Iron deficiency is the most common nutritional deficiency in the world. Women of reproductive age are disproportionately affected. And yet it remains chronically under-screened, partly because its symptoms overlap with half a dozen other conditions, and partly because the wrong test is often used to detect it. Here's what you actually need to know.
Why women are at higher risk
Women lose iron monthly through menstruation. The average menstrual cycle causes a loss of approximately 20–80ml of blood, translating to 10–35mg of iron per cycle. Women with heavier periods can lose significantly more, including those with menorrhagia or endometriosis, both of which are commonly associated with heavier blood loss. (PCOS affects menstrual patterns differently and does not consistently result in heavier periods.)
Active women compound this further. Endurance exercise causes haemolysis, the destruction of red blood cells through foot-strike impact (in runners) and sustained cardiovascular stress. Sweat also contains small amounts of iron. Women who train regularly have a meaningfully higher iron requirement than sedentary guidelines assume. (Peeling et al., European Journal of Applied Physiology, 2008)
The problem with standard testing
Standard blood tests screen for haemoglobin, the iron-containing protein in red blood cells that carries oxygen. Haemoglobin levels only drop when iron deficiency is severe. By the time haemoglobin is flagged as low, you've typically been functionally iron-deficient for months.
The marker that matters is serum ferritin, the storage form of iron. Ferritin levels drop long before haemoglobin is affected, making it a far more sensitive marker of early deficiency. Optimal ferritin for athletic performance is considered to be above 50 mcg/L; many labs only flag deficiency below 12–15 mcg/L. A result of 18 mcg/L might be reported as 'normal' by a GP, but for an active woman, it is not.
If you're experiencing unexplained fatigue or poor recovery and you haven't had your ferritin tested specifically, ask for it by name.
What it actually feels like
Iron-deficient women commonly report: persistent fatigue that sleep doesn't resolve, brain fog and difficulty concentrating, reduced exercise tolerance (sessions that used to feel manageable suddenly feel very hard), slow recovery after training, cold hands and feet, and brittle nails. These symptoms are also consistent with overtraining, thyroid dysfunction, depression, and poor sleep, which is why iron deficiency often goes unidentified for a long time.
If you've been told 'your bloods are fine' but continue to feel fatigued, ask specifically for serum ferritin. It is not routinely included in a standard blood panel.
Improving iron status
Dietary iron: Red meat, organ meat (particularly liver), shellfish, and dark leafy greens are the highest-yield sources. Plant-based iron (non-haem iron) is less bioavailable than animal-based iron (haem iron), meaning your body absorbs less of it. For both sources, vitamin C significantly enhances absorption — pair iron-rich meals with foods like oranges, bell peppers, or strawberries, or take a vitamin C supplement alongside your iron.
Avoid iron inhibitors: Tannins (tea, coffee, red wine) and calcium significantly reduce iron absorption. If you're working on iron status, avoid consuming these alongside iron-rich meals.
Supplementation: If ferritin is low, food sources alone may not be sufficient to correct it quickly. Iron bisglycinate is generally better tolerated than ferrous sulphate (which commonly causes gastrointestinal (GI) discomfort). Excessive iron supplementation can be harmful, so if in doubt, work with a GP or sports dietitian on dose and duration.
The bottom line
Iron deficiency is common, under-diagnosed, and has a direct, measurable impact on how you feel and perform. If you train regularly and experience unexplained fatigue, get your serum ferritin tested, not just haemoglobin. The numbers matter. And the fix, once identified, is often surprisingly straightforward.