If you're a woman who still menstruates, this is probably the nutrient to watch most closely. Each cycle draws iron down, and if intake doesn't keep pace, levels fall gradually over months, which is why women need more than twice what men do: around 18 mg a day versus 8 mg.
Iron carries oxygen through your blood to every part of your body. When it runs low, that's associated with persistent tiredness, weakness, and feeling short of breath or worn out by things that didn't used to tire you, a slow drain rather than a sudden one. Because it drifts down slowly, it can go a long time before anything feels clearly wrong.
Across the whole population the Global Burden of Disease study estimates 16.7% run low, but that average hides the real picture, since the shortfall sits far more heavily with women than men. Good sources include red meat, liver, and shellfish, alongside beans, lentils, and spinach.
One thing worth knowing: it's best not to exceed 45 mg a day, because excess iron is genuinely hard on the body rather than simply passed out.