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Best Supplements for Runners (2026)
Runners face unique nutritional demands that make certain supplements more relevant than for the general population. Iron is the standout: foot-strike hemolysis, sweat losses, and GI blood loss from long-distance running make iron deficiency surprisingly common, even in male runners. A 2019 meta-analysis found that iron supplementation improved VO2max and exercise efficiency in iron-depleted (not just anemic) athletes. We scored the top supplements relevant to running performance, starting with iron.
See the full Iron (Bisglycinate) scorecard →What the Evidence Says About Iron (Bisglycinate)
How A-F grades work- ATreatment of iron-deficiency anemia (confirmed by blood test)
- BBisglycinate form: fewer GI side effects vs. ferrous sulfate
- BReducing fatigue in non-anemic iron deficiency (low ferritin, normal hemoglobin)
- BCognitive function in iron-deficient adolescents and women
- BAthletic performance improvement in iron-deficient athletes
- CHair loss due to iron deficiency
- FIron supplementation in replete individuals (no deficiency)
A = strong RCT evidence · B = moderate · C = limited · D = weak · F = no evidence.
Our Top Picks
NOW Foods Iron Bisglycinate 36mg
$0.12/day at effective dose
Detailed Reviews
Nature Made Iron 65mg (Ferrous Sulfate)
ferrous sulfate tablet | 65mg/serving | 365 servings
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.
Check Price on Amazon →Thorne Iron Bisglycinate
ferrous bisglycinate chelate capsule | 25mg/serving | 60 servings
NSF Certified for Sport makes this the top choice for competitive athletes and anyone prioritizing certification rigor. Clean capsule with minimal excipients.
Check Price on Amazon →Pure Encapsulations Iron-C
iron bisglycinate + 100mg ascorbic acid, hypoallergenic capsule | 25mg/serving | 60 servings
Built-in vitamin C is a meaningful practical advantage. The go-to option for people with food allergies or multiple sensitivities. Practitioner-grade quality.
Check Price on Amazon →Also Scored
NOW Foods Iron Bisglycinate 36mg
$0.12/day | iron bisglycinate chelate vegetable capsule
Solgar Gentle Iron 25mg
$0.21/day | iron bisglycinate chelate (Albion FERROCHEL), vegetable capsule
MegaFood Blood Builder
$0.47/day | ferrous bisglycinate chelate + whole-food cofactors, tablet
Garden of Life mykind Organics Iron
$0.33/day | iron from organic plant blend (form unspecified), tablet
Flora Floradix Iron + Herbs Liquid Formula
$0.56/day | ferrous gluconate liquid with herbal extracts
What to Look For When Buying
- ✓Get ferritin levels tested before supplementing iron - iron overload is harmful, and runners should know their baseline
- ✓Iron bisglycinate is preferred for runners due to better absorption and fewer GI issues during training
- ✓Take iron away from calcium, coffee, and tea but with vitamin C for maximum absorption
- ✓Electrolyte replacement is evidence-based for runs over 60 minutes - sodium is the most important electrolyte to replace
- ✓Vitamin D status affects muscle function and bone density - runners training indoors or at high latitudes should test and supplement if needed
- ✓Caffeine at 3-6mg/kg body weight, taken 30-60 minutes before running, has strong evidence for endurance performance
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.
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.