Key points: Runners become iron deficient simply from the pounding of their muscles and muscle breakdown. Iron in muscle (myoglobin) is then lost, binding with hemosiderin and is no longer available to your body for iron’s many critical functions in oxygen carriage, and electron transport in cells and other enzymatic functions. Yes, if your ferritin is below 50, your iron stores are too low for an endurance athlete even if you are not anemic. No reason not to run with a low ferritin, though the pounding will continue, but you will underperform and feel very weak and tired.
An otherwise healthy male runner needs no other explanation for low iron stores, ie low ferritin. For women runners the problem is greatly compounded by menses. However, if the low ferritin does not respond to replacement, then you may need to be checked for diseases like celiac that further interfere with absorption or for iron loss secondary to blood loss usually from the GI tract.
As for replacement.
1) Iron is extremely poorly absorbed and if you take too much on a given day, not only will you simply not absorb it, but it paradoxically leads to a further down regulation of the absorption. Even at best you only absorb about 25% of the oral dose. You really can’t handle more than 65 mg of elemental iron per dose.
2) Iron is best absorbed on an empty stomach and in the ferrous +2 state and when soluable. Taking iron with Vit C improves all of these factors.
3) Given #1 and #2 you can’t do much better than taking 65 mg of elemental iron (325 mg of ferrous sulfate) with 500 mg of Vit C on an empty stomach every other day.
4) If your ferritin does not improve on this regimen you should be tested for abnormal blood loss beyond the running issue and diseases that further block absorption. However, because of the poor absorption of iron even under normal circumstances, it takes months for oral iron to get your stores (ferritin level) back up. You may need to be treated with IV iron if you really don’t respond to oral replacement or don’t tolerate it, and this can bypass all of these issues and replete your iron stores in 1-4 doses. Common formulations are ferraheme or venofer.