A bigger iron dose does not get you proportionally more iron
In young women low on iron, 40 mg delivered 6.7 mg absorbed and 240 mg only 18.1 mg. Doses of 60 mg and up raise a hormone that shuts absorption down for a day.
What the study found
Six times the dose, under three times the iron absorbed
In young women low on iron, 60 mg or more raises hepcidin, the hormone that shuts absorption down, and the block lasts a day, cutting uptake of the next dose by 35 to 45%. Going from 40 mg to 240 mg lifted iron absorbed only from 6.7 mg to 18.1 mg, so more is not proportionally more.
The caveat · A day's uptake, not stores rebuilt
Fifty-four young Swiss women, split across three sub-studies, all with low iron stores and none anaemic. What was measured is how much iron crossed the gut over a day or two, not haemoglobin or stores rebuilt over months. The paper argues the hepcidin timing supports alternate-day dosing but never tested such a schedule.