Lancet Haematology · 2017 · crossover · n = 40
Alternate-day dosing raised fractional iron absorption versus daily dosing.
Umbral is a wellness product, not a medical device. It does not diagnose, treat, cure or prevent any disease. Supplement information is educational; talk to a clinician before starting anything, especially if you are pregnant, under 18, managing a condition or taking medication.
These statements have not been evaluated by the Food and Drug Administration.