Why every distance runner needs Vitamin C with their iron, and almost none take it
Iron deficiency is the most common nutritional deficiency in distance runners. The fix isn't more iron. It's understanding why the iron you're already taking isn't getting through.
The problem most runners don't know they have
If your long runs feel harder than they should, if you're hitting the wall earlier than your training suggests you should, if you feel fine at rest but inexplicably exhausted under load. There's a good chance your ferritin is low.
Ferritin is your iron storage protein. When it drops, your muscles receive less oxygen, your energy production falters, and your perceived effort climbs. You can be non-anaemic with completely normal hemoglobin and still have ferritin low enough to tank your performance.
Distance runners are uniquely vulnerable. Foot-strike hemolysis, the destruction of red blood cells caused by the repetitive impact of running, depletes iron with every mile. Female runners compound this with monthly blood loss from menstruation. Plant-forward runners add a third layer: their iron sources are non-heme, the form the body absorbs at a fraction of the efficiency of heme iron from meat.
The number that matters: Sports scientists recommend distance runners maintain ferritin above 50 ng/mL for optimal performance. Most lab reference ranges flag deficiency below 12 ng/mL. You can be at 20, feel exhausted, and your doctor will tell you your iron is "normal."
Where Vitamin C enters, and why it changes everything
Non-heme iron, the kind in plant foods, iron supplements, and fortified foods, absorbs at roughly 2–10% under normal conditions. That means if you take a 20mg iron supplement, your body might actually use 1–2mg of it.
Add Vitamin C at the same time, and that absorption rate jumps to 6–30%. The mechanism is straightforward: Vitamin C converts ferric iron (Fe³⁺) into ferrous iron (Fe²⁺), the only form your intestinal cells can absorb through standard transporters. It also chelates iron to keep it soluble in the alkaline environment of the small intestine, preventing it from forming insoluble compounds that pass through unabsorbed.
The practical result: taking your iron with 100mg of Vitamin C can triple what actually makes it into your bloodstream. You don't need more iron. You need to absorb the iron you're already taking.
What to actually take
Form matters enormously with iron. Ferrous sulfate, the form in most drugstore iron supplements, absorbs reasonably well but causes significant GI distress in many people. Carbonyl iron is gentler but absorbs slowly. Iron bisglycinate is the gold standard for runners: the iron molecule is chelated to two glycine amino acids, allowing it to bypass some of the absorption inhibitors that block other forms and causing dramatically less constipation and nausea.
Every-other-day dosing is now backed by research showing higher absorption than daily dosing. The body's hepcidin response resets overnight, allowing better uptake on alternating days.
View on Amazon →What to avoid when taking iron
Coffee and tea contain tannins that block up to 60% of iron absorption. Take your iron at least an hour away from your morning coffee. Calcium competes with iron for the same absorption receptors. Don't take iron with dairy or a calcium supplement. Take it with food to reduce GI effects, but ideally a meal without these inhibitors.
Timing note: hepcidin, the hormone that regulates iron absorption, is least active in the morning. Take your iron first thing or before your morning run for best results.