What is the recommended ascorbic acid (vitamin C) dose to enhance oral iron absorption in an adult with iron‑deficiency anemia (and in children), and what safety limits should be observed?

Ascorbic Acid Dosing with Oral Iron for Iron Deficiency

Take 250–500 mg of ascorbic acid (vitamin C) together with your oral iron supplement to enhance absorption, ideally in the morning on an empty stomach. 1, 2, 3

Recommended Vitamin C Dose for Adults

• 250–500 mg of ascorbic acid taken concurrently with oral iron is the evidence-based recommendation from the British Society of Gastroenterology and multiple international guidelines. 1, 2, 3
• A practical alternative is one glass of orange juice (providing approximately 80–100 mg vitamin C), which significantly improves non-heme iron absorption when taken with the iron tablet. 2, 3, 4
• The lower threshold of 80 mg vitamin C is sufficient to enhance iron absorption, though 250–500 mg provides maximal benefit. 2, 3

Pediatric Dosing

• For children, the same principle applies: vitamin C should be co-administered with oral iron, though specific pediatric vitamin C dosing is not explicitly defined in the guidelines reviewed. 1
• The general pediatric iron dose is 2–3 mg/kg elemental iron daily, and vitamin C should be given proportionally (e.g., via age-appropriate citrus juice or chewable vitamin C). 1

Optimal Administration Strategy

• Take iron once daily in the morning on an empty stomach with 250–500 mg vitamin C (or a glass of orange juice), then wait at least 1–2 hours before eating or consuming coffee, tea, or calcium-containing foods. 2, 5, 4
• Avoid taking iron in the afternoon or evening, as circadian hepcidin elevation reduces absorption of doses taken later in the day. 2, 5, 6
• Alternate-day dosing (every other day with 100–200 mg elemental iron plus vitamin C) significantly increases fractional iron absorption and reduces gastrointestinal side effects while maintaining efficacy. 2, 6, 4

Safety Limits and Contraindications

Upper Safety Limit for Vitamin C

• No specific upper limit for vitamin C is defined for iron-deficient patients, as vitamin C itself is tightly regulated by renal excretion and does not accumulate to toxic levels in healthy individuals. 7
• Doses up to 500 mg daily are universally considered safe for co-administration with iron in iron-deficient adults and children. 1, 2, 3

Absolute Contraindications

• Patients with hemochromatosis or iron overload must completely avoid vitamin C supplements, as vitamin C accelerates iron mobilization from stores, saturates transferrin, and can cause cardiac arrhythmias, cardiomyopathy, and sudden death. 2, 3
• If hemochromatosis patients require vitamin C for other medical reasons, limit to 500 mg daily and only after physician discussion, and never during the iron depletion phase of phlebotomy therapy. 2, 3
• Patients with beta-thalassemia major or sickle cell anemia with transfusion-related iron overload should also avoid vitamin C supplementation unless under specialized hematology supervision. 7

Mechanism and Evidence Base

• Vitamin C enhances non-heme iron absorption by forming a soluble chelate with ferric iron at acidic pH that remains soluble at the alkaline pH of the duodenum, and by reducing ferric (Fe³⁺) to ferrous (Fe²⁺) iron, the more readily absorbed form. 3, 8, 4
• Non-heme iron (the form in oral supplements and plant foods) has baseline absorption of only 1–15%, compared to 15–40% for heme iron from animal sources, making vitamin C particularly important for oral iron therapy. 3, 8
• A systematic review and meta-analysis found that vitamin C co-administration with iron did not significantly increase hemoglobin or ferritin levels compared to iron alone, though the quality of evidence was very low and the practice remains widely recommended based on mechanistic data. 9

Common Pitfalls to Avoid

• Do not take iron with tea or coffee, as polyphenols are powerful inhibitors of iron absorption and should be avoided for at least 1 hour after the iron dose. 2, 3, 5
• Do not take iron with calcium-containing foods, dairy products, or calcium supplements, as calcium competitively inhibits iron absorption. 2, 5
• Do not take iron with antacids, H2 blockers, or proton pump inhibitors, as these reduce gastric acidity required for iron solubilization and absorption. 5, 4
• Do not assume all patients need vitamin C supplementation—those with normal or elevated ferritin should not receive iron or vitamin C, as this may be harmful. 2, 5

When Vitamin C Is Especially Important

• Vegetarian or vegan patients rely entirely on non-heme iron and benefit most from vitamin C co-administration. 3, 8
• Patients taking iron with food (due to gastrointestinal intolerance) require vitamin C even more, as it partially overcomes the absorption-blocking effects of dietary calcium, fiber, and phytates. 2, 3
• Patients with inflammatory bowel disease in remission (who can tolerate oral iron) should take vitamin C to maximize the limited absorption capacity. 2