IronNutrition library

Iron

mg · Minerals

Builds the maternal red-cell mass that carries oxygen to the placenta and supplies the iron stores a newborn relies on for its first months.

Second trimester absorption (recorded)

Second trimester: falling hepcidin raises uptake (~×1.5 non-heme vs T1 in this model). Isotope studies average ~9% fractional absorption at this stage.

How this interacts

Meal-resolved iron

Per meal: heme = Σ(iron × heme fraction), absorbed ~25% (× pregnancy heme factor from T2). Non-heme = remainder at 5% base × that meal's vitamin C × pregnancy non-heme factor (×1.5 T2, ×2 T3 vs T1; hepcidin-suppressed uptake). Calcium from food does not reduce non-heme uptake. Summed to a day total and compared against the trimester absorbed-iron requirement (0.8 / 4.4 / 6.0 mg for T1/T2/T3).

Calcium routing

Absorbed calcium is directed to bone by vitamin D, K2 and magnesium. It does not meaningfully block non-heme iron when eaten as food within a normal day.

Phytate vs. minerals

Phytic acid (IP6), concentrated in the bran/germ of grains, legumes and nuts, chelates zinc, iron and (more mildly) magnesium in the gut, blocking some of their absorption. The effect is dose-dependent: more phytate relative to the mineral means a larger blocked fraction. Vitamin C specifically counteracts phytate's effect on iron (not zinc) by forming an absorbable iron-ascorbate complex.

Tracked by these blood markers

Ferritinµg/L

Trimester focus

Stage	For the mother	For the baby
First trimester	Physiological need is barely above non-pregnant levels — blood volume hasn't yet expanded.	Embryonic iron needs are minimal during organogenesis.
Second trimester	Need rises sharply as blood volume expands ~50%. Maternal hepcidin falls from this trimester, so the gut absorbs more dietary iron (~×1.5 non-heme vs T1 in this model; isotope studies ~9% fractional absorption).	Fetal iron stores begin accumulating in earnest.
Third trimester	Need peaks, compounded by blood loss anticipated at delivery. Absorption is highest now (~×2 non-heme vs T1; isotope studies ~20% fractional absorption) as hepcidin stays suppressed.	The fetus accumulates the iron stores it will rely on for its first ~6 months after birth.

Connections

What links here

Evidence & sources

Evidence-backed^[1] Maternal hepcidin falls from the second trimester
In healthy pregnancies, maternal hepcidin is suppressed in the second and third trimesters, increasing dietary iron absorption and mobilisation from stores.
Evidence-backed^[2] Fractional absorption rises from T2 to T3
Stable-isotope studies: mean fractional absorption ~9% at T2 (weeks ~14–16) rising to ~20% by T3 (weeks ~32–35)—a 2.2-fold increase.
Evidence-backed^[3] How the dashboard models pregnancy uptake
Engine applies ×1.5 non-heme (T2) and ×2.0 (T3) vs T1, with modest heme uplift—anchored to the isotope and hepcidin literature, not a clinical prescription.
Evidence-backed^[4] Food calcium does not blunt non-heme iron
Whole-diet trials show milk and calcium-rich meals do not meaningfully reduce non-heme iron absorption at normal food intakes; pregnancy calcium RCTs likewise show no impact on iron status.
Evidence-backed^[5] Vitamin C enhances non-heme iron in the same meal
Ascorbic acid dose-dependently increases non-heme iron absorption; the engine boosts non-heme uptake with same-meal vitamin C (up to ×2.5 effective).
Evidence-backed^[6] Trimester absorbed-iron requirements
Absorbed-iron targets (0.8 / 4.4 / 6.0 mg/day T1/T2/T3) follow Hallberg/Bothwell modelling cited by DRI committees—distinct from the 27 mg dietary RDA.
Evidence-backed^[7] Phytate blocks non-heme iron; vitamin C counteracts it
Phytate inhibits non-heme iron absorption dose-dependently — a classic trial found 2 mg phytate-phosphorus inhibited absorption 18%, 25 mg inhibited 64%, 250 mg inhibited 82%. Ascorbic acid significantly counteracts this inhibition.

References

Fisher AL, Nemeth E. Iron homeostasis during pregnancy. Am J Clin Nutr 2016PMID 27363197
Young MF, Griffin I, Pressman E, et al.. Maternal hepcidin is associated with placental transfer of iron derived from dietary heme and nonheme sources. J Nutr 2012PMID 22695914
Tussing-Humphreys L, Pustacioglu C, Nemeth E, et al.. Hepcidin and iron homeostasis during pregnancy. Nutrients 2014PMID 25102001
O'Brien KO, Zavaleta N, Abrams SA, et al.. Fetal iron uptake from recent maternal diet and the maternal RBC iron pool. Am J Clin Nutr 2022PMID 35102365
Bothwell TH. Iron requirements in pregnancy and strategies to meet them. Am J Clin Nutr 2000PMID 10919991
Reddy MB, Hurrell RF, Cook JD. The influence of milk and milk products on iron absorption in man. Am J Clin Nutr 2007PMID 17284738
Hallberg L et al. — vitamin C enhances non-heme iron absorption (classic dose–response work)
Hallberg L, Brune M, Rossander L. Iron absorption in man: ascorbic acid and dose-dependent inhibition by phytate. Am J Clin Nutr 1989PMID 2911999

Target note

The 27 mg dietary RDA already assumes ~25% absorption — it's a proxy. The Assessment tab scores modelled ABSORBED iron (with trimester-specific uptake from T2 onward) against the physiological requirement — the more decision-relevant figure.