Three common reasons
Losing blood. Heavy periods are a common cause. Bleeding in the stomach or gut — sometimes with no obvious symptoms — is especially important to consider in men and in women after menopause. Giving blood often adds up too.
Not absorbing it. Your gut may not be taking iron in well. Celiac disease, an H. pylori stomach infection, inflammatory bowel disease, past stomach surgery and some acid-reducing medicines can all play a part.
Needing more than you take in. Pregnancy raises your iron needs. A diet low in iron can contribute, but on its own it's a less common reason in adults than the two above.
When to talk to your doctor
If you don't know why you're low, that's a conversation with your doctor — not a diet project. In men and in women past menopause, low iron without a clear cause usually needs a closer look, which can include tests of the stomach and bowel.
Food and supplements can rebuild iron while your doctor looks for a cause; they cannot replace finding that cause.