Iron
Oxygen delivery. The mineral that keeps energy from dropping off a cliff.
Iron carries oxygen in red blood cells and in myoglobin in muscle tissue. Without enough of it, energy drops, concentration slips, and physical endurance falls away. Haem iron from red meat is absorbed at around 20 to 30 percent. Non-haem iron from plants and most supplements is absorbed at 2 to 20 percent depending on what else you eat at the same time. Vitamin C taken alongside a non-haem iron supplement meaningfully increases absorption. Tea, coffee, and calcium taken at the same time reduce it.
Reference values
Daily reference values from official nutrition authorities. These are population reference figures, not personal recommendations.
| Group | RDA | Upper limit | Source |
|---|---|---|---|
| Men, 19+ | 8.7 mg | 17 mg | UK SACN DRVs (1991); EFSA UL 17 mg supplemental Fe (2004) |
| Women, 19–50 | 14.8 mg | 17 mg | UK SACN DRVs (1991) — premenopausal women include menstrual loss compensation |
| Women, 51+ | 8.7 mg | 17 mg | UK SACN DRVs (1991) — postmenopausal women align with male RNI |
Forms and bioavailability
Not all forms are absorbed equally. The form on your supplement label affects how much your body can actually use.
| Form | Absorption | With food | Timing | Evidence |
|---|---|---|---|---|
| ferrous gluconate | Typical | Either | Morning | Likely |
| ferrous sulfate | Typical | Empty stomach | Morning | Strong |
| ferrous bisglycinate | Higher | Either | Morning | Strong |
How it pairs with other supplements
Works well with
Copper is required for ceruloplasmin, which mobilises stored iron for haemoglobin synthesis. Copper deficiency can cause iron-resistant anaemia even with adequate iron intake.
Vitamin C (50-100 mg) taken with non-haem iron enhances absorption by reducing ferric to ferrous form. Stronger effect on plant-source iron.
Things to watch for
Iron and zinc share intestinal divalent-metal transporters; high-dose supplementation of either can modestly reduce uptake of the other. Spacing by a meal or supplementing on different days mitigates the effect.
Calcium reduces absorption of non-haem iron at gram-scale doses. Space supplements by 2+ hours when both are in the stack.
Iron is one of the nutrients where too much matters as much as too little. Stack Almanac monitors your total intake and flags the pairing with Vitamin C, which can help if you rely on plant-based or supplement iron sources.
Track your Iron in Stack Almanac
Last reviewed: 2026-05-21. Reference values are adult-general RDAs from UK. Individual needs may vary. This page is for informational purposes only and is not a substitute for professional medical advice.